Unsaturated-zone (UZ) 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 rainfed agriculture on water resources are recorded in chloride profiles in semiarid regions, as typified by the southern High Plains (SHP), Texas, southwestern US. 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 rainfed agriculture is recorded by downward displacement (10 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 to 89 yr) are consistent with aerial-photo records and land-owner surveys. New equilibrium volumetric recharge rates in the SHP (0.63 km3/yr) will require decades to establish and represent 1 to 8 times recharge rates for baseline pre-cultivated conditions that is focused beneath ephemeral lake or playa drainages (0.08 to 0.83 km3/yr). These chloride profiles generally represent decadal-scale monitoring of subsurface response to land-use change.
 |
Figure 1. Locations of sampled boreholes, matric potential (MP) monitoring sites, precipitation and chloride monitoring site (Muleshoe National Wildlife Refuge; MNWR), and selected urban locations in the Southern High Plains. Inset shows location of SHP in Texas and New Mexico. Generalized land cover is based on National Land Cover Data (NLCD, 1992; Vogelmann et al., 2001). Irrigation coverage is based on Qi et al. (2002). Delineated zone in southeast part of study area represents 3,400 km2 dominated by rainfed agriculture where water-table elevations have risen by an average of 7 m over an average of 40 yr (Scanlon et al., 2005). Mapped land-use categories represent 99% of area. Categories not mapped include open water, barren areas, wetlands, and forests. Rectangular outline area represents Dawson County. |
|
 |
| Expansion of irrigated areas in a 554-km2 area in the southern part of the study area. Lamesa is located in the center of the photograph for reference with Figure 1. Background image represents 2005 land cover from National Agricultural Imagery Program (2-m resolution). Red areas represent center-pivot irrigated footprint in 1995 and blue areas represent center-pivots added by 2005. |
|
|
