Soil Water Retention Curves Characterization of a Natural Forested Hillslope using a Scaling Technique Based on a Lognormal Pore‐Size Distribution

Heterogeneous water flow is known to be an important factor of hydrological processes in a natural forested hillslope. To model heterogeneous water flow, the characterization of spatial variability in water retention curve (WRC) is required. The scaling technique introduced by Miller and Miller is effective to characterize it for conditions of constant standard deviation (STD) in pore‐size distribution and porosity, but this is not necessarily appropriate for forested hillslopes. We tested the conventional scaling method and the two proposed methods that presume that field soils do not exhibit constant STD and porosity. The observed WRCs were fitted using a model, which assumes a lognormal pore‐radius distribution and contains three parameters: the matric pressure head related to the median pore radius, ψ m ; the STD of the log‐transformed pore radius distribution, σ; and the effective porosity, θ e In Method 1, which corresponds to the conventional scaling method, ψ m was optimized for each soil, whereas the values of σ and θ e were common for the whole data set. In Method 2, σ was optimized for each soil, and in Method 3, θ e was optimized for each soil, whereas the values of the remaining parameters were common for the whole data set. Method 3 produced the best description of spatial variability in the WRCs. This result indicates that in the natural forested hillslope, variability in the pore‐size distribution is characterized by variability in effective porosity. In practical aspect, we suggested an alternative simpler method to Method 3. In this method, the θ e for each location was estimated from soil penetration resistances measurable in situ. This method explained 59.3% of the spatial variability in WRCs on the studied natural forested hillslope.