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United States Department of Agriculture

Agricultural Research Service

Science Results (Summer 2010)
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Feedback between grass distribution and land surface form

Cervantes, J.H.F.              MIT
Vivoni, E.R.                       Arizona State University
Istanbulluoglu, E.            University of Nebraska  
Holifield Collins, C.D.      Southwest Watershed Research Center
Bras, R.L.                           University of California, Irvine

The relationship between topography and vegetation is poorly understood.  This work looks at interactions between vegetation and aspect, slope, and drainage area in two semiarid grasslands in the southwestern U.S. (one in Arizona and the other in New Mexico).  Digital elevation models (DEMs), biomass maps, and Normalized Difference Vegetation Index (NDVI) maps, with data spanning several years, were used in the investigation.  Results indicated that vegetation decreases on slopes that receive higher amounts of solar radiation; vegetation increases as drainage area increases; and the protection provided by vegetation cover results in steeper terrain.   Differences in the magnitude of the topographic effects on vegetation between the two field sites were found.  The effects were more significant at the Arizona site.  At this site, the effects of water accumulation along the drainage network concealed some of the slope-influenced solar radiation effects.  Finally, it was observed that the effects of topography on vegetation are relatively larger in the dry season preceding the monsoon, or early in the monsoon growing season.  These results can be used to test current numerical models in similar environments, as well as extend to different environmental settings where nutrients, light or temperature may be crucial limiting factors for vegetation growth.


Lidar remote sensing for forest canopy studies

Farid, A.               University of Arizona
Goodrich, D.C.    Southwest Watershed Research Center
Sorooshian, S.    University of California Irvine

Vegetation covering the earth’s surface, whether it be natural or managed is a critical resource for maintaining the sustainability of life on earth. Conventional remote sensing methods from airplanes or satellites have significant limitations for measuring and characterizing the height and structure of the vegetation.  The limitations of traditional methods increase with increasing density of the vegetation.  Lidar (light detecting and ranging) is an alternative remote sensing technology that is shown to improve our ability to accurately measure the structure and three dimensional aspects of vegetation canopies.  This review demonstrates that an airborne lidar (light detecting and ranging) can accurately measure features of forest canopy and leaf areas that are related to above ground biomass and plant water use.  Other applications of lidar in forest canopies are already being developed including, identification of faunal habitat, estimation of fuel loading for fires, and above ground estimates of carbon stored in vegetation canopies.


Event-based Modeling of Fecal Coliform Concentrations in Runoff from Manured Fields

Guber, A.              EMFSL
Pachepsky, Y.       EMFSL
Yakirevich, A.       Ben-Gurion University of the Negev, Israel
Shelton, D.           EMFSL
Sadeghi, A.           HRSL
Goodrich, D.C.     Southwest Watershed Research Center
Unkrich, C.L.         Southwest Watershed Research Center

Field scale models are needed to evaluate the possibility for manure-borne pathogenic and indicator microorganisms to reach surface water sources with runoff. Risk-informed management decisions based on the modeling results have to include the uncertainty in model predictions. We augmented the USDA-ARS hydraulic model KINEROS2 with add-on STWIR to develop the first mechanistic field-scale model of the release and transport of manure-borne microorganisms, successfully calibrated and tested this model with our field experimental data on manure-borne fecal coliform from the USDA-ASRS OPE3 experimental watershed, and used the model to evaluate the effect of uncertainty in bacteria distribution across the manure field on the uncertainty in model predictions. Variations in manure-borne fecal coliform concentrations were more than four orders of magnitude, and this variability was the major source of uncertainty in predicting fecal coliform in runoff. Using the average coliform concentrations in manure led to the gross overestimation of the total mass of coliforms transported to the edge of the field. The results of this work are important for the design and implementation of water conservation practices in that the margin of safety can be defined based on the simulated uncertainty in manure-borne microorganisms transport with runoff.


Evapotranspiration partitioning in semiarid creosotebush ecosystems: climate change implications of soil moisture control on shrubland transpiration

Cavanaugh, M.      Southwest Watershed Research Center
Kurc, S.A.               University of Arizona
Scott, R.L.               Southwest Watershed Research Center

The productivity of vegetation in drier parts of the world is highly sensitive to timing and amount of precipitation. Future climate scenarios suggest that the frequency and magnitude of precipitation events will change in these regions. How much and to what extent will these changes impact the water cycle in creosotebush shrublands that dominate the three North American hot deserts? In this study, we measured and examined how precipitation is transformed into moisture in the soil and then how that moisture is lost back to the atmosphere as evaporation from bare soil and transpiration from plants (T) within two creosotebush ecosystems in southern Arizona. During the dry period preceding the summer rainy season, both E, T and soil moisture were very low. With the onset of summer rains, E dominated ET; shrubs did not respond to increases in soil moisture for approximately three more weeks. A series of large precipitation events increased moisture at deeper soil layers, and triggered plant transpiration. Overall, ET was largely related to moisture levels in the shallow soil layers while T was related to moisture deeper in the soil. Under the current precipitation regime, characterized by many small storms and few large storms, soil moisture is low with most precipitation inputs lost as E. However, if climate changes lead to less frequent but larger precipitation events, these types of vegetation communities could experience shifts to more plant water use and productivity.


Short-term variations in rangeland soil moisture in response to low-tech erosion control structures

Nichols, M.H.            Southwest Watershed Research Center
McReynolds, K.        University of Arizona Cooperative Extension
Reed, C.                     Quartz Hill Water District

Rock check dams are used across the western US for erosion control. They also provide a mechanism for increasing vegetation by altering soil moisture; however, there is a lack of data for quantifying their impact on soil moisture distribution. This study was conducted to measure and document soil moisture in association with loose rock structures and wire bound rock structures in comparison with untreated control sites. A field experiment was conducted on a degraded alluvial fan in southeastern Arizona where erosion control structures were built on three small, normally dry, channels. Soil moisture was measured three times per week at depths ranging from  15 to 46 cm at six points on the upstream side of  13 loose rock structures, 13 wire bound structures, and at 13 control sites throughout the 2006 summer monsoon season. Rainfall and runoff during 2006 were above average, and differences in soil moisture measured through the soil profile on channel banks were found in association with both loose rock and wire bound check dams in comparison with controls sites. There was generally an increasing difference in the mean soil moisture at the deepest measurement depths between the lose rock structure sites and the wire/bound structure sites.  Erosion control structures are expected to increase local soil moisture in response to water impoundment. These results will aid in designing rangeland restoration projects.


Projecting avian guild responses to restoration scenarios on a large dryland river (middle Rio Grande, New Mexico, U.S.A.)

Brand, A.                    University of Arizona
Dixon, M.D.               University of South Dakota
Fetz, T.                       Hawks Aloft, Inc.
Stewart, S.                 University of Arizona
Brookshire, D.            University of New Mexico
Stromberg, J.              Arizona State University
Goodrich, D.C.            Southwest Watershed Research Center
Graber, G.                   Hawks Aloft, Inc.
Benedict, K.                University of New Mexico
Thacher, J.                  University of New Mexico
Broadbent, C.             University of New Mexico

Freshwater ecosystems, and the habitat they support are relatively rare in the semi-arid southwest and significant effort and expense is being directed at restoration of these systems. In the Middle Rio Grande, the lack of flooding due to river impoundments has contributed to the spread of exotic vegetation with dense understory fuel loads.  Restoration has focused on understory vegetation thinning but it is unclear how these actions impact bird populations. We quantified densities of five guilds of birds (canopy, midstory, and understory nesting birds; water-dependents; and spring migrants) across 12 vegetation composition-structure types. We then estimated bird population changes expected from restoration scenarios for three possible management options (mechanical clearing, hand thinning, and wetland restoration).  The estimated projections of bird population changes will help managers evaluate biological impacts of restoration measures being applied on the Middle Rio Grande.  This study is part of a larger effort to link science with valuation of non-market ecosystem services in semi-arid riparian systems.


Potential for extending Major Land Resource Area into Northern Mexico

Mann, R.S.                NRCS (Retired)
Heilman, P.               Southwest Watershed Research Center
Stone, J.J.                  Southwest Watershed Research Center

There is significant potential for cooperation between Mexico and the United States and natural resource management on rangelands. There have been a number of on-the-ground efforts demonstrating that concepts and tools developed the United States can be applied across the border in northern Mexico. Specifically the concept of Major Land Resource Area, or MLRA, a grouping of similar climate, soil, and landuse, can be applied to those areas directly south of the corresponding area in the US. For example, Ecological Site Descriptions, ESDs, are documents that catalog management information for a group of soil series with the potential to grow a common vegetation community. ESDs are defined within MLRAs and could be shared and used to guide range land management in northern Mexico. We compared ESDs used in the U.S. with those developed in Mexico by COTECOCA; there is potential for building on a similar conceptual framework.  A preliminary map was developed that represents probable boundaries of MLRA 41 if extended from the U.S. border into the states of Chihuahua and Sonora. This would facilitate the direct use of Ecological Site Descriptions across borders and improve exchange of rangeland data between the two countries.

Hydrologic response to precipitation pulses under and between shrubs in the Chihuahuan Desert, Arizona

Moran, M.S.                      Southwest Watershed Research Center
Hamerlynck, E.P.              Southwest Watershed Research Center
Scott, R.L.                           Southwest Watershed Research Center
Stone, J.J.                           Southwest Watershed Research Center
Holifield Collins,C.D.        Southwest Watershed Research Center
Keefer, T.O.                       Southwest Watershed Research Center
Bryant, R.B.                       Southwest Watershed Research Center
DeYoung, D.                      University of Arizona
Nearing, G.S.                    University of Arizona
Sugg, Z.                              University of Arizona
Hymer, D.C.                       NASA

In desert ecosystems, infrequent storms are the key drivers of plant growth and reproduction.  It follows that the areas best able to capture and retain soil water are also most conducive to vegetation establishment and stability.  In this way, the distribution of water from discrete storms can determine the vegetation species composition and distribution in arid and semiarid ecosystems.  The objectives of this analysis were to study the impact of storms on the soil moisture under and between shrubs, related to observed vegetation patterns.  Results showed that the root-zone soil moisture was about 20%  lower under shrubs than between shrubs across all storms over a 20-year period.  We concluded that the greater root density under shrubs may decrease under-shrub soil moisture.  This result provides new insight into the potential for vegetation change in arid and semiarid lands.  The study has particular relevance to changes in vegetation distribution associated with changing climate because climate models predict higher variability in the size and frequency of storm events with increasing Earth temperature. 


Using TIMSAT with AVHRR imagery to analyze phenology in China from 1982 – 2006

Wei, H.                 University of Arizona
Heilman, P.          Southwest Watershed Research Center
Qi, J.                     Michigan State University
Nearing, M.A.      Southwest Watershed Research Center
Zhihui, G.             Shenzhen University
Zhang, Y.              Beijing Normal University

Vegetation phenology parameters are indicators of seasonal biological life stages and are important for monitoring vegetation growth and agriculture production. Historical phenology parameters can be estimated from satellite remote sensing data to detect long term vegetation growth and evaluate the impacts from climate change and human activity on ecosystems. TIMESAT is a software program developed to estimate phenology parameters such as the beginning, end of growing season, and the seasonal production based on remote sensing images. In this study, we used TIMESAT to estimate phenology trends across China based on the Normalized Difference Vegetation Index (NDVI) at 8km spatial resolution from the Advanced Very High Resolution Radiometer (AVHRR) of Global Inventory Modeling and Mapping Studies (GIMMS). AVHRR NDVI images were used because they have the advantage of high temporal resolution since 1980s and are available globally.

Along with the rapid social change and economic growth that have taken place in China in the last several decades, the terrestrial ecosystems and vegetation production have also experienced dramatic changes. However, the spatial patterns of growing season production across the country and the causes of the changes remain unknown. The objective of this study is to use AVHRR data and TIMESAT to visualize the spatial patterns of vegetation growth trends across China from 1982 to 2006. Linear temporal regression was used to detect the significant trends in each pixel at the 95% significance level. Eight pairs of Landsat images at finer spatial resolutions (30-60m) were classified into vegetated area and non-vegetated area for validation.

Results showed great spatial variation in vegetation growth changes across the country during the 25 year study period. Significant decreases in vegetation production were detected in the grasslands of Inner Mongolia, and the industrializing regions including the Pearl River Delta, the Yangtze River Delta, and areas along the Yangtze River. Significant increases in vegetation production were found in Xinjiang, central China, and northeast China. Validation of the NDVI values and vegetated area changes was conducted using Landsat imagery and the results were consistent with the analyses from AVHRR data.

Moreover, we also found that although the causes of the vegetation change may vary locally, there was a coincidence of the spatial pattern of vegetation change and the scales of national policies of China launched in the 1970s such as the opening of coastal economic zones in southern China and inland cities along the Yangtze River; and the ‘Three-North Shelter Forest Programme’, which is the largest nation-wide afforestation project covering 4 million km2 in north China. The results from this study indicate the impacts of national policies on ecosystem change and that such impacts can be detected using the method described in this paper.


The Emperor Needs Clothes - Improving Agricultural Economics Data Collection, Dissemination, and Analysis

Boyle, K.P.               Retired NRCS
Heilman, P.             Southwest Watershed Research Center
Malone, R.W.         USDA-ARS Ames, Iowa
Ma. L.                      USDA-ARS Fort Collins, CO
Kanwar, R.S.            Iowa State University

Agricultural economics data is hard to collect, disseminate, and use. Most practitioners rely on data that, to varying degrees, limits the full usefulness of their findings.   Much of this data suffers from lack of standardization, antiquated software, single-user bias, and outdated approaches to information technology (IT) data collection, dissemination, and analysis. Today, most data collected by two different farm managers, researchers, or extension staffs can't be used to support modern computing (the aggregation and analysis of data, by people and machines, in a consistent manner).  The data difficulties are compounded by analysts’ reliance upon disparate climatic, landscape, demographic, agronomic, and economics data sets.

This paper investigates the contributions that modern IT can make to some of these issues, as well as limitations. The paper addresses the potential to use information technology to: 1. improve the collection and dissemination of basic economics data, 2. automate at least some economics analyses, and 3. improve how data is shared between biophysical and economics models. A case study, using two common agricultural science data sets (net returns and nitrate loadings), based on two modern software applications (Root Zone Water Quality Model, RZWQM, and DevTreks), will be used to illustrate the main points. A suite of 30 management systems were parameterized for a 30 year simulation based on 14 years of observed data from 36 one acre plots on the Northeast Iowa Experiment Station near Nashua, and analyzed for economic/environmental tradeoffs. Although the case study focuses on agricultural resource management data, the IT principles are the same for other types of economics and biophysical data.

Modern informational technology offers very feasible solutions to data management issues. The specific characteristics of an ideal, economics information collection, dissemination and analysis system were identified as: 1. Searchable, 2. Web based, 3. Accessible, 4. Easy to work with, 5. Rich, 6. Social, 8. Understandable, 9. Extensible, 10. Reusable, 11. Standards based, 12. Linked, and 13. Secure. Internet startups are building these features into hundreds, if not thousands, of web sites that, arguably, offer less meaningful services than economics has to offer.

An open source, economics web software application, DevTreks, was introduced in an agricultural case study to show that such an economics information system is feasible. Modern information technology has readily available technologies that, with high probability, can resolve many of these issues today. This study finds that professional classification and ontological systems are necessary ingredients in any meaningful economics web site.


Long-term runoff and sediment yields from small semi-arid watersheds in southern Arizona

Polyakov, V.            University of Arizona
Nearing, M.A.         Southwest Watershed Research Center
Nichols, M.H.          Southwest Watershed Research Center
Scott, R.L.                Southwest Watershed Research Center
Stone, J.J.                Southwest Watershed Research Center
McClaran, M.         University of Arizona

Soil erosion on rangelands is one of the most common problems that affects arid ecosystems of the Southwestern US. In order to preserve and manage these ecosystems one needs to understand relationships between rainfall, runoff and erosion process. Scientists at USDA-ARS Southwestern Watershed Research Center in Tucson, Arizona studied eight small, 2.5 to 9 acre, semi-arid rangeland watersheds located in Santa Rita Experimental Range on the northern slopes of the Santa Rita mountains 30 miles south of Tucson. The watersheds were equipped with instruments that recorded precipitation, surface runoff, and amount soil eroded from the area and data was collected over the period of 34 years. Average annual precipitation ranged between 354 mm and 458 mm with 53% of the total rainfall occurring from July through September. On average 9.2% of annual rainfall water run off the watersheds into channels. The amount of eroded sediment varied greatly among watersheds, ranging between 0.3 and 2.8 ton per acre per year. This may result in the loss of soil productivity and development of gullies. The authors developed mathematical relationships that help to predict amount of runoff and erosion that will occur based on the amount of precipitation. It was found that high rainfall intensity rather than the total amount of rainfall was primarily responsible for producing surface runoff. Long term monitoring was found to be essential for accurate characterization of erosion process. On one of the watersheds a single large rainfall generated nearly a quarter of all sediments eroded from the area in 34 years. Further research is ongoing to investigate the role of changes in vegetation and the affect of drought on hydrological processes.


Wind versus water erosion in a small semi-arid watershed on the Santa Rita Experimental Range in southeastern Arizona

Zhang, Y.               Beijing Normal University
Nearing, M.A.       Southwest Watershed Research Center
Liu, B.Y.                Beijing Normal University
Van Pelt, R.S.        USDA-ARS Wind Erosion and Water Conservation Unit
Stone, J.J.              Southwest Watershed Research Center

In arid and semi-arid regions, both wind and water erosion can be serious land degradation processes, and may occur contemporaneously.  Particularly where vegetation cover is sparse, rates of both water and wind erosion can be significant.  Despite the potential importance of wind and water erosion in semi-arid environments, these two forms of erosion are usually studied as two separate processes.  Meanwhile, the relative erosion rates of wind and water erosion have rarely been studied simultaneously and are poorly quantified.  Thus total measured erosion losses by wind and water erosion are difficult to compare.  In this study we developed a method for assessing net loss and gain by wind erosion and water erosion for a specified area so that net loss or gain of material within the control area due to wind and water could be directly compared.  Wind-blown sediment fluxes were obtained around a semi-arid watershed boundary from March 2008 through June 2009.  Meanwhile, runoff and sediment yields were measured at the outlet of watershed.  We are able, therefore, for the first time to report relative rates of soil erosion, in the sense of net loss or gain per unit area, by wind and by water over the same area and same time period.  The results of this research give us the capability to compare directly the significance of wind erosion and water erosion from a semi-arid rangeland watershed in southeastern Arizona.  The measurements showed that sediment yield from the study area was significantly greater for water erosion compared to wind erosion.  Our results showed that the net soil loss from wind erosion from the study area was not necessarily significant in the semiarid rangeland, southeastern Arizona USA.


Object-oriented classification to map impervious surfaces for hydrologic models

Finke, T.                University of Arizona
Moran, M. S.       Southwest Watershed Research Center
Yool, S.                 University of Arizona 

Urban growth in the southwestern U.S. and elsewhere creates impervious surfaces that increase storm water runoff and flood potential.  Here, we propose a cost-effective, automated approach for mapping imperviousness in semi-arid urban areas using images from orbiting Earth-observation satellites.  The results showed that the automated method enabled mapping of impervious areas over large urbanized watersheds with consistent accuracy.  This study contributes methodological information to the understanding of urbanization and its impacts on watershed hydrology.


Improving Estimation of Riparian Land Surface Fluxes through Root zone Groundwater Interaction in Semiarid Environments

Rajagopal, S.            University o f Arizona
Gochis, D.J.              University of Arizona
Troch, P.A.               University of Arizona
Scott, R.L.                 Southwest Watershed Research Center
Gupta, H.V.              University of Arizona               

Evaporation from vegetation is an important water source that moistens the atmosphere and affects subsequent precipitation. However, most computer models that are used for weather and climate forecasting often do not capably simulate evaporation when vegetation has access to shallow groundwater.  We make changes to a widely-used model to simulate groundwater use by vegetation and compare model simulations with measurements of evaporation made from a riparian grassland. We found that the changing the model greatly improved the simulations of evaporation. However, the model still had problems representing the observed changes in soil moisture. Results from this study confirms results from other studies and suggest that a more comprehensive observational effort needs to be undertaken in order to properly measure and then predict groundwater-vegetation behavior found in nature.


Cool-season ecosystems gas exchange of exotic and native desert grassland bunchgrasses

Hamerlynck, E.P.           Southwest Watershed Research Center
Scott, R.L.                       Southwest Watershed Research Center
Moran, M.S.                   Southwest Watershed Research Center
Huxman, T.E.                 University of Arizona         

Lehmann lovegrass invasion is a serious ecological disturbance to Southwestern US desert grasslands.  One attribute that has been considered as key to the invasive success of this South African species is its ability to maintain green biomass during the winter, which may give it access to a wider array of seasonal water than native grasses.  However, several important native bunchgrasses also have green foliage during the winter.  To determine if Lehmann’s lovegrass gains a competitive edge in using winter rainfall, we compared its whole-plant carbon dioxide and water exchange to a native winter-active grass, bush muhly, to see if this invasive grass was better able to use winter rainfall.  We found that both species effectively used winter rains to achieve significant carbon uptake through the cool-season, but that Lehmann’s lovegrass was less temperature sensitive and performed much better than bush muhly during the coldest periods of the winter, having greater carbon uptake and transpiring more water during these times.  These findings suggest that winter rainfall is more effectively used by Lehmann’s lovegrass, but also that conversion of native grasslands to lovegrass-dominated systems could seriously affect surface soil water dynamics and patterns of inter-annual productivity in these semiarid grasslands. 


The relative controls of temperature, soil moisture, and plant functional group on soil CO2 efflux at diel, seasonal, and annual scales

Barron-Gafford, G.A.          University of Arizona
Scott, R.L.                             Southwest Watershed Research Center
Jenerette, G.D.                    University of Arizona
Huxman, T.E.                       University of Arizona

A major challenge in quantifying carbon dioxide exchange dynamics within ecosystems lies in identifying whether landscapes are sources or sinks for atmospheric carbon across seasonal, annual, and decadal timescales.  Because soil respiration is such a dominant contributor to overall ecosystem efflux, it is important to quantify how temperature, moisture, and plant activity influences respiration.  We used automated measurement systems to quantify soil respiration across an entire year in three main microhabitats (under mesquite trees, under bunchgrasses, and in intercanopy soil spaces) of a semiarid grassland and relate patterns of respiration to environmental conditions. We found that microhabitat dramatically influenced the results with respiration under the mesquite trees much larger than under grasses or bare soil. We also found that respiration was not effected by temperature in the way predicted by a commonly-used theory. As mesquite plants in semiarid regions continue to expand into former grasslands, the increased carbon uptake by the more productive ecosystems commonly found in these ecosystems may be partially negated or even entirely offset by the increased carbon losses from the soil.  


Modeling erosion under future climates with the WEPP model

Bayley, T.                  University of Arizona
Elliot, W.                   US Forest Service
Nearing, M.A.          Southwest Watershed Research Center
Guertin, D.P.            University of Arizona
Johnson, T.               US EPA
Goodrich, D.C.         Southwest Watershed Research Center
Flanagan, D.C.         USDA-ARS    

Global climate change is occurring now.  Historical weather records over the last century show that precipitation is increasing both in terms of the number of days we have rain and the intensities of rain.  Statistical analyses of the records have indicated that there is a less than one in thousand chance that the changes in these patterns of precipitation could have occurred under a stable climate.  We also have good scientific reason to believe that the changes will continue into the next century as well.  Parts of the country are expected to become wetter, and parts may become drier.  As rainfall changes, so does soil erosion.  In this study we developed a computer simulation model to look at how changes in precipitation and temperatures might affect erosion rates in the United States.  This web-based tool is called the Water Erosion Prediction Project Climate Assessment Tool (WEPPCAT).  The impact of this research will be a better understanding of how climate change will affect coil erosion, as well as how we can change land use management practices such as reduced-impact tillage systems to improve conservation strategies in a future of non-stationary climate.


Headcut advance in a semiarid watershed in the southeastern United States

Reike-Zapp. D.       University of Bern, Bern Switzerland
Nichols, M.             Southwest Watershed Research Center

Headcuts can be found in many watersheds in the southwestern United States. In general, headcuts are associated with eroding areas. Within the Walnut Gulch Experimental Watershed (WGEW) near Tombstone, Arizona, three headcuts were evaluated to determine the rate of advance in relation to rainfall characteristics. Aerial photos and field measurements were used to quantify headcut advance since 1935. The data were incorporated and analyzed in a geographical information system (GIS). The headcuts have advanced persistently since 1935. The behavior of headcut advance was typical for the southwestern United States where alternating patterns of erosion and deposition are common features of the landscape. The data did not allow timing or identification of causes for headcut advance. Data suggests that all three headcuts will continue to advance in the future, even though the advance rate is affected by a layer of cemented alluvial material which acts as local base level control.


Soil moisture and ecosystem function responses of desert grassland varying in vegetative cover to a saturating precipitation pulse

Hamerlynck, E.P.          Southwest Watershed Research Center
Scott, R.L.                      Southwest Watershed Research Center
Stone, J.J.                       Southwest Watershed Research Center

Climate change models predict an increase in the intensity and frequency of extreme precipitation events.  A critical ecological and hydrological feature that will determine the impacts of such events is vegetative cover, yet how grassland soil moisture and attendant ecosystem function will respond to large, saturating events, is largely unknown.  This study showed that after application of experimental rainfall similar in total to a typical single summer monsoon growing season, net carbon balance (NEE) dynamics were nearly identical between high and low cover desert grassland plots.  This was due to the fact that ecosystem respiration (Reco, or the carbon give out by the ecosystem) in low cover plots leveled off, while Reco in high cover plots continued to rise following the pulse.  At the same time, gross ecosystem photosynthesis (GEP, or carbon uptake) rose in parallel between high- and low-cover plots, resulting in similar NEE.  For 35 days after the large pulse, soil moisture dried down and wetted up in a similar manner between high and low cover plots, then later in the season high cover plots dried out much sooner than low cover plots, probably because of greater plant water use.  These findings show that how plants balance aboveground and belowground growth varies with the degree of plant cover, and this has a strong impact on how the ecosystem responds to extreme precipitation events.


Ecological Sites of the Walnut Gulch Experimental Watershed

Heilman, P.               Southwest Watershed Research Center
Stone, J.J.                  Southwest Watershed Research Center
Robinett, D.              Retired NRCS              

Rangeland managers manage vegetation. Thus, they need a conceptual framework that defines potential vegetation communities, describes how management can shift from one vegetation community to another, and documents the expected benefits provided by the various potential vegetation communities. The most widely used conceptual unit in the range discipline is the “ecological site”. The NRCS defines an ecological site to be “a distinctive kind of land with specific physical characteristics that differs from other kinds of land in its ability to produce a distinctive kind and amount of vegetation.” Hydrologic objectives of importance on rangeland include reducing on-site runoff and erosion and off-site sedimentation, as well as the maintaining riparian ecological communities and avoiding flooding and associated expensive infrastructure investments. In rapidly urbanizing watersheds, communities will attempt to maintain natural flow regimes through a combination of designed landscape features in developed areas and vegetation management elsewhere, particularly on publicly owned land. Hydrologists will increasingly be asked to quantify the hydrologic effect of potential management options as part of integrated efforts to achieve objectives at the watershed scale. We illustrate the utility of using ecological sites for water resources management on rangelands with examples from the Walnut Gulch Experimental Watershed and explain how to access ecological site information for other locations based on the SSURGO database.


Real-time Flash Flood Forecasting Using Weather Radar and a Distributed Rainfall-Runoff Model

Unkrich, C.L.                 Southwest Watershed Research Center
Schaffner, M.               NOAA-NWS
Kahler, C.                      NOAA-NWS
Goodrich, D.C.             Southwest Watershed Research Center
Troch, P.                       University of Arizona
Gupta, H.                      University of Arizona
Wagener, T.                 Penn State University
Yatheendradas, S.       NASA/GSFC

Flash floods pose a significant danger to life and property in many areas of the world. In the United States, flash floods kill more people than any other form of severe weather, and are responsible for economic losses averaging one billion dollars per year. One way to reduce flood risk is to provide a tool that allows forecasters to better predict the timing and size of floods in high-risk areas. KINEROS2 is a computer model that can make detailed predictions of runoff based on rainfall amounts measured by weather radar. The ability of KINEROS2 to predict flash flooding was tested on one watershed in southern Arizona and six watersheds in central New York State. Based on these tests, a number of improvements to the model were recommended.


Hydrologic and erosional responses to a drought-induced shift in a desert grassland community composition

Nearing, M.A.                 Southwest Watershed Research Center
Stone, J.J.                        Southwest Watershed Research Center
Hamerlynck, E.P.           Southwest Watershed Research Center
Nichols, M.H.                 Southwest Watershed Research Center
Holifield Collins, C.        Southwest Watershed Research Center
Scott, R.L.                       Southwest Watershed Research Center

The results of the study show that spread of an exotic E. lehmanniana in a semiarid rangeland in southeast Arizona caused increased runoff and erosion. Annual sediment yield on a small watershed increased ten-fold (to 1.64 t ha-1 y-1) during the transition from native to invasive vegetation when canopy cover was at a minimum. After the recovery of vegetation and establishment of the invasive grass as a dominant species (45% relative cover) sediment yield returned to long term average level. On the hill slope scale vegetation change resulted in lasting increase in erosion rates after E. lehmanniana replaced native grasses. Despite persistent increase of sediment generation on the hill slopes, geomorphological features of the watershed helped to alleviated some of the negative effects of vegetation change on net sediment yield. Hence, conserving key morphological features in semiarid and arid land watersheds are critical in maintaining the integrity of rangeland health under changing climate conditions.


Carbon dioxide exchange in a semidesert grassland responding to drought-induced vegetation change

Scott, R.L.                          Southwest Watershed Research Center
Hamerlynck, E.P.              Southwest Watershed Research Center
Jenerette, G.D.                 University of California –Riverside
Moran, M.S.                      Southwest Watershed Research Center
Barron-Gafford, G.A.       University of Arizona

Global warming is predicted to increase drought severity and duration. Severe drought can lead to a change in plant community structure, which, in turn, may yield differences in how water and carbon dioxide are cycled in ecosystems.  We report on how the exchange of carbon dioxide between the atmosphere and a grassland in southern Arizona responded to a severe drought. When the drought ended the native grass species were replaced by an invasive African grass. The grassland was a source of carbon dioxide to the atmosphere during the drought and then became a sink when the drought ended and the exotic grass moved in. When another dry growing season occurred after the invasion, the grassland still took in more carbon than was released to the atmosphere. This study shows that invasive species may lead to more carbon sequestration in certain ecosystems and environments.


Endogenous plant rhythms drive daily fluctuations of terrestrial CO2 exchange

Resco, V.                           U. of Castilla-La Mancha, Spain
Goulden, M.                     UC Irvine
Ogle, K.                              University of Wyoming
Richardson, A.D.               Harvard
Davidson, E.A.                  Woods Hole
Hollinger, D.Y.                  USDA Forest Service
Alday, J.G.                         University of Virginia
Barron-Gafford, G.A.       University of Arizona
Carrara, A.                         CEAM, Spain
Kowalski, A.S.                   U. Granada, Spain
Oechel, W.C.                    San Diego State
Reverter, B.R.                  U. Granada, Spain
Scott, R.L.                         Southwest Watershed Research Center
Varner, R.K.                     University of New Hampshire
Moreno, J.M.                  U. of Castila-La Mancha, Spain

We tested one of the major assumptions underlying studies on the exchange of carbon dioxide between terrestrial ecosystems and the atmosphere: that daily fluctuations in the net exchange of carbon dioxide are driven almost exclusively by the immediate physiological responses to changes in environmental factors like light, humidity and temperature. However, it is increasingly recognized that photosynthesis may also vary over time in the absence of variation in external forcing because of internal plant controls from the circadian clock. We analyzed observations of carbon dioxide exchange recorded at sites around the world. We found large daily fluctuations in ecosystem carbon exchange even after removing variation in the physical environment across a wide-variety of ecosystems.  This analysis poses a new paradigm for understanding controls on carbon dioxide exchange of terrestrial ecosystems.


Validation of Advanced Microwave Scanning Radiometer Soil Moisture Products

Jackson, T.J.                   USDA-ARS Hydrology and Remote Sensing Lab
Cosh, M.H                      USDA-ARS Hydrology and Remote Sensing Lab
Bindlish, R.                     Science Systems, Inc.
Starks, P.J.                     USDA-ARS Great Plains Agroclimate and Natural Resources Research Unit
Bosch, D.D.                    USDA-ARS Southeast Watershed Research Center
Seyfried, M.S.                USDA-ARS Northwest Watershed Research Center
Goodrich, D.C.               Southwest Watershed Research Center
Moran, M.S.                   Southwest Watershed Research Center


Confirmation of a distributed, physically-based ecohydrologic model in a semiarid region using high performance computing

Bisht, G.                     MIT
Sivandran, G.            MIT
Narayan, U.               MIT
Ivanov, V.Y.               University of Michigan
Vivoni, E.R.                Arizona State University
Goodrich, D.C.          Southwest Watershed Research Center
Bras, R.L.                   University of California-Irvine                                              

When water resources are of interest, watersheds are a natural organizing unit in our landscape.  Watersheds gather rainfall, infiltrated water, and runoff and typically discharge that water at a stream location or into a body of water such as a lake or estuary.  The pathways and processes that affect runoff generation from a watershed result from a complex interaction of the climate, topography, soils, vegetation, and land use. Their explicit accounting within a computer-based watershed model is can be very difficult, thus numerous simplifications are generally made. High performance, super-computing, presents an excellent opportunity to use watershed models with fewer simplifications. “ptRIBS+VEGGIE” is such a model that couples changing vegetation with a complex watershed model. This study investigates whether this model is capable of more accurately representing runoff, infiltration, and growing vegetation in a semiarid region. The model is applied in the highly instrumented semiarid USDA-ARS Walnut Gulch Experimental Watershed in southeast Arizona for a period of 11 years. The model was able to capture the water and vegetation dynamics in Walnut Gulch.  This model has the potential to simulate the way a watershed responds and changes with a more realistic representation of living vegetation.


Projecting avian response to linked changes in groundwater and riparian floodplain vegetation along a dryland river: a scenario analysis

Brand, L.A.              University of Arizona
Stromberg, J.C.      Arizona State Univeristy
Goodrich, D.C.        Southwest Watershed Research Center
Dixon, M.D.            University of South Dakota
Lansey, K.                University of Arizona
Kang, D.                   University of Arizona
Brookshire, D.S.     University of New Mexico
Cerasale, D.J.          Cornell University                      

Natural, intact, freshwater ecosystems, and the habitat they support are relatively rare in the semi-arid southwest. The water supporting these systems is often in high demand for human or agriculture use. To address this conflict, natural scientists must evaluate how human water use decisions impact hydrologic regimes and the ecological systems this water supports.

In this study a model was developed that links groundwater changes with changes in riparian vegetation and changes in the abundance of breeding and migratory birds along the upper San Pedro River, Arizona. When groundwater levels decline, cottonwoods and willows progressively decline, and shrubbier species less dependent on permanent water sources increase.  This results in broad shifts in the composition of the breeding bird community, with canopy nesting and water birds declining but nesting birds that nest in the middle of trees increase.  When groundwater levels increase there were increases in birds which nest high in the tree canopy. When growth and groundwater pumping or recharge decisions are made, these models can be used to predict changes in riparian habitat and the bird populations they support.


The AGWA – KINEROS2 Suite of Modeling Tools in the context of watershed services valuation

Goodrich, D.C.                  Southwest Watershed Research Center
Unkrich, C.L.                      Southwest Watershed Research Center
Smith, R.E.                         Retired ARS
Woolhiser, D.                    Retired ARS
Guertin, D.P.                     University of Arizona
Hernandez, M.                 University of Arizona
Burns, I.S.                          University of Arizona
Massart, J.                         University of Arizona
Levick, L.                            University of Arizona
Miller, S.N.                        University of Wyoming
Semmens, D.J.                  USGS
Keefer, T.O.                      Southwest Watershed Research Center
Kepner, W.G.                   US EPA
Nearing, M.A.                   Southwest Watershed Research Center
Heilman, P.                       Southwest Watershed Research Center
Wei, H.                              University of Arizona
Paige, G.                           University of Wyoming
Schnaffner, M.                 National Weather Service
Yatheedradas, S.              NASA
Gupta, H.                          University of Arizona
Wagener, T.                      Penn State University
Troch, P.                            University of Arizona
Brookshire, D.                   University of New Mexico
Guber, A.K.                        USDA-ARS Beltsville, MD.
Pachepsky, Y.A.                USDA-ARS Beltsville, MD
Boyd, J.                            Resources for the Future

When water quantity or water quality is of interest, watersheds are a natural organizing unit in our landscape.  The pathways and processes that affect runoff generation from a watershed result from a complex interaction of the climate, topography, soils, land cover, and land use.  Numerous computer models have been developed to estimate how a watershed produces runoff from rainfall and snowfall.  The KINEROS2 and SWAT models are two common examples. These models often require significant data preparation and input to use them.  To expedite this task we have developed the AGWA (Automated Geospatial Watershed Assessment hydrologic modeling tool which uses nationally available spatial data sets to setup, run, and display the results from KINEROS2 and SWAT.  With these tools, natural resource managers, engineers, and scientists can estimate runoff and places in the watershed that may be prone to flood damage or water quality problems.  These users can also evaluate how conservation measures and changes in land use practices might improve water quality.  This paper provides an overview of the KINEROS2 and AGWA tools with selected examples of its application for a variety of uses and in the larger context, discusses how these tools are components of a more general watershed management framework, which embodies decision tools, scenario development and both market and non-market valuation of watershed services.


The Use of Scenario Analysis to Assess Water Ecosystem Services in Response to future Land Use Change in the Willamette River Basin, Oregon

Hernandez, M.        University of Arizona
Kepner, W.G.          US EPA
Goodrich, D.C.        Southwest Watershed Research Center
Semmens, D.J.        USGS              

The ability to assess, report, and forecast the life support functions of ecosystems is critical to our capacity to make informed decisions which will maintain the sustainable nature of our environmental services. In this study an ARS watershed modeling system was used to examine the effects projected land use/land cover change on sediment yield, and nutrient loadings from fertilizers (nitrate and phosphorus), and identify areas within the Willamette River basin in Oregon that would be most affected in the year 2050 relative to three possible future scenarios.  The three scenarios include development following a conservation strategy, existing planning trends, and an open development strategy. Results of this study suggest that the amount of forest along streams and agriculture consistently explained a high percentage of variation in nutrients. With regard to nitrate, the greatest increase was associated with agricultural land use and urban areas. Although the model predicted some improvement in headwater areas for all scenarios, nitrate loadings are expected to decrease under the conservation scenario. The largest decrease was observed in the Coast Range. With regard to phosphorus loadings, the greatest reduction was observed in predominantly forest areas. The greatest increase was observed under the open development scenario in portions of the watershed with agricultural land use. Urbanization and agriculture are presumed to be the major environmental stressors affecting watershed condition of the Willamette River Basin.


K2-O2 (KINEROS-Opus) Spatially Based Watershed Hydrologic and Biogeochemical Model

Massart, J.              University of Arizona
Guertin, D.P.          University of Arizona
Smith, R.E.              Retired ARS
Goodrich, D,C.        Southwest Watershed Research Center
Unkrich, C.L.            Southwest Watershed Research Center
Levick, L.R.               University of Arizona

K2-O2 is a computer model which can model both water and nutrient cycling of small scale watersheds over a continuous time period.  K2-O2 consists of a combination of two programs, KINEROS2 and Opus2.  Upon the development of new computer models, verification and validation are performed to determine the programs accuracy in representing reality.  Verification of a program is performed to ensure the model is correctly built to represent the processes and functions of a given environment.  Validation is performed to assess the accuracy and capabilities of the program, as well as the program’s ability depict reality based on current scientific knowledge.  This paper discusses the verification and validation of K2-O2 using two different watersheds with different climatic and hydrologic characteristics.   Also, this paper shows how K2-O2 will provide a useful tool for watershed managers and natural resource planners when assessing the present and future management of watersheds.


Patterns of grain-sized dependent sediment transport in low-ordered, ephemeral-channels

Yuill B.                 University of New Orleans
Nichols, M.H.     Southwest Watershed Research Center

Sediment is a primary source of water pollution in surface water in Arizona. Sediment samples are collected on the Walnut Gulch Experimental Watershed in southeastern Arizona to further our understanding of sedimentation processes. Sediment particles travel in differing modes depending on their distribution, and source, and characteristics of runoff. Sediment data were collected during 37 runoff events and were analyzed to determine grain-size dependant factors affecting sediment transport in a low ordered, ephemeral watershed. Two distinct grain-size dependent transport patterns were observed, that of the finer grain-size fraction (< 0.25 mm) and that of a coarser grain-size fraction (≥ 0.25 mm). The concentration of the fine fraction was correlated to flow duration, peaking near the beginning of a flow event and declining thereafter. The concentration of the fine fraction showed a slight trend with seasonal and recovery period. These differences based on grain size are important because many models of sediment transport are based on a single grain size to represent sediment characteristics. This may lead to erroneous estimates of sediment loads, especially when a large range of sediment grain sizes are present.


Temperature response curves of photosynthesis and ecosystem respiration across vegetation types

Yuan, W.                   Beijing Normal University
Luo, Y.                       University of Oklahoma
Liu, H.                        Jackson State University
Gianelle, D.              IASMA Research and Innovation Center
Sottocornola, M.     IASMA Research and Innovation Center
Scott, R.L.                 Southwest Watershed Research Center
Rodeghiero, M.       IASMA Research and Innovation Center
Goulden, M.L.         University of California Irvine
Montagnani, L.        Forest Services of Autonomous Province of Bolzano, Bolzano, Italy
Bernhoefer, C.         Technical University of Dresdan, Germany
Gough, C.M.             Virginia Commonwealth University
Martin, T.                 University of Florida

Ecosystem models are used to quantify the U.S. and global carbon balance, and they all have an inherent response of both carbon dioxide uptake (photosynthesis) and release (respiration) to temperature. But, this response can differ between widely used models and is not directly tested due to the historical lack of observations to test them. We characterized the temperature curves of ecosystem photosynthesis and respiration across various geographical regions and ecosystem types using newly available measurements from 30 sites with multiple years of data.  We found that observed temperature response curves often differed from those found in widely-used models, challenging current modeling practice.


Repeat Photography using GigaPan Imagery in the San Simon Watershed, Arizona, USA

Nichols, M.H.           Southwest Watershed Research Center
Crimmins, M.           University of Arizona
Brandau, B.              University of Arizona                               

Rangeland monitoring often includes repeat photographs as a basis for documentation. Research was conducted to evaluate the use of new technologies for extending the use of photography in rangeland science. High resolution panoramic photography using the Gigapan system was employed to re-photograph landscapes in the San Simon Watershed in Arizona that were documented in the early 1940’s using traditional black and white print photography. The system creates composite images that address the limitations of traditional photography that require each picture to be a compromise between resolution and area covered. Photo stations were re-located on the ground, and example Gigapan images were taken and posted to the Gigapan website where they are available for additional interrogations. Based on the successful initial test, high resolution panoramic photography will be used to evaluate landscape change in the San Simon Watershed.


Bed-load and Morphological Evolution for Sediment Transport down Eroding Rills

Zheng, T.                 Loughborough University, UK
Sander, G.C.           Loughborough University, UK
Heng, B.C.P.           Loughborough University, UK
Barry, D.A.              Environmental Engineering Institute, Switzerland
Polyakov, V.O.        Southwest Watershed Research Center

This study explores mechanisms of suspended and bed-load sediment transport and bed evolution using an extended Hairsine-Rose model. The results showed a good match with experimental data obtained for two boundary conditions: transport limiting and detachment limiting. It is concluded that erosion models may need to incorporate bed morphological effects so that they are able to capture the key physical mechanisms controlling the flow and associated sediment transport.


Temperature and precipitation controls over leaf- and ecosystem--level CO2 flux along a woody plant encroachment gradient

Barron-Gafford, G.A.      University of Arizona
Scott, R.L.                          Southwest Watershed Research Center
Jenerette, G.D.                 University of Arizona
Hamerlynck, E.P.              Southwest Watershed Research Center
Huxman, T.E.                    University of Arizona

The worldwide phenomenon of the conversion of many historic grasslands to shrublands and savannas has the potential to alter how ecosystems will respond to changes in global climate because the dominant plants, either grasses and woody plants, have different responses to these changes.  We used a combination of leaf and ecosystem-level meteorological measurements to quantify the temperature sensitivity of carbon dioxide exchange in a riparian sacaton grassland and a mesquite woodland across seasonal periods of differing precipitation input in southeastern Arizona. We found that the sensitivity of net ecosystem plant productivity to precipitation was greater in the grassland, but that the woodland was more productive for almost all of the temperature ranges experienced by both ecosystems in both the rainy and dry periods. By maintaining plant function across a wider range of temperatures during periods of limited precipitation, woody plants were more productive, while grass productivity was limited to a narrower temperature range. This higher capacity for assimilation may have significant implications for an ecosystem’s response to projected climate change scenarios of higher atmospheric temperatures and more variable precipitation, particularly as semiarid regions worldwide experience a conversion of dominance from grasses to shrubs.

Last Modified: 8/17/2010
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