Estimating adsorbed water film thickness in unsaturated soil

Abstract

In saturated soil, water fills every crack and crevice between the individual grains of dirt. As the water evaporates or drains away, some water can become trapped between soil particles by capillary pressure or be adsorbed onto the surface of the grains in extremely thin films. The variable thickness of these adsorbed films affects contaminant reaction and transfer rates, stored water volumes, and the rate of water transfer through the soil. As such, proper parameterizations of adsorbed film thickness are important in hydrologic models. However, techniques that estimate the film thickness by averaging the total water volume by the soil surface area or by relating the film thickness to the size of the individual soil grains tend to underestimate or overestimate the thickness of the adsorbed films. Many of these previous approaches use macroscopic properties to estimate a quantity that is largely controlled by small‐scale interactions.