From Bureau of Economic Geology, The
University of Texas at Austin (www.beg.utexas.edu).
Bureau Seminar, January 13, 2006
Impact of Climate Variability and Land Use/Land Cover Change on Water Resources in Southwestern U.S.
Understanding potential impacts of climate variability and land use/land cover changes on groundwater recharge is critical for water resources in semiarid regions. The impact of interannual climate variability related to El Nino Southern Oscillation (ENSO) was evaluated using nonvegetated and vegetated weighing lysimeters in the Mojave Desert, Nevada. The effect of converting natural to agricultural ecosystems on recharge was archived in unsaturated and saturated zone pressure head and tracer data in the High Plains in Texas.
Results of the lysimeter studies indicate that vegetation controls water cycle response to climate variability. Extreme El Nino winter precipitation (2.3–2.5 times normal) typical of the U.S. Southwest would be expected to increase groundwater recharge, which is critical for water resources. However, lysimeter data indicate that rapid increases in vegetation productivity in response to elevated winter precipitation reduced soil water storage to half of that in a nonvegetated lysimeter, thereby precluding deep drainage below the root zone that would otherwise result in groundwater recharge. Vegetation dynamics have been controlling the water cycle in interstream desert areas throughout the U.S. Southwest, maintaining dry soil conditions and upward soil water flow since the last glacial period (10,000–15,000 yr ago), as shown by soil water chloride accumulations. Although measurements are specific to the U.S. Southwest, correlations between satellite-based vegetation productivity and elevated precipitation related to ENSO indicate this model may be applicable to desert basins globally. Understanding the two-way coupling between vegetation dynamics and the water cycle is critical for predicting how climate variability influences hydrology and water resources in water-limited landscapes.
The US High Plains is one of the largest agricultural areas in the U.S. The impact of changes from natural to agricultural ecosystems is documented in pressure head and chloride profiles in unsaturated and saturated zones. Changing from natural to agricultural ecosystems changed the system from discharging (evapotranspiration) to recharging. Evidence of discharge (no recharge, upward fluxes < 0.1 mm/yr) under natural 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 – 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.