Humans have exerted large-scale changes on the terrestrial biosphere, primarily through agriculture; however, the impacts of such changes on the hydrologic cycle are poorly understood. The purpose of this study was to test the hypothesis that the conversion of natural rangeland ecosystems to agricultural ecosystems impacts the subsurface portion of the hydrologic cycle by changing groundwater recharge and flushing salts to underlying aquifers. The hypothesis was examined through point and areal studies investigating the effects of land use/land cover (LU/LC) changes on groundwater recharge and solute transport in the Amargosa Desert (AD) in Nevada and in the High Plains (HP) in Texas, US. Studies use the fact that matric (pore-water-pressure) potential and environmental-tracer profiles in thick unsaturated zones archive past changes in recharging fluxes. Results show that recharge is related to LU/LC as follows: discharge through evapotranspiration (i.e., no recharge; upward fluxes o0.1 mm yr À1 ) in natural rangeland ecosystems (low matric potentials; high chloride and nitrate concentrations); moderate-to-high recharge in irrigated agricultural ecosystems (high matric potentials; lowto-moderate chloride and nitrate concentrations) (AD recharge: $ 130-640 mm yr À1 ); and moderate recharge in nonirrigated (dryland) agricultural ecosystems (high matric potentials; low chloride and nitrate concentrations, and increasing groundwater levels) (HP recharge: $ 9-32 mm yr À1 ). Replacement of rangeland with agriculture changed flow directions from upward (discharge) to downward (recharge). Recent replacement of rangeland with irrigated ecosystems was documented through downward displacement of chloride and nitrate fronts. Thick unsaturated zones contain a reservoir of salts that are readily mobilized under increased recharge related to LU/LC changes, potentially degrading groundwater quality. Sustainable land use requires quantitative knowledge of the linkages between ecosystem change, recharge, and groundwater quality.

Computer models are widely used to simulate ground-water flow for evaluating and managing the ground-water resources of many aquifers, but few are designed to also account for surface flow in streams. A computer program was written for use in the U.S. Geological Survey modular finitedifference ground-water flow model to account for the amount of flow in streams and to simulate the interaction between surface streams and ground water. The new program is called the Streamflow-Routing Package.The Streamf low-Routing Package is not a true surface-water flow model but rather is an accounting program that tracks the flow in one or more streams which interact with ground water. The program limits the amount of ground-water recharge to the available streamflow. It permits two or more streams to merge into one with flow in the merged stream equal to the sum of the tributary flows. The program also permits diversions from streams.Streams are divided into segments and reaches. Each reach corresponds to individual cells in the finite-difference grid used to simulate groundwater flow. A segment consists of a group of reaches connected in downstream order. Leakage is calculated for each reach on the basis of the head difference between the stream and aquifer and a conductance term. It is subtracted or added to the amount of streamflow into the reach. The stage in each reach can be computed using the Manning formula under the assumption of a rectangular stream channel.The amount of leakage in each reach (either into or out of the aquifer) is incorporated into the ground-water flow model by adding terms to the finite-difference equations. Recharge to the aquifer in a reach ceases when all the streamflow in upstream reaches has leaked into the aquifer and the stream is dry. A stream is permitted to flow again in downstream reaches if the head in the aquifer is above the elevation of the streambed.Results from the program have been compared to results from two analytical solutions. One assumes time varying areal recharge to the aquifer and discharge only to a stream and the other assumes recharge to the aquifer from a change in stream stage. Results from the program reasonably duplicated the analytical solutions.

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A new Unsaturated-Zone Flow (UZF1) Package was written for use with the U.S. Geological Survey (USGS) MODFLOW-2005 groundwater model. The UZF1 Package is designed to simulate percolation through an unsaturated zone between land surface and the water table. The performance of this computer program has been tested in models of hypothetical groundwater flow systems; however, future applications of the programs may reveal errors that were not detected in the test simulations.