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Science Results (Summer 2007)
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The role of precipitation uncertainty for soil property estimation using soil moisture retrievals in a semi-arid environment


Peters-Lidard, C.D.      NASA/Goddard Space Flight Center

Mocko, D.M.               NASA/Goddard Space Flight Center

Santanello, Jr., J.A.       NASA/Goddard Space Flight Center

Tischler, M.A.              US ARMY Corps of Engineers

Moran, M.S.                SouthwestWatershedResearchCenter

Garcia, M.                    NASA/Goddard Space Flight Center

Wu, Y.                         NASA/Goddard Space Flight Center


Soil moisture is a critical component of atmospheric, land-surface, and hydrologic models that impacts weather forecasts on daily to seasonal timescales.  However, accurate prediction of the moisture conditions in the soil is limited by insufficient information and representation of soil type and textural properties.  In this study, the problem of estimating soil moisture and soil properties is approached from a unique perspective.  It is based upon a larger effort where NASA, USDA, and our university partners are helping the US Army Corps of Engineers develop techniques to use microwave satellite measurements to derive soil wetness and properties at very high resolution (tens of meters) for assessment of human and vehicle mobility.  The first testbed for this experiment is the Walnut Gulch Experimental Watershed in southeastern Arizona, where 6 daily estimates of near-surface soil moisture across the watershed were derived from passive microwave data using established techniques.  Then, a land-surface model was run to determine which soil types and properties are required in the model to simulate the soil moisture conditions that match those from remote sensing.  By adjusting the sand, clay, and silt contents (i.e. the properties that control the flow of moisture) of the soil in a physically consistent manner, errors in model simulated versus observed soil moisture were minimized.  Overall, this study demonstrates the potential to gain physically meaningful and much-needed soils information at high-resolution using repeated soil moisture data combined with accurate precipitation inputs to the models. 


Preface:  Fifty Years of Research and Data Collection U.S.D.A. Walnut Gulch Experimental Watershed


Moran, M.S.                SouthwestWatershedResearchCenter

Stone, J.J.                  SouthwestWatershedResearchCenter

Renard, K.G.               Retired ARS

Heilman, P.                 SouthwestWatershedResearchCenter

Goodrich, D.C.            SouthwestWatershedResearchCenter

Keefer, T.O.                SouthwestWatershedResearchCenter


This Special Section of Water Resources Research and the associated web site ( describe 50 years of data collection and the most recent research results at the USDA Walnut Gulch Experimental Watershed (WGEW) in southeast Arizona.  The goal of this compilation is to encourage cooperative, interdisciplinary studies of semiarid ecohydrology at WGEW based on continuing long-term measurements of hydrology, climate and vegetation.


Sediment yields from unit-source semi-arid watershed at Walnut Gulch


Nearing, M.A.              SouthwestWatershedResearchCenter

Nichols, M.H.              SouthwestWatershedResearchCenter

Stone, J.J.                  SouthwestWatershedResearchCenter

Renard, K.G.               Retired ARS

Simanton, J.R.             Retired ARS


Information on sediment export rates from small, upland watersheds in semi-arid regions is limited.  The nature of rainfall in semi-arid climates is such that rainfall is infrequent, highly variable both in space and time, and typically very intense.  Due to this high variability and infrequency, long measurement records are generally necessary in order to be able to characterize sediment export rates from watersheds.  The Walnut Gulch Experimental Watershed has operated since the mid-1950s.  This study reports results of eleven recent years of data collection of sediment from small watersheds in Walnut Gulch.  The results show that the erosion rates in this area are much greater than rates typically reported for rangelands of Arizona, and that a large fraction of the sediment that is generated comes from a few isolated storms.  The erosion rates that we measured appeared to be related both to the vegetation and the general geology of the area.  These data will help land managers, government agencies, and conservation groups understand the rates of erosion that can occur on rangelands, and some of the factors that control these rates.


Reduction of ecosystem energy following precipitation


Jennerette, G.D.           University of Arizona

Scott, R.L.                    SouthwestWatershedResearchCenter

Huxman, T.E.               University of Arizona


Carbon dioxide (CO2) concentrations in the atmosphere have risen dramatically since the start of the Industrial Revolution.  There is major concern about how this rise in CO2 has and will affect global climate.  All ecosystems take in and release CO2 and so there has also been a large research effort to understand how these CO2 increases will affect and modify the cycling of carbon dioxide within ecosystems.   This study developed a method to separate measurements of total ecosystem carbon dioxide exchange into carbon dioxide release (respiration) and uptake (photosynthesis) and applied this method to a data from a network of CO2 flux measurement sites in southern Arizona.  Results indicate that the influence of precipitation on respiration varies depending on landscape position and ecosystem type.  Therefore, future changes in precipitation patterns associated with global warming will likely result in widely variable ecosystem responses in CO2 cycling in this region, and future research efforts will seek to better understand how ecosystem characteristics give rise to these variable effects.


Quantifying the ecohydrologic significance of hydraulic redistribution in a semiard savanna


Scott, R.L.       SouthwestWatershedResearchCenter

Cable, W.L.     University of Wyoming

Hultine, K.R.    University of Utah


Plant roots capture, store and transport soil water, and in doing so alter ecosystem water, energy, and nutrient balance. One potentially important process facilitated by roots is the redistribution of water from moist to dry soil layers. This scientific paper investigates the movement of water in the roots of mesquite trees in a semiarid rangeland. It was found that when the surface soil layers were wet, mesquite trees moved water from the surface and downward in their taproot even during the winter when the canopy was leaf-less. When the surface was dry, mesquites brought deep moisture up to the surface in their taproot, and this moisture was either used to support transpiration or, at night, it was sent out laterally into the near surface soil. These results directly show that mesquites can move significant amounts of water both upward and downward in the soil. Redistribution has important ecological and hydrological consequences, and so, this process should be accounted for in many plant/soil models.


A prototype SDSS for TMDL development in rangeland watersheds


Duan, Y.          Tetratech

Heilman           SouthwestWatershedResearchCenter

Guertin, D.P.    University of Arizona


Water quality in many water bodies does not meet designated uses. Under the Clean Water Act, plans to improve water quality, known at Total Maximum Daily Loads (TMDLs), need to be developed for non-attainment water bodies. Such plans can be very complex, so decision support tools are needed to manage spatial data, ensure consistency, and to meet planning deadlines under limited budgets. A prototype web-based spatial decision support system was developed to better understand to cost to the rancher of controlling sediment on rangelands. The user can estimate the effect of implementing best management practices and see the results in a ranch budget, graphics showing the cost as the amount of sediment leaving the ranch is reduced, and maps. An operational tool will probably require integration with other rangeland erosion models, when they become available. The prototype was implemented for the Walnut Gulch Experimental Watershed in Arizona and a sample analysis created.   


Integrated water management in Mexico:  Building a framework for research


Sanchez-Cohen, I.                   INIFAP

Diaz, G.                                   INIFAP

Villenueva, J.                            INIFAP

Benavides, J.D.                        INIFAP

Gonzalez, J.L.                          INIFAP

Gonzalez, G.                            NIFAP

Heilman, P.                              SouthwestWatershedResearchCenter

Mann, R.                                  NRCS Retired

Estrada, J.                                INIFAP


Many areas in Mexico suffer from water quality and quantity problems. To address these issues, a national effort to study water problems on a watershed basis is planned. The lead agency on the effort is the National Institute for Forestry, Agriculture and Animal Husbandry Research of Mexico (INIFAP). This paper presents the national objectives of research, the scientific questions to be addressed, criteria used for decision making regarding national watersheds, the deliverables in short, medium, and long time scales. As both Canada and the United States have similar efforts, in Canada known as Watershed Evaluation of BMPs, and in the US as the Conservation Effects Assessment Program, interaction with these related projects is proposed.


Flow prediction in an arid catchment in Oman


Al-Qurashi, A.              ImperialCollegeLondon

McIntyre, N.                ImperialCollegeLondon

Wehater, H.                 ImperialCollegeLondon

Unkrich, C.L.               SouthwestWatershedResearchCenter


It is difficult to predict the amount and rate of runoff resulting from rain storms in areas with arid climates.  When using computer models to predict runoff, the type of input data available is a primary consideration when choosing which model to use.  This study evaluated the performance of a relatively complex model, Kineros2, by comparing its runoff predictions against measured runoff from 27 storms in Oman.  It was found that the model's performance was generally poor, mainly because there were not enough rain gauges to adequately describe the spatial variability of rainfall during the storms.