Exploring controls on rainfall–runoff events: spatial dynamics of event runoff coefficients in Iran

This study investigates the controls that influence the spatial variability of event runoff coefficients (RCs) based on the comparative hydrology concept. We analysed 18 468 storm events in 963 Iranian catchments ranging in size from 101 to 8467 km². The mean RCs vary spatially due to long-term hydro-climate conditions and other catchment descriptors (CDs). The controls on spatial variability of RCs are investigated using a correlation analysis of the statistical moments of the RCs and CDs. The results indicate (1) catchment elevation is the most important control on RCs at the country scale, and hydro-climatic attributes such as mean annual precipitation, ratio of actual evaporation to precipitation, and the ratio of potential evaporation to precipitation are other important controls on RCs; and (2) at the regional scale, catchment area is a major control on RCs, although it is not as important as hydro-climatic controls at the national scale.