Analysis of sources of variability of runoff volume in a 40 plot experiment using a numerical model

Runoff volumes from held plots can be quite variable. but the reasons for t his variability are not completely understood. Such variations can be impor tant for understanding the hydrologic system, and for evaluating the effect iveness of infiltration, runoff and sediment models. In this study, we inve stigated the sources of variability among 40 replications in a previously r eported experiment on fallow plots located on a claypan soil in Missouri, U SA. A numerical model was calibrated using data from the experiment and fro m other published data on the variability of soil properties. The results d escribe qualitatively the trend in the observed relationship between the co efficient of variation (CV) and mean runoff volume per event, as well as th e lack of stability in time of the relative differences in runoff volume am ong plots. Quantitatively, approximately 50% of the observed coefficients o f variation among the replicated plots were explained by the spatial variab ility of K,, surface storage, and the depth to claypan. The remaining 50% m ay be due to the variability in rainfall among plots, measurement error in runoff, the fact that some published rather than site specific information was used in the analyses, and simplifications introduced in the modeling pr ocess. Our results suggested that changes in the relative differences in ru noff volumes between plots during the season might be explained by the modi fication of the spatial distribution of K, and surface storage which occurs during tillage. The introduction of these sources of variability in the mo del formulation produced a realistic description of the variance of the obs erved values of runoff volume, as well as a relatively clear delineation be tween the explained and unexplained variability. The results may also serve as an index of model performance in predicting observed data. (C) 2001 Els evier Science B.V. All rights reserved.