Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
AGU Fall Meeting, San Francisco, California, December 5–9, 2005
Impacts of the Shift from Natural to Agricultural Ecosystems Archived in Unsaturated and Saturated Zone Systems in the Southern High Plains, USA
Agriculture has resulted in large-scale changes on the terrestrial biosphere, with associated changes in the water cycle. The purpose of this study was to evaluate the impacts of converting natural rangeland ecosystems to agricultural ecosystems on subsurface components of the water cycle. Unsaturated zone water pressure (matric potential) profiles, environmental tracer profiles, and groundwater level and solute monitoring were used to provide an archive of system response to land use/land cover (LU/LC) changes. Results show a change from discharge (evapotranspiration) under natural rangeland ecosystems to recharge under cultivated (dryland and irrigated) ecosystems. Evidence of discharge (no recharge, upward fluxes < 0.1 mm/yr) under rangeland ecosystems is provided by low matric potentials, upward hydraulic head gradients, high chloride concentrations, and no change in groundwater levels over time. Recharge fluxes (5 to 65 mm/yr) under dryland agricultural ecosystems are shown by high matric potentials, downward hydraulic head gradients, low chloride concentrations, and rising groundwater levels. Recharge under irrigated agricultural ecosystems is shown by high matric potentials and downward hydraulic head gradients. Unsaturated zone chloride and nitrate concentrations are increasing beneath irrigated regions as a result of evapoconcentration and may ultimately result in soil salinization. Replacement of natural rangeland with dryland ecosystems is documented by downward displacement of chloride fronts in some profiles. Thick unsaturated zones under natural conditions contain a reservoir of salts that are mobilized by recharge caused by cultivation resulting in degradation of groundwater quality. Although past LU/LC changes resulted in unintended impacts on the water cycle, a comprehensive understanding of these impacts could be used to manage water resources through future LU/LC changes.