Groundwater Recharge in Juniper Woodlands: Insights from Long-Term Monitoring of Cave Drip Rates

The Edwards Plateau in Central Texas covers some 230,000 square kilometers. It is underlain by limestone and dolomite, and these karst parent materials give the region its distinct hydrologic character. Although the climate is semiarid, springs are abundant and support many perennial rivers. In particular, the Edwards Plateau is the source area for the prolific and regionally important Edwards Aquifer, the main water source for much of central Texas. Compared with other semiarid regions, the Edwards Plateau has abundant water resources; but an expanding population is now taxing those resources, and ways are being sought to increase groundwater recharge. Over the past 150 years, the expansion of juniper and oak have turned the Plateau into a shrub-dominated semiarid rangeland-a legacy of the historical overgrazing that took place from around 1880 to 1950. Many believe that reducing this woodland cover would lead to higher groundwater recharge. We undertook our study to try to understand the influence of juniper on groundwater recharge. For six years, we monitored drip rates in caves underlying juniper forests (these rates are surrogate measurements for groundwater recharge). After four years of monitoring, we removed the juniper from the study site and continued monitoring. Surprisingly, our results indicate that removing the trees had a negative effect on recharge. Several mechanisms may contribute to the reduction in recharge: for example, surface runoff increased following removal of the trees; and it is possible that the trees assist recharge by funneling water to their roots, hastening its movement through the substrate.