Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
BEG Seminar, Friday, February 14, 2003
Understanding the water balance at the land atmosphere interface is critical for evaluating the partitioning of precipitation into evapotranspiration and soil water storage. The relationship between climate and vegetation dynamics on the water balance of the near surface zone was examined in this study by comparing soil water storage monitoring data from nonvegetated and vegetated weighing lysimeters over an 8 yr period (19942001) in the Mojave Desert in Nevada. Vegetation included shrubs such as creosote (Larrea Tridentata), four wing saltbrush, (Atriplex canescens) underlain by grasses. The water balance was simulated using the UNSATH code to determine controls on the near surface water balance.
Monitoring results from the nonvegetated lysimeter indicated that soil water storage generally increased in response to winter precipitation and decreased gradually during the spring and summer as a result of evaporation. Soil water storage increases were similar in the vegetated lysimeter; however, soil water was removed much more rapidly in the vegetated lysimeter in response to evapotranspiration relative to the nonvegetated lysimeter. Numerical simulations accurately reproduced variations in soil water storage in the nonvegetated lysimseter, root mean square error was about 0.16 m which represents ~ 10% of the range in water storage fluctuations. Temporal variations in water storage in the vegetated lysimeter could be reproduced by varying the leaf area index vegetation parameter in response to soil water storage changes. These results indicate that simulators cannot prescribe the temporal variability in vegetation parameters a priori but must incorporate the dynamic response of vegetation to soil water storage changes. The opportunistic behavior of vegetation plays a critical role in limiting subsurface water flow in desert ecosystems. The ability of vegetation to capture all the infiltrated water and return it to the atmosphere is substantiated by the lack of temporal variability in subsurface pressures recorded in long-term monitoring in many desert sites. These results have important implications for system response to climate change, water resources, and contaminant transport.