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Science Results (Spring 2005)
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Using airborne lidar to discern age classes of cottonwood trees


A. Farid                       University of Arizona

David C. Goodrich       Southwest Watershed Research Center

S. Sorooshain               University of Calif.Irvine


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.  To maintain the economic, social, and ecological viability of these areas it is essential that decision makers and resource managers have a solid scientific basis on which to make watershed based decisions including management of riparian vegetation.  Riparian trees use water in proportion to their size, and are especially large users of water in flood plains along rivers in semi-arid environments. It's difficult to measure tree size using traditional ground-based techniques to determine their water use. For this reason new techniques that are more accurate and efficient need to be developed. This study demonstrated that a lidar (light detecting and ranging) system mounted in an airplane can accurately measure features of forest canopies that are related to water use. Such information is not readily available from other remote sensing methods. The results illustrate the potential of airborne lidar data to differentiate different age classes of cottonwood trees for riparian areas quickly and quantitatively. This information can be used in many forestry, ecological and hydrologic applications that will improve management of hydrologic resources and ecological models.


This paper describes the challenges of truly collaborative, scientifically-based, watershed management.  These challenges are compounded in the bi-national SanPedroRiver Basin.  Based on our experiences in working with policy and decision-makers in the San Pedro we propose a process for fostering collaboration in binational basin.  A key aspect of this process is the successful engagement of scientists with community decision-makers and land managers. This requires a long-term commitment by both parties to build trust and establish effective communication to approach problem solving and resource management.



A graphical tool for water quality conservation planning 



P. Heilman                    Southwest Watershed Research Center

G. Armendariz              Southwest Watershed Research Center

L. Ma                           Great Plains Systems Research

R. Malone                    Agricultural Land &Watershed Management Research


Information available to farmers describing the potential effects of management on both yields and water quality is usually a limiting, as available information is qualitative, not customized for the farmer's conditions, and often focused only on erosion. This paper describes an approach to supporting decision makers facing water quality problems by agricultural pollutants and illustrates the approach for an area in northeastern Iowa. Observed data from an intensively monitored research site near Nashua, Iowa with 36 one-acre tile-drained plots were used to parameterize the Root Zone Water Quality Model. Management system effects were simulated on crop yields, water and nitrogen budgets, and pesticide losses. These results were put into a database and an internet interface built to analyze the results. The interface presents a series of graphics to the conservationist and farmer showing how alternative management systems affect the agricultural system to reduce the quantity of agricultural pollutants leaving agricultural fields.



Planning agricultural research looking ahead from 2005


P. Heilman        Southwest Watershed Research Center

L. Lane            Retired ARS


Agricultural research planning requires a strategic, long-term perspective. The long lead-time to implement a research program and apply the results implies that research projects will be meeting an anticipated need, perhaps decades in the future. Agricultural research is relatively mature and future public budgets will probably require a continuing concentration of focus on the part of agricultural research institutions. Productivity growth in the rest of the economy will require continued salary increases for personnel in agricultural institutions. Although the objectives and constraints of national research planning will vary widely, future world prices for agricultural goods will, to a large extent, determine the benefits of any research programs implemented today. A number of price projections are available. Research planners can review available model projections, and their underlying assumptions, and then determine the likelihood of agricultural prices continuing to fall, level off, or increase. Future price levels will have implications for long-term agricultural research programs.


Slope shape effects on erosion: A laboratory study


D. Ricke-Zapp            UNIV. OF SWITZERLAND

M. Nearing                   Southwest Watershed Research Center


Slopes in nature have complex shapes, but for reasons of convenience and reproducibility, data is almost exclusively collected on plots with flat, uniformly sloped surfaces.   We undertook this study to see what happens when rain falls on soil that is on complex shaped slopes as might occur in nature.    Artificial rainfall was applied for 90 minutes to a silty soil in a 4 by 4 m box.  Five slope shapes were formed: uniform, down-slope concave, down-slope convex, cross-slope concave, and cross-slope convex.  We found that both erosion rates and the spatial patterns of erosion were very different between the different slope shapes.  The shapes that had flat areas near the bottom of the slopes had a lot of sediment deposition on those parts of the slopes.  Yet, erosion rates within the box on some of those treatments were very high.  This result illustrates the fact that just measuring what comes off a slope or plot does not necessarily tell us about the erosion that is occurring on the plot.  We also found that these plots had a natural tendency to self-organize in terms of how the energy of the flowing water across the slope was expended.  This essentially means that even though erosion processes on this small scale are very different than those that occur in large river basins, some basic principles of how these systems use energy and change over time are similar.  This work helps us understand the basic physics of soil erosion, which in the long term will give us a better capability to conserve soil for future agriculture.


Modeling response of soil erosion and runoff to changes in precipitation and cover


M. Nearing                   Southwest Watershed Research Center

V. Jetten                       UNIV. UTRECHT

C. Baffaut                     UNIVERSITY OF MISSOURI

O. Cerdan                    BRGM ARN FRANCE

A. Couturier                 BRGM ARN FRANCE

M. Hernandez              University of Arizona

Y. Le Bissonnais           University of Arizona

M. Nichols                   Southwest Watershed Research Center

J. NunesS                     DEPT ENV. SCI. PORTUGAL

C. Renschler                 UNIVERSITY OF BUFFALO

V. Souchere                 UNIV. UTRECHT


The consensus of atmospheric scientists is that the earth is warming, and as global temperatures increase the hydrologic cycle is becoming more vigorous.  The Intergovernmental Panel on Climate Change (IPCC) has reported that there has been a very likely increase (probability 90 to 99%) in precipitation during the 20th century in the mid-to-high latitudes of the Northern Hemisphere.  Much of the increase in precipitation that has been observed worldwide has been in the form of heavy precipitation events.  For example, Karl and Knight (1998) reported that from 1910 to 1996 total precipitation over the contiguous U.S. increased, and that 53% of the increase came from the upper 10% of precipitation events (the most intense precipitation).    Soil erosion rates may be expected to change in response to changes in climate for a variety of reasons, the most direct of which is the change in the erosive power of rainfall.  Soil erosion responds both to the total amount of rainfall and to differences in rainfall intensity, however, the dominant effect appears to be rainfall intensity and energy rather than rainfall amount alone.  In this study seven soil erosion models were used to evaluate the effects that future changes in rainfall intensities and amounts, as well as plant cover, can be expected to impact soil erosion rates.  The results of this study are alarming.  If the trends reported for precipitation in the United States and Europe over the last century continue, significant consequences will incur.  If rainfall amounts during the erosive times of the year were to increase roughly as they did during the last century in the United States, the increase in rainfall would be on the order of 10%, with greater than 50% of that increase due to increase in storm intensity.  If these numbers are correct, and if no changes in land cover occurred, erosion could increase by something on the order of 25 to 55% over the next century.  Correspondent values for runoff are 23 to 31%.  Both storm water runoff and soil erosion are likely to increase significantly under climate change unless offsetting amelioration measures are taken.


Potential effects of climate change on rainfall erosivity in the yellow river basin of China


G. Zhang          BeijingNormalUniversity

M. Nearing       Southwest Watershed Research Center

B. Liu               BeijingNormalUniversity


Global climate change is occurring now.  Historical weather records over this 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.  Changes in runoff can have major impacts on soil erosion and flooding, which has been a serious problem in recent years in many parts of the U.S. and China. This study used output from large General Circulation Models that predict the future of climate under various greenhouse gas scenarios to estimate the potential changes in the power of rainfall (termed erosivity) to cause erosion in the Yellow River, Loess Plateau area of China, where soil erosion rates are extremely high and cause significant downstream pollution problems for China.  The results showed significant increase in rainfall erosivity across Yellow River basin for the coming century.  The general trend of increases in erosivity decreased from east to west. The average predicted increases in rainfall erosivity ranged from 8 to 30% by the year 2080. Changes in soil erosion may mean that China will need changes in conservation strategies. The impact of this research will be better and more targeted conservation strategies for the future, which will ultimately result in a better soil resource base for growing food.   


Vegetation-hydrology interactions:  Dynamics of riparian plant water use along the San Pedro River, Arizona


D. Williams       University of Wyoming

R. Scott            Southwest Watershed Research Center


Riparian vegetation intercepts surface and sub-surface water flowing from drainage basins and forms a functionally important interface between terrestrial and aquatic ecosystems. The influence of riparian vegetation on hydrological processes, and conversely, the impact of hydrological processes on riparian vegetation have been the focus of considerable scientific investigation. Through such investigations, ecologists and hydrologists have formed productive, collaborative relationships and together have generated broad conceptual understanding of hydrological factors controlling riparian ecosystem structure and function and associated feedbacks with stream hydrology and geomorphology. This chapter describes the current state of knowledge of vegetation-hydrology interactions in riparian ecosystems of arid and semiarid basins focusing on studies conducted along the Upper San Pedro River in southeastern Arizona. We highlight some key concepts related to the influence of riparian plants on stream hydrology, describe patterns of water use by dominant vegetation communities along the San Pedro River, and evaluate some of the principal methodologies used to make these determinations.


Spatial patterns of soil erosion and deposition in two small semiarid watersheds


A. Kimoto         Kyoto University Japan

M. Nichols       Southwest Watershed Research Center

M. Nearing       Southwest Watershed Research Center

J. Ritchie          Hydrology & Remote Sensing Laboratory      


This study was undertaken to use radioactive Cesium in soils to measure the distribution and rates of soil erosion in two small semi-arid watersheds located in southeastern Arizona.  The radioactive Cesium that we measured 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.  For a long time scientists have been measuring the amount of sediment that leaves a watershed.  This 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.  However, the amount of sediment that a watershed generates to river and stream systems does not tell the whole story about soil erosion.  What scientists are generally not able to measure on a routine basis are the hillslope erosion rates and the spatial distributions of erosion in watersheds.  Yet such knowledge is important for two reasons.  First, in order to treat watersheds so as to reduce sediment generated, we must know in general where the sediment is coming from.  Secondly, since we use computer models to help us assess erosion in watersheds, and the positive impacts of the application of conservation efforts on the landscape, we need data to help us know whether the models are working correctly.  Our results indicate that erosion rates in these watersheds were actually much greater on average than we expected for rangelands.  Just as interesting was the fact that the measurement of sediment leaving a watersheds told us very little about the amounts of erosion that were taking place on hillslopes within the watersheds.  In this case the amount of sediment measured in the traditional manner at the watershed outlets were extremely different from watershed to watershed,  while erosion rates on the hillslopes inside the two watersheds were not that much different at all.  This was due to the fact that most of the sediment generated in one watershed was deposited before it left the outlet, while nearly all of the sediment in the other watershed left the watershed outlet.  This means that we must be very careful when we interpret sediment yield rates from watersheds.  This study has significant implications for improving our ability to 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.


Canopy temperatures variability as an indicator of crop water stress severity


M. Gonzalez-Dugo                   CIFA, SPAIN

M. Moran                                Southwest Watershed Research Center

L. Mateos                                IAS, SPAIN

R. Bryant                                  University of Arizona


Irrigation is a significant means of raising production in agricultural crops. It is essential in arid environments, and is often used to increase crop productivity in semi-arid and humid areas.  This study uses the within-field variability of canopy temperature, measured by airborne sensors, to measure field-scale water stress and improve irrigation scheduling.  This approach worked well to identify low and moderately stressed crops, and thus, assist with irrigation timing.  On the other hand, the approach was also sensitive to irrigation uniformity, field root zone water holding capacity, and some basic meteorological conditions such as wind speed, vapor pressure deficit and air temperature.  These sensitivities could limit the operational application of this remote sensing approach for irrigation scheduling.  Nonetheless, for large irrigation districts, this may be an economical option for minimizing water use and maximizing crop yield.


Moisture controls on trace gas fluxes in semiarid soils


D. Martens       Southwest Watershed Research Center

J. McClain        Irrigation & Water Quality Research


In semiarid soils, variability in seasonal soil moisture (SM) and temperature (T) can alter ecosystem/atmosphere exchange of CO2 and the trace gases N2O and CH4 potentially acting as a negative or a positive feedback to global warming. The impact of SM inputs (warm summer monsoon vs. cool winter rain) on fluxes of these gases was monitored in three vegetation zones from July 2002 ' September 2003 in southeastern Arizona. The soil C content (0-5 cm) in the vegetation zones ranged from 5.2 g C in the bare site, 13.4 g C in the sacaton (Sporobolus wrightii) site to 30 g organic C kg-1 soil under established mesquite (Prosopis velutina). Carbon dioxide and N2O emissions during the 15 month study were highly dependent on available SM and T. During heavy rains of the 2002 monsoon (81% of 2002 rainfall), large differences in soil C content did not correlate with variations in CO2 production, as efflux from the three sites averaged 124.4 ? 1.9 g CO2 m-2. During the fall through spring period, CO2 production efflux for the three sites averaged 93.5 ? 14.7 g m-2. In 2003, limited monsoon rain (59% of 2003 rainfall) CO2 emissions were reduced (compared with 2002) by 19%, 40% and 30% in the mesquite, open, and sacaton sites, respectively to 88.4? 16.3 g m-2 (average 29% reduction). Isotopic analysis of CO2 respired showed that the majority of C respired (50 to 98%) reflected the isotopic signal of the site vegetation. Nitrous oxide emissions during the 2002 monsoon season averaged 21.1 ? 13.4, 2.1 ? 4.4, and 3.9 ? 5.2 ug N2O m-2 h-1 in the mesquite, open, and sacaton sites for an average N2O monsoon flux of 16.2 mg m-2. During the fall through spring period, N2O production efflux for the three sites averaged 33.7 ? 18.6 mg m-2. Limited rainfall during the 2003 monsoon reduced N2O emissions by 47% in the mesquite, but N2O fluxes increased in the sacaton (5%) and open (55%) compared with 2002 for an average monsoon emission of 12.3 ? 6.7 mg m-2 (24% reduction). Following a dry winter and spring 2002 (15 mm rain), premonsoon CH4 consumption in the three vegetation zones was close to zero, but following monsoon precipitation (238 mm rain), the CH4 sink averaged 45.4 ? 15.8 mg m-2. From October through April 2003 across all sites the CH4 sink averaged 146.1 ? 23.6 mg m-2. Methane oxidation was a subsurface process as oxidation rates were measured at the 5-50 cm depth in laboratory incubations suggesting that as the soil surface dried, CH4 oxidation activity shifted deeper in the sandy soils, thus allowing for high net oxidation flux rates while the surface soil was extremely dry. This study measured ecosystem greenhouse gas potential (GHGP) that averaged 128.5 ? 23.6 g m-2 with the 2002 monsoon with a postmonsoon GHGP of 100.1 ? 19.1 g m-2. A 60% reduction in monsoon precipitation in 2003 reduced GHGP to 91.0 ? 18.1 g m-2 (29% reduction) that suggest predicted shifts in annual precipitation patterns from a majority as summer rain to greater winter precipitation may reduce soil CO2 and N2O emissions while promoting CH4 oxidation rates in semiarid zones of the Southwest, potentially acting as a negative feedback for future global warming.


Application of the HEC6T model to assess post-fire channel remediation alternatives


H. Canfield       Southwest Watershed Research Center

C. Wilson         Los Alamos National Lab

L. Lane            Retired ARS



A. Earles          WRIGHT WATER ENGINEERS       


Increased flood peak and erosion have occurred following the Cerro Grande Fire in Los Alamos, New Mexico. Because of concerns about erosion and transport of radionuclides, computer model, HEC6T, was used to describe the observed scour, deposition and sediment movement following the fire.  The model was then used to study whether small dams built across the channel would reduce the erosion of the radionuclide bearing sediment in the channels.  A computer simulation that simulated fifty-years of channel flow was used to test the effectiveness of these small dams - relative to taking no action.  The simulation showed that the most upstream dams trapped the most sediment.  By causing sediment to deposit on the upstream side of the dam, the water flowing over the structure is able to scour below the dam because the water is able to pick up some sediment, because it has deposited some upstream of the structure. However, overall the simulations showed that the small dams can be an effective method for reducing scour.  However, the design and emplacement of the dams requires analyses and interpretation accounting for hydrology, hydraulics, sediment transport and sediment yield.


Selection of parameter values to model post-fire runoff and sediment transport at the watershed scale in Southwestern forests


H. Canfield       Southwest Watershed Research Center

D. Goodrich     Southwest Watershed Research Center

I. Burns            University of Arizona


Erosion and flooding increase following wildfire. Land managers and specialists who develop plans to reduce the erosion and flooding on people and facilities below the burned watersheds (Burned Area Emergency Rehabilitation (BAER) teams) must be able to estimate these post-fire changes.  This paper describes how values in computer models must be changed to account for the impacts of fire, so that they can be useful to people needing to design rehabilitation plans. Since wildfire primarily impacts soils and vegetation cover on hillslopes, it is appropriate to assume that changes in hillslope conditions will result in changes in flood peak, flood volume and eroded sediment.  The AGWA (Automated Geospatial Watershed Assessment) hydrologic modeling tool employs both a model based mostly on statistical relationships (SWAT) and a model that attempts to describe the processes of erosion and flooding mathematically (KINEROS2).  The models were applied on two burned watersheds.  Analysis of data from the Marshall Gulch watershed near Tucson, Arizona, showed that flood peak changes following fire are greater than flood volume changes. Changing model input values in the KINEROS2 model to make it better describe the flooding pattern Starmer Canyon near Los Alamos, New Mexico shows a pattern of change over time that is consistent with watershed recovery.  The input values that best fit the flooding pattern show that the hillslopes must have more vegetation and litter which slows the water, reducing flooding and erosion.  Therefore, the most important change the models must incorporate is a change from very smooth, bare conditions following the fire.


Partitioning of evapotranspiration and its relation to carbon dioxide exchange in a Chihuahuan Desert shrubland


R. L. Scott       Southwest Watershed Research Center

T. E. Huxman      University of Arizona

W. L. Cable       Southwest Watershed Research Center

W. E. Emmerich    Southwest Watershed Research Center


Encroachment by woody plants into former grasslands has been a widely reported phenomenon across many semiarid landscapes around the world. We do not yet understand how this pervasive on-going change in vegetation will affect water and nutrient cycling, and so we cannot predict the outcomes of this change on society. We investigated the how water and carbon dioxide are cycled in a Chihuahuan Desert shrubland in southeastern Arizona to begin to collect data that will help scientists to decipher this issue.  We used innovative measurements to better understand specifically how the shrubs affected water and carbon dioxide exchange.  Results suggest that the ecosystem lost the most carbon at the start of the summer rainy season when the shrubs were still dormant, but once the shrubs became active they were able to efficiently acquire both water and carbon dioxide throughout the growing season.


Evapotranspiration on western U.S. rivers estimated using the Enhanced Vegetation Index from MODIS and data from eddy covariance and Bowen ratio flux towers


P. L. Nagler      University of Arizona

R. L. Scott       Southwest Watershed Research Center

C. Westenburg     University of New Mexico

J. R. Cleverly    University of Arizona

A. R. Huete       University of Arizona


Evapotranspiration (ET) by riparian vegetation is an important component of the water budget of arid and semi-arid watersheds. While accurate estimates of riparian zone ET are needed to properly and soundly apportion river water for human and environmental needs, these estimates are not widely available and are costly to produce using current on-the-ground technologies.   This study develops and applies a method that uses information readily available from satellites to estimate ET over portions of three western U.S. rivers.  The method is shown to accurately reproduce estimates made from on the ground instruments and to derive river reach estimates that fit within the range of recent estimates made by other approaches.  The results of this study suggest that this relatively simple approach might provide accurate and cost effective riparian ET estimates in basins with similar climate and vegetation.


Trends in Precipitation, Runoff, and In-Channel Vegetation on the USDA-ARS Walnut Gulch Experimental Watershed.
M. Nichols         Southwest Watershed Research Center
M. Nearing        Southwest Watershed Research Center
C.Shipek           Southwest Watershed Research Center

Precipitation and runoff play a critical role in adjustments to channel morphology and in channel vegetation establishment in semiarid regions. In southeastern Arizona, more than 60% of the precipitation occurs during July, August, and September, and almost all of the runoff is produced during these months. Data collected from the network of raingages and runoff measuring flumes on the USDA-ARS Walnut Gulch Experimental Watershed were analyzed to quantify trends in rainfall and runoff.  Since the mid 1950s there has been an increase in precipitation during non-summer months. There was a decrease in annual runoff and a decrease in the magnitude of the maximum annual runoff event during the same time period.  Air photos and field measurements were combined to quantify vegetation changes in the main Walnut Gulch channel. In channel vegetation was found to have increased since the mid 1930s. The vegetation increase may be a response to a combination of increased available moisture during non-summer months and fewer large magnitude flow events during the summer months.  Although the trends identified may represent short term fluctuations in rainfall and runoff, an improved understanding of the relationships among precipitation, runoff, and channel vegetation establishment is important for understanding the potential impacts of larger-scale climate changes on water quality and quantity.

Measuring sediment yield rates from semiarid rangeland watershed.

M. Nichols         Southwest Watershed Research Center

Sediment yields from eight subwatersheds within the United States Department of Agriculture - Agricultural Research Service Walnut Gulch Experimental Watershed (WGEW) were computed from stock pond sediment accumulation measurements, water level records, and estimates of sediment transported in pond overflows. Sediment accumulation records ranging from 30 to 47 years were evaluated for subwatershed ranging from 35.2 to 159.5 ha. Within the 150 km sq WGEW, sediment yield from upland watersheds ranged from 0.5 m3/ha/yr to 3.0 m3/ha/year, with a mean of 1.4 m3/ha/year and a standard deviation of 1.0 m3/ha/year. Although sediment yields were temporally and spatially variable, with the exception of runoff volume, no significant relationships were found to explain sediment yield variability. The spatial and temporal variability in sediment yields adds to the complexity of generalizing sediment yield rates across rangeland regions.