Prediction of wetting front stability in dry field soils using soil and precipitation data

Abstract

A need exists for information regarding the stability of wetting fronts in field soils because they increase the vulnerability for groundwater contamination. In this study, we develop a simple approach for the evaluation of wetting front stability in dry soils. We show that the stability of wetting fronts in the top layer of a soil depends both on the type of soil and the intensity of the precipitation. Our approach distinguishes stability criteria for wetting events that are different for a high, intermediate, and low infiltration rate. At high infiltration rates, wetting fronts are stable if the infiltration rate exceeds or equals the saturated hydraulic conductivity of the soil. The stability criterion for low infiltration rates (less than approximately 0.2 cm/h for sand soils) is based on two characteristics times[ a gravitational time and an infiltration time. The gravitational time, t grav, indicates when gravity and capillarity each contribute equally to the process of infiltration. The infiltration time, t infil, is the duration of the infiltration event. Experimental and literature data show that in well-sorted laboratory sands, wetting fronts are stable when t infil < 0.002 t grav. This expression can also be expressed as Wi < 0.002 S 2 with W the total amount of precipitation, i its intensity, and S the sorptivity at a slightly positive soil-water pressure. For intermediate infiltration rates, wetting fronts remain stable as long as W is smaller than the amount of water needed to wet a distribution layer near the surface. The application of the stability criteria is demonstrated with a case study from the Sevilleta dunes near Socorro, NM.