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Science Results (Winter 2009)
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Characteristics of Coarse Sediment Transported in Low-order Channels on the Walnut Gulch Experimental Watershed, Arizona


Nichols, M.                 Southwest Watershed Research Center


Sediment is a primary pollutant across the southwestern United States. Despite its importance in addressing water quality issues, sediment data are rare because data collections is difficult and labor intensive, and runoff events are infrequent. This research was conducted to collect and quantify the loads of sediment carried during thunderstorm generated flows on the USDA-ARS Walnut Gulch Experimental Watershed in southeastern Arizona. The yield of coarse sediment from 12 runoff events with runoff volumes ranging from 19.6 to 98.1 m3 ranged from 24 to 343 kg. During individual runoff events, coarse particles make up approximately 20% of the total load. A method was developed to account for this coarse fraction when computing event based sediment loads using traditional traversing slot sediment samplers to collect sediment data.


Crop Stress


Moran, M.S.                Southwest Watershed Research Center

Maas, S.                      Texas Tech University

Vanderbilt, V.C.         NASA


Crop stress is the plant response to environmental factors that ultimately results in sub-optimal crop production.  This encyclopedia contribution covers the role that satellite imaging plays in providing information about crop stress.  Current technologies are suitable for monitoring the environmental factors of primary interest to U.S corn, cotton, soybean and wheat producers: water, nutrients, weeds, and insects The technologies of the future will probably include sensors to measure natural and genetically induced fluorescence related to crop vigor.  Assimilation of satellite-based information in crop yield models could lead to a turn-key solution for agronomic decisions suited to both specialists and non-specialists.


Latitudinal patterns of interannual variability in net ecosystem exchange


Yuan, W.                     University of Oklahoma

Luo, Y.                        University of Oklahoma

Richardson, A.            University of New Hampshire

Oren, R.                      Duke University

Luyssaert, S.               University of Antwerpen

Janssesn, I,A.              University of Antwerpen

Grunwald, T.               Institute of Hydrology and Meteorology

Ceulemans, R.             University of Antwerpen

Aubinet, M.                 Unite de Physique

Bernhofer, C.              Institute of Hydrology and Meteorology

Baldocchi, D.D.          University of California Berkeley

Cehn, J.                       University of Toledo

Dunn, A.L.                  Worcester Stare College

Deforest J.D.               University of Toledo

Goldstein, A.H.          University of California Berkeley

Moors, E.                    Alterra

Munger, J.W.              Harvard

Monson, R.K.             University of Colorado

Suyker, A.E.               University of Nebraska

Starr, G.                      University of Alabama

Scott, R.L.                  Southwest Watershed Research Center

Tenhunen, J.                University of Bayreuth

Verma, S.B.                University of Nebraska

Versala, T.V.               University of Helsinki

Wofsy, S.D.                Harvard          


In the face of climate change, in part caused by increases in atmospheric carbon dioxide due to human activities, it is important to properly account for amount of carbon dioxide that is released or taken in by Earth's biosphere.  This study used measurements of carbon dioxide exchange between the atmosphere and the land surface from a large diversity of ecosystems in the northern hemisphere to determine the year-to-year magnitude and variation in components of this exchange. Results were compared across a range of latitudes.  It was found that the yearly amount of photosynthesis in evergreen forests largely determined the net carbon dioxide exchange whereas the amount of respiration largely controlled the variability of the net change for deciduous forests. For grasslands, the net exchange was found to increase with latitude and precipitation was found to be the dominant factor that controlled the net exchange as well as the variability of the component fluxes. This study takes a broad look across a large range of latitudes to help better understand what environmental factors help to dictate the amount and variability of carbon dioxide exchange of ecosystems around the world.



An alternative constraint to the U.S. carbon sink based on MODIS and Ameriflux


Xiao, J.                        Purdue University

Zhuang, Q.                  Purdue University

Law, B.E.                    Oregon State University

Baldocchi, D.D.          University of California Berkeley

Chen, J.                       University of Toledo

Richardson, A.D.        University of New Hampshire

Melillo, J.M.                Marine Biological Laboratory

Wharton, S.                 University of California Davis

Oren, R.                      Duke University

Noormets, A.              North Carolina State University

Fischer, M.L.               Lawrence Berkeley National Laboratory

Verma, S.B.                University of Nebraska

Cook, D.R.                  Argonne National Laboratory

Sun, G.                        USDA Forest Service

McNulty, S.                USDA Forest Service

Wharton, S.                 University of California Davis

Wofsy, S.C.                Harvard

Bolstad, P.V.              University of Minnesota

Burns, S.P.                  University of Colorado

Curtis, P.S.                  Ohio State University

Drake, B.G.                 Smithsonian Environmental Research Center

Falk, M.                       University of California Davis

Foster, D.R.                Harvard

Gu, L.                          Oak Ridge National Laboratory

Hadley, J.                    Harvard

Hollinger, D.               USDA Forest Service

Katul, G.G.                 Duke University

Litvak, M.                   University of New Mexico

Martin, T.A.                University of Florida

Matamala, R.               Argonne National Laboratory

Meyers, T.P.                NOAA/ARL

Monson R.K.              University of Colorado

Munger, J.W.              Harvard

Oechel, W.                  San Diego State University

Paw U., K.T.               University of California Davis

Schmid, H.P.               Indiana University

Scott, R.L.                  Southwest Watershed Research Center

Starr, G.                      University of Alabama

Suyker, A.E.               University of Nebraska

Torn, M.S.                   Lawrence Berkeley National Laboratory


More accurate projections of future carbon dioxide concentrations in the atmosphere and associated climate change as well as carbon accounting and climate policy-making depend on improved scientific understanding of the terrestrial carbon cycle. Despite the consensus that U.S. terrestrial ecosystems act as a carbon sink, the size, distribution, and interannual variability of the sink remain uncertain. Here we report total terrestrial carbon uptake in the conterminous U.S. at -0.68 Pg C yr-1 with the majority of the sink in regions dominated by evergreen and deciduous forests and savannas based on novel estimates of net ecosystem carbon exchange with high spatial (1km) and temporal (8-day) resolution derived from flux tower measurements and wall-to-wall satellite observations. We find that the U.S. terrestrial ecosystems could offset 40% of the fossil-fuel carbon emissions. The dominant sources of interannual variation of the carbon sink include extreme climate events and disturbances, with droughts in 2002 and 2006 reducing U.S. net ecosystem carbon uptake by ~20% relative to a normal year, and disturbances including wildfires and hurricanes resulting in reduced carbon uptake or carbon release into the atmosphere.



Watershed Research Approach In Mexico: Results In Contrasting Watersheds


Sanchez-Cohen, I.                   INIFAP

Gonzalez Barrios, J.L.             INIFAP

Valle, M.V.                             INIFAP

Padilla, G.D.                           INIFAP

Benavides, J.                           INIFAP

Heilman, P.                             Southwest Watershed Research Center


Mexico faces a number of water quality and quantity problems. To provide a better scientific foundation to address water problems an effort to support making decision on the watershed scale efforts is underway with the goal of developing a national watershed research network, to be called MEDS, for Mexican Decision Support. This paper presents initial results at 3 study watershed in central Mexico for the Nazas, Patzcuaro, and Tapalpa watersheds, where soil erosion, water quality and building paleo-climate data bases are the main topics of research. The paper also proposes the creation of a North American Integrated Water Management consortium linking similar national efforts, namely CEAP (USA), BMP (Canada) and MEDS (Mexico). If such an international collaboration could be achieved the sharing of scientific approaches should benefit all three countries.


Soil Evaporation Response to Lehmann Lovegrass (Eragrostis lehmanniana)

Invasion in a Semiarid Watershed


Moran, M.S.                Southwest Watershed Research Center

Scott, R.L.                  Southwest Watershed Research Center

Hamerlynck, E.P.        Southwest Watershed Research Center

Green, K.N.                University of Arizona

Emmerich, W.E.          Southwest Watershed Research Center

Holifield Collins, C.    Southwest Watershed Research Center


The invasion of the exotic grass, Lehmann lovegrass, into native desert grasslands is of great concern to ranchers and land managers throughout the Southwestern United States.  Lehmann lovegrass displaces native grasses and reduces plant and animal diversity.  There is far less known about the impact of Lehmann lovegrass invasion on ecosystem hydrology, despite the fact that it is a common invasive species in the desert southwest where water is scarce.  The goal of this study was to use multiyear measurements of a naturally occurring vegetation transition to quantify the change in surface water balance associated with Lehmann lovegrass invasion.  Results showed that the water loss from soil evaporation over the growing season doubled with Lehmann lovegrass invasion, which in turn will determine the persistence and management of Lehmann lovegrass in desert grasslands.