Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
AGU Fall Meeting, San Francisco, California, December 5–9, 2005
Integration of Physical and Chemical Data to Assess Impacts of Changes from Natural to Agricultural Ecosystems on Subsurface Flow and Transport in Semiarid Regions
Agriculture is a dominant factor driving large-scale changes in the terrestrial biosphere, including changes to the hydrologic cycle. The purpose of this study was to integrate soil physics and environmental and applied tracers to quantify impacts on subsurface flow of land use/land cover (LU/LC) change from natural to agricultural ecosystems. Matric potential (pore-water pressure) and tracer profiles in unsaturated zones provide an archive of past LU/LC changes on recharging fluxes. The study focused on the Southern High Plains, one of the largest agricultural regions in North America. Methods included measurement of matric potential, chloride, and nitrate in the unsaturated zone and analysis of water levels and chloride and nitrate concentrations in the saturated zone. Unsaturated zone matric potential data provide a sensitive indicator of LU/LC change. Average matric potentials range from -130 to -270 m in 6 rangeland area profiles and from -2 to -59 m in 15 cultivated area profiles. Total head gradients change from upward under natural conditions to downward under cultivated conditions, indicating a change from discharge through evapotranspiration to groundwater recharge. Matric potential monitoring indicates that fallow periods associated with cultivated areas most likely cause the change from discharge to recharge conditions. Chloride profiles provide a time-integrated estimate of water flux. Chloride concentrations decrease by orders of magnitude from natural conditions (>500 mg/L) to rainfed (dryland) agricultural conditions (<10 mg/L). The shapes of the chloride profiles also change from bulge shaped under natural conditions resulting from long-term accumulation due to evapotranspiration to low chloride profiles under dryland conditions, indicating flushing of chloride. Recharge rates from chloride profiles under dryland agriculture range from 5 to 65 mm/yr (average 27 mm/yr). Chloride profiles under irrigated agricultural conditions are also bulge shaped, though related to evapoconcentration of irrigation water associated with the low application rates (~0.3 to 0.6 m/yr). Unsaturated zone nitrate profiles were generally low under natural and agricultural conditions, with the exception of irrigated agricultural conditions where elevated nitrate was related to evapoconcentration of irrigation water. Rises in groundwater levels ranged from 1.5 to 23 m (average 6.7 m) during the last few decades beneath dryland agriculture in the Southern High Plains, resulting in an average recharge rate of 24 mm/yr. These recharge rates are consistent with estimates from unsaturated zone chloride data. Increases in groundwater chloride and nitrate concentrations are attributed to flushing of chloride that previously accumulated in the unsaturated zone and addition of nitrate in fertilizers.