Arid-site recharge, while generally low, can be highly variable. Recharge under similar climate and soil conditions but with different plant cover and topography can vary from zero to more than the annual precipitation. Simple estimates of recharge based on fixed fractions of annual precipitation are misleading because they do not reflect the plant and soil factors controlling recharge. Detailed water balance models, successful for irrigated agriculture, fail to predict evapotranspiration accurately under conditions where plants suffer seasonal water stress and cover is sparse. Recharge, when estimated as a residual in water balance models, may be in error by as much as an order of magnitude. Similar errors can occur when soil water flow models are used with measured o r estimated soil hydraulic conductivities and tension gradients. Lysimetry and tracer tests offer the best hope for evaluating recharge at arid sites, particularly in siting waste disposal facilities, where reliable recharge estimates are needed. Quantification of drainage using lysimetry over several years under a given set of soil, plant, and climate conditions for a specific site can provide a basis for calibrating models for recharge prediction. Tracer tests using such long-lived tracers as '"CI or perhaps stable isotopes ("0, deuterium) can provide qualitative estimates of recent recharge at a given site.
KEY WOKIX
Rechargc Watcr halancc Lysimetry Traccr tcsts
A simple modification of the Day (1965) hydrometer method is suggested for routine textural analysis. The simplified method requires no calibration, graphs or tables and uses a weighted average of the 1.5‐hour and 24‐hour readings to determine the 2µm clay fraction. The relationship is:
urn:x-wiley:03615995:saj2sssaj197903615995004300050038x:equation:saj2sssaj197903615995004300050038x-math-0001 whereP2µm = summation percentage for the 2µm clay fraction.P24, P1.5 = summation percentage for settling time of 24 hour and 1.5 hour, respectively.K = weighting factor, used to approximate the actual relation of the summation percentage curve over the clay percentage range of interest.This equation was found to be satisfactory in the range of 5% ⩽ P2µm ⩽60% by weight for over 70 North Dakota soil and subsoil materials when K=0.876.Sensitivity analyses were performed for the Day, Bouyoucos, and the Simplified Day hydrometer methods to determine which of several measured parameters contribute to the greatest possible error; hence which parameters should be controlled with the greatest precision. The hydrometer scale reading error contributes more error than any other single parameter.
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, 3H, and 36Cl) have been the most successful in estimating ground‐water recharge in dry regions. Lysimeters, which can directly measure root‐zone drainage, have been useful in quantifying recharge, particularly for coarse 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.
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