The impact of climate variability on the water cycle in desert ecosystems is controlled by biospheric feedback at interannual to millennial timescales. This paper describes a unique field dataset from weighing lysimeters beneath nonvegetated and vegetated systems that unequivocally demonstrates the role of vegetation dynamics in controlling water cycle response to interannual climate variability related to El Niñ o southern oscillation in the Mojave Desert. Extreme El Niñ o winter precipitation (2.3-2.5 times normal) typical of the U.S. Southwest would be expected to increase groundwater recharge, which is critical for water resources in semiarid and arid regions. However, lysimeter data indicate that rapid increases in vegetation productivity in response to elevated winter precipitation reduced soil water storage to half of that in a nonvegetated lysimeter, thereby precluding deep drainage below the root zone that would otherwise result in groundwater recharge. Vegetation dynamics have been controlling the water cycle in interdrainage desert areas throughout the U.S. Southwest, maintaining dry soil conditions and upward soil water flow since the last glacial period (10,000 -15,000 yr ago), as shown by soil water chloride accumulations. Although measurements are specific to the U.S. Southwest, correlations between satellite-based vegetation productivity and elevated precipitation related to El Niñ o southern oscillation indicate this model may be applicable to desert basins globally. Understanding the two-way coupling between vegetation dynamics and the water cycle is critical for predicting how climate variability influences hydrology and water resources in water-limited landscapes.climate change ͉ ecohydrology ͉ El Niñ o

Although water conservation programs in the arid southwestern United States have prompted prudent landscaping practices such as planting low water use trees, there is little data on the actual water use of most species. The purpose of this study was to determine the actual water use of two common landscape tree species in Tucson, Ariz., and water use coefficients for two tree species based on the crop coefficient concept. Water use of oak (Quercus virginiana `Heritage') and mesquite (Prosopis alba `Colorado') trees in containers was measured from July to October 1991 using a precision balance. Water-use coefficients for each tree species were calculated as the ratio of measured water use per total leaf area or per projected canopy area to reference evapotranspiration obtained from a modified FAO Penman equation. After accounting for tree growth, water-use coefficients on a total leaf area basis were 0.5 and 1.0 for oak and mesquite, respectively, and on a projected canopy area basis were 1.4 and 1.6 for oaks and mesquites, respectively. These coefficients indicate that mesquites (normally considered xeric trees) use more water than oaks (normally considered mesic trees) under nonlimiting conditions.

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and canyon sediments in several canyon reaches are contaminated with low-levels of radionuclides from his-Following the May 2000 Cerro Grande fire at Los Alamos, NM, toric LANL operations (Reneau et al., 1998). To mitisurface water control structures were constructed near Los Alamos to mitigate the transport of contaminant-bearing sediment toward the gate potential off-site migration of these sediments, the Rio Grande river due to increased runoff caused by the removal of U.S. Army Corps of Engineers constructed a low-head vegetation by the fire. A low-head weir was constructed in Los Alamos weir (rock-and-mesh gabion) in Los Alamos Canyon Canyon, 5 km to the east of Los Alamos, to capture contaminant-bear-Published