Determination of unit watershed size for use in small watershed hydrological modeling

Abstract

Since the uniform rainfall over the watershed is the most fundamental assumption in small watershed modelling, the limitation on watershed size should be investigated. This study defines the unit watershed size as a dimensinal criteron which is associated with the storm size, and the extent and frequency of storm exclusion ( called spatial and temporal errors). Two approaches of determining average storm cell radius were proposed. One is related with the spatial variation in storm rainfall (DSIP), while another considers both spatial variation and storm exclusion events (RVIP). Both analytical and empirical solutions are obtained and the effect of multiplestorm events is discussed. The storm radius for Walnut Gulch is determined as 4.6 miles which is close to others' results. Given storm radius, a relationship between unit watershed size and the spatial and temporal errors is developed analytically. Based on this relationship, both selection and evaluation of unit watershed size are made possible. If the error levels are known, then the proper watershed size can be selected and if the watershed size is given, then the error levels can be evaluated. By using unit watershed size, the models of small watersheds may be extended to those of large watersheds.