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Evaluating Weathering Characteristics of Water-harvesting Catchments from Rainfall-runoff Analyses¹

ABSTRACT

Better ways are needed to evaluate the weathering properties of water-repellent and membrane-covered water-harvesting field catchments. Normally, only the ratio of runoff to precipitation from natural storms (percentage of runoff) is calculated, which provides useful information on treatment efficiency and durability, but little about why or how a treatment actually failed. We used linear regression analyses of precipitation vs. runoff-yield data collected over several years (storms with no runoff omitted) to obtain (i) threshold values, i.e., the minimal rainfall needed to produce runoff, and (ii) the runoff-efficiencies of treatments after threshold. These analyses showed that silicone and wax-treated water-repellent catchments weathered quite differently. The silicone treatment showed a uniform deterioration throughout the treated zone while the wax treatment showed a progressive deterioration beginning at the top of the treated profile. A gravel-covered membrane catchment gradually increased runoff threshold from 1- to 2-mm water, which was ascribed to a gradual accumulation of dust entrapped in the gravel. The weathering properties of several other common type catchments were evaluated using the technique. A brief discussion is included on the application of the technique to evaluate the accuracy of precipitation- and runoff-measuring devices in water-harvesting systems.

¹ Contribution from the Western Region, ARS—USDA.

² Soil Scientist and Research Hydraulic Engineer, respectively, U.S. Water Conservation Laboratory, 4331 East Broadway, Phoenix, AZ 85040.

Received for publication December 30, 1976. Accepted for publication February 1, 1977.