Dependence of annual runoff coefficients on basin size and other properties in a climate transition zone from semi-humid to arid and semi-arid on the Loess Plateau, China

The dependence of the runoff coefficient (RC) on basin size has been recognized since decades as a big challenge. However, how such region-specific scale effect varies with basin properties still remains poorly understood. This seriously limits the ability of runoff prediction in ungauged watersheds with different environments and the integrated management of watershed under climate change and land use/cover change. In this study, the scaling relationships of annual RC and its variations with basin properties were quantified based on annual precipitation and runoff data series in twenty-five sub-basins belonging to two large basins on the Loess Plateau, Northwest China. The scaling behavior of RC was found to be significant, and the mean annual RC range significantly decreased as basin size increase. And this decrease of mean annual RC was faster in areas with thin loess cover than that in areas with thick loess cover. Area thresholds, beyond which the mean RC was no longer sensitive to basin size change, were observed for the scaling behavior of annual RC and it was approximately 10,000 km(2) in thick loess areas and 25,000 km(2) in thin loess areas. This indicated that the effects of basin properties on RC scaling behavior was related with scales. Soil thickness might be one of the key basin properties that determined the scaling behavior. An exponential regression equation (R-2 = 0.71, p < 0.01) was built for the annual runoff (R) according to the corresponding geomorphic area (a(n)) and annual precipitation (P) for basins with multiple geomorphic types (n). The results imply that the basin properties not only control the form of the scaling relationships of annual RC but also limit its variations under different geomorphic conditions. These findings may help us assess and manage water resources at the basin scale more accurately.