VADOSE-ZONE TECHNIQUES FOR ESTIMATING GROUNDWATER RECHARGE IN ARID AND SEMIARID REGIONS

Both physical and chemical methods have been used to estimate recharge in arid and semiarid areas. Our review indicates that indirect, physical approaches, such as water balance and Darcy flux measurements, are the least successful, while methods using tracers (e.g., Cl, H-3, and Cl-36) have been the most successful in estimating groundwater recharge in dry regions. Lysimeters, which can directly measure root-zone drainage, have been useful in quantifying recharge, particularly for course soils, but are costly to construct and operate. Of the tracer techniques available, Cl balance techniques appear to be the simplest, least expensive, and most universal for recharge estimation. In Australian studies, under native vegetation in semiarid areas, Cl profiles were found to be remarkably uniform, indicating very low and relatively uniform rates of groundwater recharge. Following changes in land use, recharge appeared to become much more variable, increasing more than two orders of magnitude. Methods for scaling point estimates of recharge to large areas using indirect techniques (such as nondestructive electromagnetic induction) have also been developed. In deep unsaturated zones, the pressure response in the soil water may be recorded in the profile, and simple field measurements may be used to obtain semi-independent verification of recharge rates determined by using Cl balance techniques.