Effect of grid size on runoff and soil moisture for a variable-source-area hydrology model

Soil chemical and biological dynamics in mixed use landscapes are dependent on the distribution and pattern of soil moisture and water transport. In this paper we examine the effect of different grid sizes on soil water content for a spatially explicit, variable-source-area hydrology model applied to a watershed in central New York. Data on topography, soil type, and land use were input at grid sizes from 10 to 600 m. Output data consisted of runoff and spatial pattern of soil moisture. To characterize the spatial variability at different grid sizes, information theory was used to calculate the information content of the input and output variables. Simulation results showed higher average soil water contents and higher evaporation rates for large grid sizes. During a wet year, runoff was not affected by grid size, whereas during a dry year runoff was greatest for the smallest grid size. While the information content (i.e., spatial variability) of soil type and land use maps was not affected by the different grid sizes, increasing grid sizes caused the information content of the slope gradient to decrease slightly and the Laplacian (or curvature of the landscape) to decrease greatly. In other words, increasing grid cell size misrepresented the curvature of the landscape. During wet periods the decrease in information content of the soil moisture data was the same as for the Laplacian as grid size increased. During dry periods, when local fluxes such as evaporation and runoff determine the moisture content, this relation did not exist. The Laplacian can be used to provide a priori estimates of the moisture content deviations by aggregation. These deviations will be much smaller for the slowly undulating landscapes than the landscape with steep valleys simulated in this study.