Inferring Water Retention Parameters from Instrumented Infiltration Tests.

Water retention controls soil water dynamics in unsaturated conditions, and must be accurately quantified in order to predict water movement and storage in unsaturated soils. Despite its importance, water retention is rarely measured in the field due to various logistical and technological challenges. For example, it is essentially impossible to measure the entire range of soil matric potentials using one or even two different technologies, so previous work for the most part has either focused on measuring a portion of the total range (e.g., using tensiometers to understand near-saturated water retention behaviors), or on collecting samples for laboratory determination of approximate water retention parameters. Here we present an alternative method for determining water retention parameters, in situ, by using single ring infiltration tests augmented with an additional measurement of lateral changes in water content through time. We present a theoretical framework by which the rate of lateral wetting front progression is directly related to the water retention characteristics of the soil, and therefore can be used to deduce these properties in situ. These measurements are simple to perform and interpret and are non-destructive to the soil, and therefore can be used to infer water retention properties at different temporal and spatial scales. As a result, this approach can be used to increase our understanding and prediction of soil water characteristics in a variety of systems and environments.