Impacts of Land-Use Changes on Water Resources Archived in Unsaturated Zone Tracer Profiles, Southern High Plains, USA

Unsaturated zone tracer profiles provide an excellent record of the impacts of past land-use changes on water resources. Chloride tracer profiles record the absence of groundwater recharge beneath natural ecosystems (grasslands and shrublands) and the increase in recharge caused by conversion to rain-fed agriculture in the southern High Plains, Texas. Natural ecosystems are characterized by bulge-shaped chloride profiles that reflect evapotranspirative enrichment of atmospherically deposited chloride during the Holocene (10,000 to 15,000 yr). In many areas sulfate profiles are comparable to chloride profiles and reflect a similar source and process. Flushing of chloride is attributed to increased recharge related to conversion of natural ecosystems to rain-fed agriculture. A total of 10 of 20 sampled profiles beneath rain-fed agriculture have chloride completely flushed throughout the sampled depth (4.0 to 9.2 m). Chloride flushing in these profiles is consistent with an average 7 m groundwater-level rise in a 3,400 km2 area of predominantly rain-fed agriculture during the past 4 decades. This groundwater-level rise is equivalent to a median recharge rate of 21 mm/yr. Increases in chloride at the base of the remaining 10 profiles record the transition from no recharge beneath natural ecosystems to recharge beneath cultivated areas. Water fluxes based on the chloride mass balance approach for the flushed zone in all profiles range from 3 to 92 mm/yr (median 23 mm/yr), similar to estimates based on groundwater- level changes. The chloride mass balance approach can also be used to time the transition from natural to agricultural ecosystems. Results indicate that the chloride in the flushed zone above the chloride displacement front represents 30 to 90 years in different profiles, which are generally consistent with estimates of the beginning of cultivation provided by land owners and/or bracketed by aerial photo coverages. An understanding of impacts of past land-use changes on subsurface water resources archived in unsaturated zone tracer profiles can be used to help manage future water resources through land-use management.