INFILTRATION AND PERCOLATION PROCESSES IN SHALLOW SOILS UNDERLAIN BY FRACTURED BEDROCK

Understanding the processes involved in infiltration and percolation in shallow soils is important in evaluating the potential for recharge, the generation of runoff, and the supply of soil water for uptake of water by plants. Shallow soils generally occur at higher altitudes in mountainous basins that tend to receive the greatest precipitation and snow fall, yet are the least monitored and most poorly understood parts of the basin. If the rate of run-on, rainfall, or snowmelt is greater than the infiltration capacity of the surface soil then runoff is generated from that location (Hortonian overland flow). During runoff, infiltration will still occur at the infiltration capacity of the soil, which decreases with increasing infiltration until it reaches a minimum, gravity-dominated, infiltration rate (the saturated hydraulic conductivity). In shallow soils the water may percolate and reach the soil-bedrock interface, generating perched or ponded conditions if the percolation rate exceeds the bedrock infiltration capacity. The perched water level will rise toward the surface and start moving downgradient on sloping surfaces. If the perched water reaches the surface, the infiltration rate of the soil is reduced to the infiltration capacity of the bedrock and the remaining surface-applied water will run off downslope (saturated overland flow) or rise above the land surface. Depending on the permeability of the bedrock, the hydraulic properties of the soil, and factors controlling evapotranspiration, the soil and bedrock drainage will continue at a near-constant rate, even after run-on, rainfall, or snowmelt stops, until the soil becomes unsaturated. The change in the rate of soil drainage as a function of soil water content is a critical factor in determining the amount of potential recharge and the supply of soil water for plant water uptake. These processes and concepts are illustrated with examples from field experiments in bedrock environments under varying conditions.