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

Soil chemical and biological dynamics in mixed use landscapes are dependent on the distribution and pattern of soil moisture and water transport. In t his paper we examine the effect of different grid sizes on soil water conte nt for a spatially explicit, variable-source-area hydrology model applied t o 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 run off and spatial pattern of soil moisture. To characterize the spatial varia bility at different grid sizes, information theory was used to calculate th e 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 sli ghtly 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 t he soil moisture data was the same as for the Laplacian as grid size increa sed. During dry periods, when local fluxes such as evaporation and runoff d etermine the moisture content, this relation did not exist. The Laplacian c an be used to provide a priori estimates of the moisture content deviations by aggregation. These deviations will be much smaller for the slowly undul ating landscapes than the landscape with steep valleys simulated in this st udy.