Spatio-temporal variability of event runoff coefficients

SummaryRunoff coefficients are widely used as a diagnostic variable of runoff generation in process studies and as an important input parameter in hydrologic design. In the present study runoff coefficients have been back calculated from hourly runoff data, hourly precipitation data and estimates of snowmelt. A total of about 50,000 events in 337 Austrian catchments with catchment areas ranging from 80 to 10,000 km2 have been analysed over the period 1981–2000. The results indicate that the spatial distribution of runoff coefficients is highly correlated with mean annual precipitation but little correlated with soil type and land use. The temporal distribution of runoff coefficients can be accurately represented by a Beta distribution. The parameters of this distribution exhibit spatial patterns that match six climatic regions of Austria. In each of the regions, event runoff coefficients increase with event rainfall depth and with antecedent rainfall but the differences between the regions are larger than those between events of different sizes. An analysis of the runoff coefficients by flood types indicates that for flash floods, runoff coefficients are smallest, and they increase, in that order, for short rain floods, long rain floods, rain-on-snow floods and snowmelt floods. It appears that in this type of climate and at the scale of the catchments examined here the main controls on event runoff coefficients are the climate and the runoff regime through the seasonal catchment water balance and hence antecedent soil moisture in addition to event characteristics. Catchment characteristics such as soils and land use affect runoff coefficients to a lesser degree.