Impact of land use and land cover change on groundwater recharge
and quality in the southwestern U.S.

Bridget Scanlon, principal investigator

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 o0.1mmyr_1) 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: _ 130640mmyr_1); and moderate recharge in nonirrigated (dryland) agricultural ecosystems (high matric potentials; low chloride and nitrate concentrations, and increasing groundwater levels) (HP recharge: _ 932mmyr_1). 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.

Spatial distribution of (a) dominant land use/land cover (LU/LC) category buffer zones and (b) water level changes in the Southern High Plains in Texas and New Mexico. Borehole locations symbolized by LU/LC category are shown for the HP1, HP2, and HP3 sites. Area A is 3400 km2 and dominated by dryland farming that was analyzed for recharge (Eqn (1)). Area B is 32 400 km2 and includes irrigated and dryland agricultural areas that were analyzed for the impact of LU/LC on groundwater quality. Bank areas in the water-level-change map indicate areas with no data. Actual area percentages are irrigated, 11%, Dryland, 41%; Rangeland, 46%; and Other, 2%.

 

Reference
Scanlon, B. R., R. C. Reedy, D. A. Stonestrom, D. E. Prudic, and K. F. Dennehy. 2005. Impact of land use and land cover change on groundwater recharge and quality in the southwestern USA. Global Change Biology 11:1577-1593.

March 2006