Relationship between Topography, Land Use and Soil Moisture in Loess Hill Slopes

The relationship between topography, land use, and topsoil moisture storage was investigated for a small catchment with undulating deep loess hill slopes in the south of the Netherlands. For a period of 10 months, soil moisture profiles were measured weekly at 15 locations throughout the catchment. A Generalized Additive Model was employed to find relationships between various factors influencing soil moisture. The model defines a water balance as a sum of non-linear components. The water balance was applied to our data at various spatial (catchment, response unit, hillslope and plot), and temporal (monthly, weekly and daily) scales. Each of the water balance components was parameterized as a function of topographic, land use, weather and antecedent soil moisture variables. The model framework is hierarchical: it starts at the coarsest spatio-temporal resolution, the water balance components found here act as constraints when identifying models at finer resolutions. It turned out that the importance of land-use variables varied considerably with temporal resolution. At coarse resolutions land-use was unimportant, whereas at finer resolutions it became more relevant. Land use was equally important over all spatial resolutions (response unit and finer). Topography was mostly relevant at the plot scale. The water balance terms became increasingly non-linear at finer scales. Evapotranspiration depended mainly on reference evapotranspiration and crop cover. Drainage to deeper layers depended mainly on soil moisture and to a lesser extent on topography. Lateral transport was weakly dependent on topography. It appeared that autoregressive components became increasingly important at finer temporal resolutions.