Semiarid unsaturated zone chloride profiles: archives of past land use change impacts on water resources in the southern High Plains, United States

1 Unsaturated zone chloride profiles in semiarid regions provide a decadal- to century-scale record of past environmental changes, similar to climate change records provided by tree rings and ice cores. Impacts of conversions from natural ecosystems to rain-fed agriculture on water resources are recorded in chloride profiles in semiarid regions, as typified by the southern High Plains (SHP), Texas, southwestern United States. Large chloride accumulations beneath natural grassland and shrubland ecosystems (3 profiles) reflect evapotranspirative enrichment of atmospherically derived chloride during the Holocene, indicating no recharge in interdrainage areas. Conversion to rain-fed agriculture is recorded by downward displacement (9 profiles) or complete flushing (10 profiles) of chloride bulges, indicating increased recharge. Increased recharge associated with cultivation (median 24 mm/yr, 5% of precipitation, 19 profiles) was quantified using chloride mass balance calculations. The timing of land use change was estimated using chloride data, and results (43-89 years) are consistent with aerial photo records and landowner surveys. New equilibrium volumetric recharge rates beneath rain-fed agriculture in the SHP (0.63 km3/yr) will require decades to establish and represent one to eight times recharge rates for baseline precultivated conditions that are focused beneath ephemeral lake or playa drainages (0.08-0.83 km3/yr). These chloride profiles generally represent decadal-scale monitoring of subsurface response to land use change.