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

Runoff volumes from field plots can be quite variable, but the reasons for this variability are not completely understood. Such variations can be important for understanding the hydrologic system, and for evaluating the effectiveness of infiltration, runoff and sediment models. In this study, we investigated the sources of variability among 40 replications in a previously reported experiment on fallow plots located on a claypan soil in Missouri, USA. A numerical model was calibrated using data from the experiment and from other published data on the variability of soil properties. The results describe qualitatively the trend in the observed relationship between the coefficient of variation (CV) and mean runoff volume per event, as well as the lack of stability in time of the relative differences in runoff volume among plots. Quantitatively, approximately 50% of the observed coefficients of variation among the replicated plots were explained by the spatial variability of Ks, surface storage, and the depth to claypan. The remaining 50% may 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 process. Our results suggested that changes in the relative differences in runoff volumes between plots during the season might be explained by the modification of the spatial distribution of Ks and surface storage which occurs during tillage. The introduction of these sources of variability in the model formulation produced a realistic description of the variance of the observed values of runoff volume, as well as a relatively clear delineation between the explained and unexplained variability. The results may also serve as an index of model performance in predicting observed data.