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United States Department of Agriculture

Agricultural Research Service

Science Results (Winter 2012)
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The Role of Water Subsidy on Vegetation Dynamics in a Semiarid Grassland Catchment: Comparison between Field Measurements and 3-D Ecohydrological Modeling


Niu, G.Y.                    University of Arizona

Troch, P.A.                  University of Arizona

Paniconi, C.                 Université du Québec

Scott, R.L.                  Southwest Watershed Research Center

Durcik, M.                   University of Arizona

Zeng, X.                      University of Arizona

Huxman, T.                 University of Arizona

Goodrich, D.C.           Southwest Watershed Research Center


In dry regions, the exchange of water and carbon between the land and atmosphere is strongly governed by the amount of water in the soil. The amount of soil water may vary considerably across a landscape due to variations in terrain slope, aspect, soils, and vegetation conditions.  However, many models used in climate and weather forecasting do not consider this type of soil water variation.  This study examines key factors controlling soil water availability in a semiarid experimental watershed in southeastern Arizona using a model that was augmented to include and predict these variations in soil water. The model results were compared with numerous measurements and found to reproduce the observations better than a model that did not predict soil water spatial variation.  Further analyses indicate that the wetter soils in lowland areas along stream channels provide plants with favorable conditions to take in more carbon and use more water in times of drought and toward the end of the summer growing season.  These results indicate that it can be important for weather and climate models to include the effects of surface hydrology for certain applications.


Modern pasture monitoring: An example from the Balkhash region of Kazakhstan


Lebed, L.        Kazakhstan Research Institute of Ecology and Climate

Qi, J.                Michigan State University

Heilman, P.     Southwest Watershed Research Center


After the fall of the Soviet Union the scientific establishments in the Central Asian Republics also collapsed. This paper reports on an effort to re-establish a regional rangeland monitoring program in Kazakhstan using an example from the southern Balkhash area. The vegetation community in the example is from a foothill plain with a sagebrush-ephemeral vegetation community and a sand plain with sand loving vegetation in the Turan Desert. The monitoring approach is based on remotely sensed imagery and meteorological data, an archive of soil and vegetation information, and periodic ground sampling. The Pasture simulation model was used to calculate indices to assess pasture land condition. The end product of the monitoring method presented is a set of maps of seasonal forage production and the seasonal capacity of large areas to support grazing. Other Central Asian Republics with similar historical databases developed under the U.S.S.R. could also use the same modern monitoring approach.


Sediment source identification in a semiarid watershed at soil mapping unit scales


Rhoton, F.E.               USDA-ARS National Sedimentation Laboratory

Emmerich, W.E.          Southwest Watershed Research Center (Retired)

McChesney, D.S.        USDA-ARS National Sedimentation Laboratory

Nearing, M.A.             Southwest Watershed Research Center

Ritchie, J.C.                USDA-ARS Hydrology and Remote Sensing Laboratory


Sediment source identification is a scientific approach used to locate the primary sources of sediment and chemical pollutants in watersheds. Given the generally strong association of pollutants with the fine sediment sizes eroded from upland fields, identification of sediment source areas within watersheds is important for targeting areas of the watershed that are the greatest contributors of the pollutants and may require a change in management systems. We developed an accurate soil geomorphology-pedology approach to quantifying soil erodibility in terms of a soil aggregation index which was used to calculate a potential sediment yield index for each individual soil type mapped in a watershed. This ability to identify primary sediment source areas at soil mapping unit scales will permit site specific location of best management practices, and more efficiently reduce sediment and chemical contaminant loadings in watersheds.


Hydrologic response to pulse storm events during a native to exotic transition in a semiarid grassland


Sugg, Z.P.                   University of Arizona

Moran, M.S.                Southwest Watershed Research Center

Scott, C.A.                  University of Arizona

Van Leeuwen, W.       University of Arizona

Nearing, G.                 University of Arizona

Holifield Collins, C.    Southwest Watershed Research Center

Heilman, P.                 Southwest Watershed Research Center

Bryant, R.B.                Southwest Watershed Research Center


An important threat to our national grasslands has been the introduction of African grasses in both humid and arid regions in the Americas. Introduced primarily to revegetate degraded rangeland and to quickly develop pasture for forage, non-native African grass species have in many cases expanded beyond their initial planted area into areas of native vegetation where their presence is unwanted.   This invasive species story is true for Lehmann lovegrass which was introduced to the U.S. Southwest from South Africa in 1932.  This study reports on the impact of Lehmann lovegrass invasion on ecosystem water use by comparing post-storm water loss before, during and after a natural grassland transition. Results indicate that, compared to an ecosystem dominated by native grasses, dominance of Lehmann lovegrass is associated with an increase in water loss through evaporation after storms in the summer growing season.  This cumulative effect contributed to observed increases in post-invasion evaporation over the growing season.  These results offer an insight into the impact of invasive species on water cycle in semiarid rangelands in the western United States.  Greater evaporation from soil could lead to a “drying” of the ecosystem, which could have implications for the persistence of Lehmann lovegrass and the resilience of native species.


A radarsat-2 quad-polarized time series for monitoring crop and soil conditions in Barrax, Spain


Moran, M.S.                Southwest Watershed Research Center

Aonso, L.                    University of Valencia

Moreno, J.                   University of Valencia

Mateo, C.                    University of Vaencia

De la Cruz, F.              Instituto Técnico Agronómico Provincial (ITAP), Albacete, Spain

Montoro, A.                Instituto Técnico Agronómico Provincial (ITAP), Albacete, Spain


New satellite sensors to measure radar backscatter over broad agricultural regions have been recently launched and more are planned for launch in the next decade.  There is great interest in using radar images for monitoring crop and soil condition, including monitoring crop biomass, leaf area, crop residue, plant water content, crop growth stage, soil tillage and soil water content.  In this study, we analyzed a time-series of 57 radar images for large fields of wheat, barley, oat, corn, onion and alfalfa in an agricultural area near Madrid Spain.  We found that radar images provided information that could be used to map the distribution of crops over large areas.  Further, images also provided valuable information about crop growth stage and soil moisture condition that could be used for crop management and yield prediction.  The results from this study provided recommendations for planning future satellite systems to optimize the collection of meaningful information about crop and soil conditions.


Extending results from agricultural fields with intensively monitored data to surrounding areas for water quality management


Heilman, P.                 Southwest Watershed Research Center

Malone, R.                  USDA-ARS Ames, IA

Ma, L.                         USDA-ARS  Fort Collins, CO

Hatfield, J.                  USDA-ARS Ames, IA

Ahuja, L.                     USDA-ARS Fort Collins, CO

Boyle, K.                     NRCS Retired

Kanwar, R.                  Iowa State University


This paper addresses the general question of how the understanding of management effects on pollution loads and farm income developed from intensively monitored agricultural study sites can be applied to a larger area. The specific question is how information about the physical and biological processes at Iowa State University’s Northeast Research Farm near Nashua, Iowa, could be applied over a large area to help farmers select management systems to reduce nitrogen loading in tile drained areas.  Previous research has documented the parameterization and calibration of the RZWQM model at Nashua to simulate 35 management system effects on corn and soybean yields and N loading in tileflow from 1990 to 2003. As most management systems were studied for a 6 year period and in some cases weather had substantial impacts, a set of 30 alternative management systems were also simulated using a common 1974-2003 input climate dataset. To integrate an understanding of the economics of N management, we calculated net returns for all management systems using the DevTreks social budgeting tool. We ranked the 35 observed systems in the Facilitator decision support tool using N loading and net returns and found that rankings from simulated results were very similar to those from the observed results from both an onsite and offsite perspective. We analyzed the effects of tillage, crop rotation, cover crops, and N application method, timing, and amount for the 30 long term simulations on net returns and N loading. Our approach is an example of a proposed framework to create a quality assured database to systematically extend the intensively studied results to the larger area it represents. Currently used approaches to quantifying management effects are either not specific to tile drained agriculture, not site-specific, or focused on reducing N application without quantifying N loading.


Energy exchange and evapotranspiration over two temperate semi-arid grasslands in North America


Krishnan, P.                NOAA

Meyers, T.P.                NOAA

Scott, R.L.                  Southwest Watershed Research Center

Kennedy, L.                National Audubon Society

Heuer, M.                    NOAA


The exchange of energy and water between the land surface and the atmosphere is a key driver of the Earth’s climate system. Understanding the relative roles of climate versus vegetation or land cover on energy exchange processes is critical for predicting how ecosystems will respond to future physical and biological changes, especially to predictions of vegetative and climatic changes.  The seasonal and year-to-year change in surface energy and water exchange of two semiarid grasslands in southern Arizona, USA were investigated using measurements collected from 2004 through 2007. One of the grasslands was a post-fire site and the other was an unburned site.  Although there was large variation between yearly rain totals and differences in site characteristics like vegetation amount, both sites responded similarly to changes in environmental conditions.  The exchanges of energy and evaporation were mainly governed by the amount of rainfall and vegetation growth.


Invasion of shrubland by exotic grasses:  Ecohydrological consequences in cold vs. warm deserts


Wilcox, B.P.                Texas A & M

Turnbull, L.                 Arizona State University

Young, M.H.               University of Texas

Williams, J.                  USDA-ARS

Ravi, S.                       University of Arizona

Seyfried, M.S.             USDA-ARS

Bowling, D.R.             University of Utah

Scott, R.L.                  Southwest Watershed Research Center

Germino, M.J.             Idaho State

Cadwell, T.                 Desert Research Institute

Wainwright, J.             University of Sheffeild


Across the globe, many savannas and woodlands are undergoing conversion to grasslands due the invasion of foreign grass species. Here we summarize the current state of knowledge concerning the hydrological and ecological consequences of this conversion for the desert regions of North America. We surveyed and summarized the peer-reviewed scientific literature and used mathematical models to arrive at the following conclusions. When shrublands are invaded by grasses, many changes take place: rooting depths, canopy cover, species heterogeneity, water use and fire regimes are radically altered. Obviously, then, grass invasion has the potential to alter key ecohydrological processes. With respect to the processes of runoff and erosion, we find that grass invasion influences cold deserts and warm deserts in different ways. In cold deserts, runoff and erosion will increase following invasion; in particular, erosion on steep slopes will be greatly accelerated following burning. In addition, evaporation will be lower and infiltration will be higher which after several decades could affect groundwater levels. For warm deserts grass invasion may actually reduce runoff and erosion (except for periods immediately following fire), and is likely to have little effect on either evaporation or soil water. Significant gaps in our knowledge do remain, primarily because there have been no comprehensive studies measuring all components of the water and energy budgets at multiple scales. How these changes may affect regional changes like weather patterns are still uncertain.


Thermal Optimality of Net Ecosystem Exchange of Carbon Dioxide


Niu, S.                         University of Oklahoma

Luo, Y.                        University of Oklahoma

Fei, S.                          University of Oklahoma

Yuan, W.                     University of Oklahoma

Zhang, Z.                    University of Oklahoma

Schimel D.                  NEON

Amiro, B.                    University of Manitoba

Ammann, C.                Federal Research Station Agroscope Reckenholz-Tänikon, Reckenholzstr

Altaf Arain, M.           McMaster University

Arneth, A.                   Lund University

Aubinet, M.                 Gembloux University of Agronomy

Barr, A.                       Climate Research Division

Beringer, J.                  Monash University

Bernhofer, C.              Technische Universität Dresden

Black, A.                     University of British Columbia

Buchmann, N.             Zurich Institute of Plant Science

Cescatti, A.                 European Commission, Joint Research Center

Chen, J.                       University of Toledo

Davis, K.                     Pennsylvania State University

Dellwik, E.                  Pennsylvania State University

Desai, A.                     University of Wisconsin

Dolman, H.                 University Amsterdam

Drake, B.                     Smithsonian Environmental Research Center

Etzold, S.                    Zurich, Institute of Plant Science

Francois, L.                 Unité de Modélisation du Climat et des Cycles Biogéochimiques

Gianelle, D.                 IASMA, Research and Innovation Centre

Goldstein, A.              University of California Berkeley

Gu, L.                          Oak Ridge National Laboratory

Hanan, N.                    South Dakota State University

Helfter, C.                   Centre for Ecology and hydrology

Hirano, T.                    Hokkaido University

Hollinger, D.               USDA Forest Service

Lindroth A.                 Lund University

Janssens I.                   University of Antwerpen

Jones, M.                     Trinity College

Kiely, G.                     University College Cork, Republic of Ireland

Kolb, T.                       Northern Arizona University

Kutsch, W.                  Johann Heinrich von Thünen-Institute (vTI), Institute for Climate Research

Lafleur, P.                   Trent University

Law, B.                       Oregon State University

Litvak, M.                   University of New Mexico

Loustau, D.                 Research Unit EPHYSE

Lund, M.                     Lund University

Marek, M.                   Institute of Systems Biology and Ecology, Academy of Sciences of the Czech Republic

Matteucci, G.              National Research Council, Institute of Agroenvironmental and Forest Biology

Martin, T.                    University of Florida

Montagnani, L.           Forestry Services, Agency for the Environment, Province of Bolzano Moors, E.                 ESS-CC, Aterra,Wageningen, UR

Munger, J.W.              Harvard University

Noormets, A.              North Carolina State University

Oechel, W.                  San Diego State University

Oejnik, J.                     Poznan University of Life Sciences

Paw U, K.T.                UC Davis

Pilegaard, K.               Technical University of Denmark      

Rambal, S.                   DREAM, CEFE, CNRS

Raschi, A.                   CNR – Istituto di Biometeorologia (IBIMET)

Saleska, S.                   University of Arizona

Scott, R.L.                  Southwest Watershed Research Center

Seufert G.                   Institute for Environment and Sustainability, Joint Research Center European Commission

Spano, D.                    Universita degli Studi di Sassari

Stoy, P.                       Montana State University

Sutton, M.                   Centre for Ecology and Hydrology (CEH)

Varlagin, A.                Russian Academy of Sciences

Vesala, T.                    University of Helsinki

Wohlfahrt, G.              University of Innsbruck

Yakir, D.                     Weizman Institute of Science

Yang, B.                      Oak Ridge National Laboratory


Ecosystems provide multiple services to humans. Models that represent ecosystem functioning, such as the cycling of carbon, are based on relationships with temperature that have been developed for enzyme- or leaf-level processes. The response of the ecosystem in total to changes in temperature, however, is not known.  In this paper, data from a large network of carbon dioxide exchange measurement sites are used to see how this net carbon dioxide exchange varies with temperature. It is found that peak uptake of carbon dioxide by an ecosystem is strongly related to a site’s average growing season temperature, which may indicate an adaptation of plants and organisms within an ecosystem to temperature.  This finding is a novel property of ecosystems that was unknown prior to this synthesis effort and can be useful for the testing of ecosystem models used to predict the response to global climate change.


Water-use-efficiency of annual- and bunchgrass-dominated savanna intercanopy space


Hamerlynck, E.P.        Southwest Watershed Research Center

Scott, R.L.                  Southwest Watershed Research Center

Cavanaugh, M.L         Southwest Watershed Research Center         

Barron-Gafford           Southwest Watershed Research Center


Ecosystem water use efficiency (WUEe), the ratio between net ecosystem carbon dioxide exchange (NEE) and evapotranspiration (ET) is an important functional feature of terrestrial ecosystems.  Savanna ecosystems, which are dynamic shifting mosaics of grass and tree dominated patches, annual or perennial bunchgrasses can dominate tree intercanopy spaces.  These growth forms have very different physical structures, life cycles, and physiological characteristics, all of which could strongly affect savanna WUEe.  We tracked soil moisture content (q25cm), ET and plant transpiration (T), and the constituent fluxes of NEE, ecosystem respiration (Reco) and gross ecosystem photosynthesis (GEP) and plant community water use efficiency (WUEc = GEP/T) to see how the controls to NEE and WUEe changed under these contrasting growth forms.  We specifically expected annual plot WUEe to be lower than in bunchgrass plots, mainly because of lower WUEc.  WUEe was indeed lower in annual dominated plots, but WUEc was the same.  WUEe in annual plot was lower because 1) these plots had higher soil evaporation contributions to ET, since T was similar to bunchgrasses, and 2) Reco was higher in annual plots, even though GEP was similar to bunchgrass plots, resulting in lower NEE.  These findings show that differences in plant community structure dramatically altered the basic controls to ecosystem water and carbon fluxes in intercanopy savanna.


Convergence of Dynamic Vegetation Net Productivity Responses to Precipitation Variability from 10 Years of MODIS


Ponce, G.                    University of Arizona

Moran, M.S.                Southwest Watershed Research Center

Huete, A.                    University of Technology, Sydney Australia

Bresloff, C.                 University of Arizona

Huxman, T.                 University of Arizona

Bosch, D.                    USDA-ARS Southeast Watershed Research Laboratory

Bradford, J.                 US Forest Service

Buda, A.R.                  USDA-ARS Pasture and Watershed Mgmt. Research Unit 

Gunter, S.                    USDA-ARS Southern Plains Range Research Station

McNab, H.                  US Forest Service

McClaran, M.              University of Arizona

Peters, D.                    USDA-ARS Jornada Experimental Range

Sadler, J.                     USDA-ARS Cropping Systems & Water Quality Research Unit

Seyfried, M.                USDA-ARS Northwest Watershed Research Center

Starks, P.                     USDA-ARS Grazing Lands Research Laboratory

Sutherland Montoya, D. USDA Forest Service

Heartsill, T.                 USDA-FS


There is great scientific and public interest in the impact that climate change will have on vegetation across the United States. Due to the great diversity of vegetation at the continental scale, scientists have turned to satellites for information about how changes in precipitation will influence plant growth.  In this study, decades of measurements of climate and vegetation at 13 USDA experimental sites were synthesized to generalize the potential impact of climate change on vegetation ranging from desert grassland in Arizona to tropical forest in Puerto Rico.  First, results showed that satellite measurements offer a viable surrogate for time- and labor-intensive field measurements of plant growth.  Second, it appears that the lower precipitation predicted with climate change will cause all biomes (grassland to forest) to utilize water more efficiently.  This study offers a new paradigm for climate change research based on interpretation of satellite measurements over large regions.  The results of this study will influence the management of our national renewable resources with a better understanding of how vegetation will respond to predicted climate change.


The Potential of Carbonyl Sulfide as a Proxy for Gross Primary Production at Flux Tower Sites


Blonquist, J.                University of Utah

Montzka, S.                 NOAA

Munger, J.W.              Harvard University

Yakir, D.                     Weizmann Institute of Science

Desai, A.                     University of Wisconsin

Griffis, T.                    Indiana University

Monson, R.                 University of Arizona

Scott, R.L.                  Southwest Watershed Research Center

Bowling, D.                University of Utah


Regional and continental scale studies of the seasonal dynamics of atmospheric carbonyl sulfide (OCS) mole fractions and leaf-level studies of plant OCS exchange have shown a close relationship with those for CO2. CO2 has sinks and sources within terrestrial ecosystems, but the primary terrestrial exchange for OCS is thought to be leaf uptake, suggesting potential for OCS uptake as a proxy for gross primary production (GPP). We examined the utility of OCS uptake as a GPP proxy in micrometeorological studies of biosphere-atmosphere CO2 exchange. Theoretical concepts from earlier OCS studies were combined to relate net ecosystem exchange (NEE) and vertical mole fraction gradients of CO2 and OCS to GPP. At the Harvard Forest AmeriFlux site, measured CO2 and OCS vertical gradients were correlated and were related to NEE and GPP, respectively. Estimates of GPP from OCS-based NEE partitioning were similar to GPP derived with established techniques, providing evidence that OCS uptake can potentially serve as a proxy for GPP. Measured vertical CO2 mole fraction gradients at five other AmeriFlux sites were used to project vertical OCS mole fraction gradients to provide indication of potential OCS signals at other sites where no OCS data were available. At the three forest sites, projected OCS gradients were similar in magnitude to those at Harvard Forest. At the two sites with short canopies (C4 grassland and soybean), projected OCS gradients were larger than those in forests, indicating greater potential for OCS uptake as a GPP proxy at these sites.


An Integrated Model Framework of Catchment-Scale Ecohydrological Processes


Niu, G.Y.                    University of Arizona

Paniconi, C.                 Université du Québec

Troch, P.A.                  University of Arizona

Scott, R.L.                  Southwest Watershed Research Center

Durcik, M.                   University of Arizona

Zeng, X.                      University of Arizona

Huxman, T.                 University of Arizona

Goodrich, D.C.           Southwest Watershed Research Center


An accurate prediction of the response of atmospheric, hydrological, and ecological processes in the face of climate change and other challenges requires accurate computer simulation models. In general it has been difficult to create models that are capable of simulating all these processes at the same time.  A complex model was created to couple these processes together and then tested against observations collected in a cold and moist watershed from Vermont and a dry and hot watershed in Arizona.  The coupled model, with minor changes, performed well in simulating the observed stream flow, snow amounts, soil moisture, surface energy, water, and carbon dioxide exchange.  The resulting model provides a basis for pulling together other types of Earth system models (e.g., soil chemistry and erosion models) and for assessing the impacts of climate change on watershed hydrological and ecological processes.





Goodrich, D.C.           Southwest Watershed Research Center

Guertin, D.P.               University of Arizona

Burns, I.S.                   University of Arizona

Nearing, M.A.             Southwest Watershed Research Center

Stone, J.J.                    Southwest Watershed Research Center

Wei, H.                        University of Arizona

Heilman, P.                 Southwest Watershed Research Center

Hernandez, M.            University of Arizona

Spaeth, K.                   NRCS

Pierson, F.                   USDA-ARS

Paige, G.B.                  University of Wyoming

Miller, S.N.                 University of Wyoming

Kepner, W.                  US EPA

Ruyle, g.                      University of Arizona

McClaran, M.P.           University of Arizona

Weltz, M.A.                USDA-Reno

Jolley, L.                     NRCS


America’s rangelands cover about 80% of the western U.S. and provide habitat for wildlife, recreational opportunities, forage for livestock, a source of minerals and raw materials, and water resources for irrigating crops and the rapidly urbanizing western states.  Effective rangeland management requires the ability to assess the potential impacts of management actions on soil erosion and sediment yield at both the hillslope and watershed scales. Many of the current tools1 for assessing and evaluating the effects of rangeland management practices on soil and water resources were originally developed for traditional cropland agricultural practices.  New Decision Support Tools (DSTs) that are easy-to-use, incorporate ecological concepts and rangeland management practices, use readily available data, and are designed to represent rangeland hydrologic and erosion processes are the focus of this article.  The recently developed RHEM (Rangeland Hydrology and Erosion Model) and the Automated Geospatial Watershed Assessment tool (AGWA) form the foundation of this DST.  RHEM is applicable at the hillslope scale.  AGWA enables application of RHEM at the watershed scale, allowing assessments of larger areas.


Modeling climate change effects on runoff and soil erosion in southeastern Arizona rangelands and implications for mitigation with rangeland conservation practices


Zhang, Y.                    University of Arizona

Hernandez, M.             University of Arizona

Anson, E.                    Southwest Watershed Research Center

Nearing, M.A.             Southwest Watershed Research Center

Wei, H.                        University of Arizona

Stone, J.J.                    Southwest Watershed Research Center

Heilman, P.                 Southwest Watershed Research Center


Climate change is expected to change precipitation patterns in the southwestern United States. This study was done to evaluate the potential impacts of precipitation changes on soil erosion and surface water runoff in southeastern Arizona that will occur as a result of rainstorms.  We used the outputs from seven models of climate change for the projected time periods of the 2050s and 2090s in order to run a model to assess what these changes might be compared to 1970 through 1999 conditions.  We used a USDA-ARS model called the Rangeland Hydrology and Erosion Model (RHEM).  Our results suggested no significant changes in annual precipitation across the region, but projected mean annual runoff and soil loss approximately doubled. These dramatic increases in runoff and soil loss were attributed to the increase in the frequency and intensity of extreme events.  Predicted erosion from shrub communities increased more than that for other plant communities under the three scenarios.  This may be a problem, because future increasing runoff and soil erosion could accelerate the transitions of grassland to shrublands or to more eroded states that has already been occurring on the area over the past century.  Rangeland management policies and practices should consider these possible changes and adapt to the increased risk of runoff and soil erosion under a changing climate.


Effects of antecedent soil moisture on runoff modeling in small semiarid watersheds of southeastern Arizona


Zhang, Y.                    Beijing Forestry University

Nearing, M.A.             Southwest Watershed Research Center

Wei, H.                        University of Arizona


The amount of water in the soil prior to a rainstorm can greatly affect the amount of water that infiltrates and the amount of water that runs off the surface of the soil.  This effect can be important to understand when we apply runoff and soil erosion models in order to make assessments of soil condition and the effects of rainstorms, including soil erosion and flooding.  In this study we took measured soil moisture, rainfall, and runoff data from watersheds in dry-lands of Arizona and applied a model to determine how important it is to know the soil moisture in order to obtain good estimates off runoff from storms.   Our results indicate that the overall effect in terms of quantifying runoff in the long term were relatively minor.  The reason for this is that in arid environments the soils are generally very dry nearly all of the time, and they dry very quickly after any rain that falls in days immediately prior to the rainstorms being studied.  We hypothesize that the effect could be much more important in humid areas where soil moisture levels are generally higher much of the time.


Opportunities to Engage with NASA’s Soil Moisture Active Passive (SMAP) Mission Applications


Brown, M.E.               NASA Goddard Space Flight Center

Moran, M.S.                Southwest Watershed Research Center

Escobar, V.                 Sigma Space/NASA Goddard Space Flight Center

Entekhabi, D.              Massachusetts Institute of Technology

O’Neill, P.E.               NASA Goddard Space Flight Center

Njoku, E.                     NASA Jet Propulsion Laboratory

Doorn, B.                    NASA Headquarters

Entin, J.                       NASA Headquarters


NASA is planning to launch the Soil Moisture Active Passive (SMAP) mission to provide global measurements of soil moisture and freeze/thaw state for applications such as flood forecasting, drought monitoring, and numerical weather prediction. The SMAP mission is leading a new effort to increase and sustain the interaction between users and scientists involved in mission development. This has resulting in unique opportunities for users to engage with the SMAP Mission even before launch.  These opportunities include becoming an official Early Adopter of SMAP data, attending SMAP applications, calibration/validation and algorithm workshops, and using SMAP soil moisture datasets for applications research.


The Soil Moisture Active Passive (SMAP) Applications Activity


Brown, M.E.               NASA Goddard Space Flight Center

Moran, M.S.                Southwest Watershed Research Center

Escobar, V.                 Sigma Space/NASA Goddard Space Flight Center

Entekhabi, D.              Massachusetts Institute of Technology

O’Neill, P.E.               NASA Goddard Space Flight Center

Njoku, E.                     NASA Jet Propulsion Laboratory


NASA is planning to launch the Soil Moisture Active Passive (SMAP) mission to provide global measurements of soil moisture and freeze/thaw state for applications such as flood forecasting, drought monitoring, and numerical weather prediction. The SMAP mission is leading a new effort to increase and sustain the interaction between users and scientists involved in mission development. The objective of the effort is to improve the pace at which new measurements are incorporated into systems and processes used in decision making throughout the country. Even before launch, Early Adopters of SMAP data are making plans to use SMAP data for cropland soil moisture monitoring, weather predictions, food security warning, and mapping dust storms.


Shrub encroachment alters sensitivity of soil respiration to temperature and moisture


Cable, J.M.                  University of Alaska

Barron-Gafford, G.A. University of Arizona

Ogle, K.                      Arizona State University

Pavao-Zuckerman, M. University of Arizona

Scott, R.L.                  Southwest Watershed

Williams, D.G.            University of Wyoming

Huxman, T.E.              University of Arizona


Shrub encroachment into grasslands has been occurring over the past century around the world.  Shrub encroachment creates patchier landscapes, but it is unclear how this change affects soil processes like soil carbon dioxide respiration.  We quantified the response of soil respiration to different understory conditions created by large mesquite shrubs, medium-sized mesquite, sacaton bunchgrasses, and open spaces in a semiarid riparian shrubland.  Field and incubation respiration data were analyzed in a complex statistical framework, which revealed that the magnitude of respiration is higher under large shrubs, but medium shrubs have a lower magnitude than grasses. The temperature sensitivity of respiration is higher near the trunk of large shrubs compared to the other locations, but this differed from the moisture sensitivity. This study highlights the need to more completely understand changes in the spatial patterns in soil processes as a function of shrub encroachment within riparian areas, and the manner in which these processes combine to influence landscape carbon balance.


Understanding ecohydrological connectivity in savannas: A system dynamics modeling approach


Miller, G.R.                 Texas A & M University

Cable., J.M.                 University of Alaska

McDonald, A.K.         Texas A & M University

Bond, B.                     Oregon State University

Franz, T.E.                  Princeton

Wang. L.                     Princeton

Gou, S.                        Texas A & M University

Tyler, A.P.                   University of Arizona

Zou, C.B.                    Oklahoma State University

Scott, R.L.                  Southwest Watershed Research Center


Rates of movement and exchange of water depend upon the characteristics and connectivity of pathways in an ecosystem, such as the soil-plant-atmosphere continuum (vertical connectivity), soil properties (both vertical and horizontal connectivity), and the distribution of plants on the landscape (horizontal connectivity). We created a computer model that represents the primary hydrological pathways within an ecosystem with the goal of examining how ecosystem processes are impacted by the connectivity between ecosystem components for savanna sites in the U.S. and Africa.  The model successfully reproduced seasonal patterns of soil moisture dynamics and evaporation. It also demonstrated more complex, system-level behaviors, such as interactions that lead to the persistence of drought that depends on the degree of ecosystem connectivity with the atmosphere and groundwater resources. These results highlight the need to further explore mechanisms associated with ecosystem resilience and recovery from changes in water supply.


DevTreks:  Social budgeting software t improve agricultural economic data collection


Boyle, K.                     DevTreks

Heilman, P.                 Southwest Watershed Research Center

Malone, R.W.              USDA-ARS

Ma, L.                         USDA-ARS

Kanwar, R.S.              USDA-ARS


Agricultural economics needs more and better data. We propose a new approach, social budgeting, for building and analyzing agricultural economic datasets. Social budgeting employs modern information technologies, such as social networks and cloud computing data centers, to collect and analyze economics data over the Internet. DevTreks, an open source, social budgeting web software program, is introduced as an example. A case study involving Midwestern corn soybean production uses DevTreks and the process-based Root Zone Water Quality Model (RZWQM) to illustrate how modern information technology can be used to study economic-pollutant tradeoffs.  We recommend starting or joining open source, on-line efforts aimed at integrating the physical, chemical, biological, and economic aspects of agriculture.


Commonalities of carbon dioxide exchange in semiarid regions with monsoon and Mediterranean climates


Scott, R.L.                  Southwest Watershed Research Center

Serrano-Ortiz, P.         Estación Experimental de Zonas Áridas

Kowalski, A.S.            Universidad de Granada

Domingo, F.                Estación Experimental de Zonas Áridas


Arid and semiarid areas occupy around one third of the Earth’s land surface and store around 25% of the world’s carbon. In these regions, water is a major limiting element on the exchange of carbon between the atmosphere and land. We measured and analyzed carbon fluxes of semiarid ecosystems that were dominated by either summer or winter precipitation to understand how precipitation seasonality influences patterns of photosynthesis and net carbon dioxide flux. We found that precipitation strength and timing were the dominant controls of carbon exchange, but temperature also played an important role in regulating plant growth and net carbon uptake. While all the systems were active in the warm parts of the year if water from precipitation was available, photosynthesis and uptake were not responsive to precipitation in the coolest parts of the winter. Rather, precipitation likely accumulated in the soil and fueled springtime growth after it had warmed. The commonalities that we found across these contrasting climates bode well for later modeling efforts in that these exchanges carbon are likely regulated by similar ecosystem processes across major differences in climate.


Coupling Diffusion and Maximum Entropy Models to Estimate Thermal Inertia and Soil Moisture


Nearing, G.S.              University of Arizona

Moran, M.S.                Southwest Watershed Research Center

Scott, R.L.                  Southwest Watershed Research Center


Information about the amount of water in the soil available for plant growth is important for day-to-day decisions made by crop producers and natural resource managers.  In this study, we developed an approach for mapping soil moisture over large areas with satellite measurements of surface temperature and available data from weather stations.  The approach is operational and provides most accurate estimates of soil moisture in wet soil conditions. 


How do more extreme rainfall regimes affect ecosystem fluxes in seasonally water-limited Northern hemisphere temperate shrublands and forests?


Ross, I.                        National Council for Scientific Research

Mission, L.                  National Council for Scientific Research

Rambal, L.                  National Council for Scientific Research

Arneth, A.                   Lund University, Sweden

Scott, R.L.                  Southwest Watershed Research Center

Carrara, A.                  Fundacion CEAM

Cescatti, A.                 European Commission Joint Research Center

Genesio, L.                  National Research Council


During the 20th century, there has been change in global patterns of precipitation.  Likewise, climate change in this century is expected to result in fewer and larger rainfall events in many temperate regions of the world. There are few and contradictory conclusions about how this change will affect ecosystem productivity.  Data from a global network of sites that measure carbon dioxide exchange between the land and the atmosphere were analyzed to investigate the effects of differences in rainfall distribution on the carbon balance of seasonally water-limited shrubland and forest sites.  Results show that sites where rainfall events are larger but more widely spaced have significantly lower gross vegetation productivity, slightly lower respiration rates, and consequently a smaller net ecosystem productivity.   With the forecasted changes in climate of less, more spread out, rain events, these results imply poor consequences for ecosystem productivity.


Evaluating the increase in storm runoff from residential development in a semi-arid environment using the KINEROS2 modeling framework


Kennedy. J.R.                         USGS

Goodrich, D.C.                       Southwest Watershed Research Center

Unkrich, C.L.                          Southwest Watershed Research Center


Growth and urbanization occurred rapidly in the American Southwest and is projected to exceed the growth of other regions of the United States in the future.  Computer models used to predict the effects of urbanization on runoff typically account for the impervious areas (e.g. roads, roofs, driveways) but not the effect of changes in the soil’s ability to absorb rainfall in the constructed area (e.g. yards, common areas).  In this study detailed hydrologic measurements collected in a residential development and adjacent natural watershed in southeast Arizona were used with a novel ARS watershed model (KINEROS2) to demonstrate that compaction of soils done for construction resulted in roughly a 50% decrease in the amount of water that can infiltrate into the soils. Because of this change, about 25-30% of the total runoff due to urbanization was caused by soil compaction with the remainder caused by constructed impervious areas.  Overall, the urbanization resulted in a nearly 20-fold increase in runoff over the natural watershed. The study also validated the new KINEROS2 model urban element which simplifies how a sub-division can be represented in the watershed computer model.  In recent years, the increase in runoff associated with urbanization in the Southwest has begun to be considered as a potentially renewable water source so it is important to accurately estimate the amount of this extra, manageable water.


Quantification of terrestrial ecosystem carbon dynamics in the conterminous United States combining a process-based biogeochemical model and MODIS and AmeriFlux data


Schwalm, C.R.                        Clark University

Wiliams, C.A.                         Clark University

Schaefer, K.                            University of Colorado

Baldocchi, D.                          UC Berkeley

Black, T.A.                              University of British Columbia

Goldstein, A.H.                      UC Berkeley

Law, B.E.                                Oregon State University

Oechel, W.C.                          San Diego State University

Paw U, K.T.                            UC Davis

Scott, R.L.                              Southwest Watershed Research Center         


Quantification of net carbon exchanges between the terrestrial ecosystems and atmosphere is scientifically and politically important. Yet, this quantification is difficult and remains highly uncertain. A carbon exchange model was combined with satellite and land-based measurements of carbon exchange and ecosystem properties. This approach was applied over the United States from 2000 – 2005. The new version of TEM was an improvement from a previous version and generally captured the expected patterns of regional carbon dynamics in time and space.   This study provides a new independent and more adequate measure of carbon fluxes for the conterminous United States, which will benefit studies of carbon-climate interactions and facilitate policy-making for carbon management and climate.


Environmental Controls on Ecosystem Respiration in Drylands: The Role of the

Vertical Distribution of Soil Moisture


Neil, A.L.                                University of Arizona

Kurc, S.A.                               University of Arizona

Brooks, P.D.                           University of Arizona

Scott, R.L.                              Southwest Watershed Research Center


Respiration, the generation of carbon dioxide, from plant growth and decomposition is a major component of the global carbon cycle.  It is not well understood how water limitations in arid lands impact these respiration processes.  We examined the relationship between several ecosystem characteristics (soil temperature, vegetation type, and soil moisture) and respiration measurements from a network of sites throughout the southwestern US. The amount of moisture in the soil provided the best prediction of respiration and was most useful in the context of a wet/dry classification scheme based on shallow and deep soil zones.  In contrast, we found that

soil temperature alone was a poor predictor for respiration and that including information about vegetation type did little to improve that prediction. Vertical soil moisture classification

also indicates different sensitivity to moisture among vegetation types. These results underscore the importance of water-limitation for both soil microbial and plant respiration in arid lands and provide a useful tool for future analyses of carbon cycling under climate or vegetation change in these landscapes.


A sediment budget for a small semiarid Watershed in southeastern Arizona, USA 


Nichols, M.H.             Southwest Watershed Research Center

Nearing, M.A.             Southwest Watershed Research Center

Polyakov, V.O.           Southwest Watershed Research Center

Stone, J.J.                    Southwest Watershed Research Center


A sediment budget was developed for a 47 ha watershed based on hydrologic, geomorphic, erosion, and sediment data collected from 1963 through 2006 on the USDA-ARS Walnut Gulch Experimental Watershed in the southwestern US. Although the channel network is well developed and incising in the upper end of the watershed, hillslopes are the dominant source of sediment, contributing 97% of the overall total. Because the study watershed is controlled by an earth dam at the outlet, the sediment budget was balanced with a high degree of confidence. With the 44 year study period, temporally variable precipitation and runoff patterns create spatially variable sediment transfers. Watershed outlet sediment yield measurements are not sensitive to temporal and spatial variability in watershed sediment dynamics.


Ecosystem gas Exchange in native and non-native desert grassland bunchgrasses


Hamerlynck, E.P.                    Southwest Watershed Research Center

Cavanaugh, M.L.                    Southwest Watershed Research Center

Scott, R.L.                              Southwest Watershed Research Center


The South African bunchgrass, Lehmann lovegrass, dominates large areas of Southwest U.S. grasslands.  High productivity is important to the invasive success of this grass, yet short-term pulse studies and single season ecosystem-level studies have resulted in contradictory findings regarding the mechanisms underlying higher productivity, and the consequences to ecosystem water and carbon processes to U.S. rangelands.  To address this, we measured water balance (i.e. soil water content, evapotranspiration and transpiration) and carbon-balance components (net ecosystem carbon exchange, and its parts, ecosystem respiration and photosynthesis) and their interaction (water use efficiency, or photosynthesis per transpiration) over a forty week period for lovegrass and two important native grasses.  We found that lovegrass dominance increased soil evaporation throughout the year, and that cool season conditions limited lovegrass and cottontop net carbon exchange throughout the year, while bush muhly effectively used cool season rain, and subsequently fixed more carbon during the summer monsoon.  Soil water was always lower under bush muhly, but this likely reflected differences in soil bulk density, not high plant water use or differences in canopy structure that could influence rainfall interception.  Water use efficiency in lovegrass increased, as did bush muhly WUE, while cottontop WUE did not.  These findings showed that the invasive lovegrass did not use cool season soil water to achieve higher productivity, as past researchers have conjectured, but that its ability to rapidly maximize WUE following early monsoon season rain likely enhances its productivity compared to most native grasses.  Future climate predictions suggest warmer temperatures and infrequent, though larger, rainfall events across the arid Southwest, and these conditions are likely to favor the spread of this invasive grass across arid and semi-arid U.S. rangelands.


Assessing satellite-based rainfall estimates in semi-arid Watershed using the USDA-ARS Walnut Gullch gauge network and TRMM-PR


Amitai, E.                    NASA

Unkrich, C.L.              Southwest Watershed Research Center

Goodrich, D.C.           Southwest Watershed Research Center

Habib, E.                     University of Louisiana

Thi, B.                         Chapman University


Arid and semi-arid regions account for approximately one-third of the land mass of earth.  These regions are experiencing continued pressure from population growth in many parts of the world.  Water is a critical resource in these regions and is often in short supply.  Detailed study of water resources and the hydrology of semi-arid regions is important if we are to continue to populate and use these regions.  Rainfall estimates from National Weather Service (NWS) radar shown daily on popular news forecasts are now being used for water resource decisions and models.  In addition, satellite based measurement from the Tropical Rainfall Measuring Mission (TRMM) are also being assessed for their ability to estimate rainfall rates. TRMM data has the potential provide estimates over large areas that are not blocked by mountains as is the ground-based NWS Radar in much of the western US. Rainfall observations from the USDA- ARS Walnut Gulch Experimental Watershed (WGEW) were used to assess the ability of  TRMM satellite estimates  for 25 rainy overpasses which occurred during 1999-2010. Preliminary results indicate a very good agreement between the TRMM and WGEW estimates of rainfall rates.  This is an important finding as rainfall is critical in arid and semiarid regions and it not well measured over large parts of the globe.


A comparison of two stream gauging systems for measuring runoff and sediment yield on semi-arid watershed


Polyakov, V.               Southwest Watershed Research Center

Nearing, M.A.             Southwest Watershed Research Center

Hawdon, A.A.             CSIRO Townsville, Australia

Wilkinson, S.N.           CSIRO Townsville, Australia

Nichols, M.H.             Southwest Watershed Research Center


Measuring and sampling are critical for understanding the relationships among rainfall, runoff, and transported sediment. A supercritical flume with a total load traversing slot sediment sampler used on several sites at the Walnut Gulch Experimental Watershed (WGEW) near Tombstone, AZ has proven to be a reliable way to measure flow and sediment discharge from small watersheds. However, it requires installation of a costly permanent structure that interferes with erosion and is only suitable for relatively small flows. The Australian Commonwealth Scientific and Industrial Research Organization (CSIRO) developed an alternative in-channel fully automated pump system for measuring water velocity, depth, turbidity and collecting runoff samples. Research was conducted to test the CSIRO system in comparison with standard measurement equipment at WGEW. The instruments were operated side-by-side on a 3.7 ha watershed. Overall, the CSIRO pump system underestimated the amount of sediment transported in comparison with the standard slot sampler. Although the fine fraction of sediment was comparable, the pump system consistently underestimated the coarse sediment fraction. This study outlines the benefits and limitations of the pump sampler based system for monitoring sediment concentration and yield in high-energy headwater catchments, and makes recommendations for improvement of its performance.


Mapping total vegetation fractional cover across western rangelands with the MODIS sensor           


Hagen, S. C.                Applied Geosolutions, LLC

Heilman, P.                 Southwest Watershed Research Center

Marsett, R.                  Ranges, LLC

Torbick, N.                  Applied Geosolutions, LLC

Salas, W.                     Applied  Geosolutions, LLC

van Ravensway J.       Michigan State University

Qi, J.                            Michigan State University


Budgetary pressures will increasingly limit the time available for field monitoring of publicly owned rangelands in the West. Remote sensing has long had the potential to complement field monitoring. In practice, the use of remote sensing has been limited, in part because the variables provided by remote sensing did not correspond to variables monitored in the field, in part because of the cost of image processing, and also because remote sensing has not been integrated into the workflow of public land management agencies. This paper describes progress on the first two of those issues. A method to scale green and senescent (total) vegetation cover field measurements to Landsat imagery (30 m) and then to the MODIS scale (500 m) is described. Canopy cover is one of the variables that public rangeland managers collect in the field, but remotely sensed vegetation products are often limited to green cover. With the method described in this paper, as composite MODIS imagery is available every 8 days, it is now possible to inexpensively create a cover time series across a region or state. Landsat imagery could be processed once or twice a year to complement the frequent, but spatially coarse, MODIS imagery. Both sets of imagery would need to be interpreted with some amount of field monitoring data. Additional time and work is still required to integrate the total vegetation imagery products into agency policies and workflow.


Effect of small-scale spatial variability of precipitation measurements on synthetic weather generation and outputs of a runoff and soil erosion model


Anson, Eric                 Southwest Watershed Research Center

Nearing, M.A.             Southwest Watershed Research Center


Models that land managers use to manage watersheds, quantify soil erosion and runoff of water during rainstorms, and evaluate or design conservation practices require weather information as input information.  Often this is done with the use of weather simulators that have the capability of building very long sequences of weather in order to statistically represent a measured weather record for a relatively stationary climate.   In this study we looked at a weather simulator commonly used for driving soil erosion models, called CLIGEN, in order to understand its accuracy relative to the measured data we are trying to represent.  We could do this at the Walnut Gulch Experimental Watershed because at this location we have a very dense set 88 of rain gauges measuring rainfall over a relatively small area of 50 square miles.  This allowed us to have many experimental replicates of rainfall measurements in order to see how much variation and error is created in our runoff and erosion estimations just as a function of rainfall variability over short distances.  The results indicated that in the end, runoff and erosion predictions can be in error by between 9 and 17% just due to natural variation in rainfall measurements.


Scales and Erosion


Cerdà, A.                    Universitat de Valencia, Spain

Brazier, R.                   University of Exeter, United Kingdom

Nearing, M.A.             Southwest Watershed Research Center

De Vente, J.                Desertification and Geoecology Department, America, Spain


Different processes of soil erosion dominate on different sizes of land areas, e.g., different spatial scales.  Thus, our scientific studies of erosion at the plot scale do not necessarily allow us to understand erosion and sediment delivery to streams and rivers at the scale of watersheds.  Likewise, simple measurements of sediment yields in rivers and streams do not allow us to understand how erosion takes place on a single hillslope.  This is a problem for not only understanding erosion and sediment production, but also in developing mathematical models that are used to manage lands for soil conservation.  In order to understand and manage lands for soil conservation at the watershed scale, we need to be able to understand and mathematically describe the integration of erosion processes across all the scales of interest, from a small portion of a hillslope to large watersheds.  This paper is an introduction to a special issue of the scientific journal Catena, wherein is collected a series of reports on this subject.   


Sediment tracers in water erosion studies: Current approaches and challenges


Gómez, J.A.                Instituto de Agricultura Sostenible

Guzman, G.                 Instituto de Agricultura Sostenible

Quinton, J.                  University of Lancaster, UK

Nearing, M.A.             Southwest Watershed Research Center

Mabit, L.                     Soil and Water Management and Crop Nutrition Laboratory, Austria


This paper is a review of the use of various types of tracers used to study soil erosion and sediment movement in the field.  These tracers include the use of radionuclides, rare earth elements, fingerprinting by use of soil physical and chemical characteristics, magnetism and others.  These tracers help us understand where sediment comes from is important to know because sediment caused by soil erosion is a major contributor to non-point source pollution of America's rivers, streams, and reservoirs.  The use of radioactive Cesium is common; because it was deposited in soils across the entire world as a result of atmospheric atomic bomb testing that was conducted by various nations in the period largely around the early 1960s.  After that time bomb testing was largely restricted to underground tests, specifically to reduce the release of such radioactive material into the atmosphere.  Loss or gain of this material in the environment allows identification of erosion or deposition since the early 1960s.  Another technique discussed is the use of rare earth oxide materials. These materials bind strongly to soil and sediment particles and can be measured in sediment and soil samples, thus they are ideal for tracking soil movement in the field.  This report has significant implications for improving our ability to measure attributes that help us manage the soil and water resources of this nation by improving our knowledge of erosion rates in rangelands of southern Arizona and providing spatial data needed to test and improve the tools we use for conservation planning.


On the Effects of Improved Cross-Section Representation in One Dimensional Flow Routing Models Applied to Ephemeral Rivers


Hutton, C.                   University of Exeter

Brazier, R.                   University of Exeter

Nicholas, A.                University of Exeter

Nearing, M.A.             Southwest Watershed Research Center


Accurate flood prediction in Semi-Arid Environments is difficult due to uncertainties in rainfall and in model calibration, yet the hazard and cost of flash flooding still remains. The paper has demonstrated that incorporating high resolution topographic data into the structure of a flow routing model can improve on existing approaches for representing river morphology in such models, and lead to more robust flash flood prediction in ephemeral semi-arid rivers. Incorporating such datasets into existing models applied for flash flood prediction is increasingly feasible given the now wide availability of topographic products from Light Detection and Ranging (LiDAR) platforms. Such datasets can help contribute to improved flash flood prediction, and therefore more robust flood warnings


National assessment of soil erosion on non-federal rangelands


Weltz, M.A.                USDA-ARS

Jolley, L.                     Natural Resources Conservation Service

Hernandez, M.             University of Arizona

Spaeth, K.                   Natural Resources Conservation Service

Talbot, C.                    Natural Resources Conservation Service

Nearing, M.A.             Southwest Watershed Research Center

Stone, J.J.                    Southwest Watershed Research Center

Goodrich, D.C.           Southwest Watershed Research Center

Wei, H.                        University of Arizona

Pierson Jr., F.              USDA-ARS

Morris, C.                    USDA-ARS


The USDA-Natural Resources Conservation Service (NRCS) has used resource inventories for over 65 years to assess the Nation’s natural resources on non-Federal lands. Since 1995, an interagency group composed of the NRCS, Agricultural Research Service, and Geological Survey have worked together to develop a robust field approach for the National Resource Inventory (NRI) on rangeland.  The new NRI protocols are designed to detect long-term, years to decades, changes in the condition of rangeland ecosystems, and monitor short-term impacts, which may be of immediate concern. A new process based model was developed by ARS for assessing soil erosion rates on rangelands in support of the Conservation Effects Assessment Project and the NRI.  The Rangeland Hydrology and Erosion Model (RHEM) was developed on data collected exclusively from rangeland erosion experiments. The RHEM tool was used to estimate runoff and erosion at the hillslope scale for over 10,000 NRI sample points in the 17 western states on non-Federal rangelands.  National average annual erosion rate on non-Federal rangeland is estimated to be 1.41 ton ha-1 year-1.  Nationally 20% of non-Federal rangelands generate over 65% of the average annual soil loss. Over 18% of the non-Federal rangelands might benefit from treatment to reduce soil loss to below 2.24 ton ha-1 year-1.  National average annual erosion rates combine areas with low and accelerated soil erosion.  Evaluating data in this manner can misrepresent the magnitude of the soil erosion problem on rangelands. We estimate currently that between 23 and 29% of the Nation’s rangelands are vulnerable to accelerated soil loss (soil erosion > 2.24 ton ha-1 event -1) if assessed as a function of vulnerability by using the risk of a runoff event of a given magnitude (25 or 50 year return storm event).  NRCS has not evaluated potential erosion risk in National reports in the past and adaptation of this technique will allow USDA and its partners to be proactive in preventing accelerated soil loss on rangelands.


Evidence for positive and negative soil moisture-vegetation feedbacks across southwestern U.S. deserts


Hamerlynck, E.P.        Southwest Watershed Research Center

Scott, R.L.                  Southwest Watershed Research Center

Kurc, S.A.                   University of Arizona

Duniway, M.S.            USGS

Snyder, K.A.               USDA-ARS


Recent research, primarily from an extensive rainfall gradient across the Kalahari desert in Africa, and from a Long Term Ecologial Research (LTER) located in New Mexico, have suggested woody plants canopies exert a strong, positive effect on sub-canopy soil moisture, and that this improved soil moisture condition is a primary organizing force in aridland ecological processes.  We tested this hypothesis by gathering a wide range of multi-year soil moisture data from under and between plant canopies across the warm S.W. United States deserts (the Mojave, Sonoran, and Chihuahuan deserts) and determining the probability that under canopy soil moisture exceeded intercanopy soil moisture across each data set.  We found that soil moisture at most depths under woody aridland plants was lower than the intercanopy, but, following highly infrequent, heavy rains, under canopy soils were wetter than in the intercanopy, but only in very shallow surface soils, and for very short time periods.  These findings led us to suggest that aridlands may be characteristically negative, where the net effect of plant canopies are to reduce soil moisture, or positive, where net canopy effects can enhance subcanopy soil moisture.  Such fundamental distinctions are important, in that aridland plant community, biogeochemical, and ecosystem processes are strongly determined by hydrological context, and will likely be sensitive indicators of changing climate conditions. 


Last Modified: 1/19/2012