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Science Results (Fall 2005)
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Controls on transpiration in a desert riparian cottonwood forest


Gazal, R.M.                  UtahState

Scott, R.L.                   SouthwestWatershedResearchCenter

Goodrich, D.C.            SouthwestWatershedResearchCenter

Williams, D.C.  University of Wyoming


Cottonwood forests are located along many streams and rivers of western U.S.  Unfortunately, little is known about how much water they use and the factors that control this amount. This study found cottonwood water use along the SanPedroRiver in southeastern Arizona was much higher at a site located near a portion of river that had water in it all year than at a site near a river reach that dried out for part of the year.  Trees along the occasionally dry reach were more water-stressed due to the deeper water table, and this resulted in decreased water use that was due to a reduction in tree sap flow and in the amount of green leaves.  Since cottonwoods depend upon the water table being near the land surface, their existence is threatened in western U.S. basins where water tables are decreasing. These measurements of cottonwood water use improve our understanding of the role of riparian vegetation in the water cycle of many semiarid basins and help to improve models that are used to manage the allocation of water resources in the West.


Atmospheric carbon mitigation potential of agricultural management in the southwestern USA

Martens, D.A.                          SouthwestWatershedResearchCenter
Emmerich, W.E.                       Southwest Watershed Research Center
McLain, J.                                SouthwestWatershedResearchCenter
Johnsen, T.N.                           Retired ARS

Agriculture in the southwestern USA is limited by water supply due to high evaporation and limited seasonal precipitation. Where water is available, irrigation allows for production of a variety of agricultural and horticultural crops. This review assesses the impacts of agriculture on greenhouse gas emission and sequestration of atmospheric C in soils of the hot, dry region of the southwestern USA. In Texas, conservation tillage increased soil organic C by 0.28 Mg C ha?1 year?1 compared with more intensive tillage. Conversion of tilled row crops to the conservation reserve program or permanent pastures increased soil organic C by 0.32 ? 0.50 Mg C ha?1 year?1. Soil organic C sequestration was dependent on rotation, previous cropping, and type of conservation tillage employed. Relatively few studies have interfaced management and C cycling to investigate the impacts of grazing management on soil organic C, and therefore, no estimate of C balance was available. Irrigated crop and pasture land in Idaho had soil organic C content 10-40 Mg C ha?1 greater than in dryland, native grassland. Soil salinity must be controlled in cropland as soil organic C content was lower with increasing salinity. Despite 75% of the region's soils being classified as calcic, the potential for sequestration of C as soil carbonate has been only scantly investigated. The region may be a significant sink for atmospheric methane, although in general, trace gas flux from semiarid soils lacks adequate characterization. Agricultural impacts on C cycling will have to be better understood in order for effective C sequestration strategies to emerge.

Spatial patterns of soil erosion and deposition in two small semi-arid watersheds


Nearing, M.A.                          SouthwestWatershedResearchCenter

Kimoto, A.                               University of Kyoto, Japan

Nichols, M.H.                          SouthwestWatershedResearchCenter

Ritchie, J.C.                              ARS Beltsville, MD


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.  For a long time scientists have been measuring the amount of sediment that leaves a watershed.  This is important to know, 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.  Our results indicate that erosion rates in these watersheds were actually much greater on average than we expected for rangelands.  Also, we found 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 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.