On the definition and modelling of streamflow drought duration and deficit volume

Abstract

The threshold level approach is used to define drought characteristics, i.e. drought duration and deficit volume from time series of daily streamflow. Three different procedures for pooling dependent droughts are compared: a method based on an inter-event time and volume criterion (IC), a moving average procedure (MA), and a method based on the sequent peak algorithm (SPA). The extreme values of drought duration and deficit volume are analysed using both an annual maximum series (AMS) and a partial duration series (PDS) approach. Two Danish catchments with very different flow regimes were used in the study. The IC and MA methods provided virtually the same sample statistics of the AMS of drought duration and deficit volume for all thresholds considered. The results of the SPA method differed significantly from the other two methods for high thresholds due to the presence of multi-year droughts. For analysis of seasonal droughts the SPA method is restricted to low thresholds. The occurrence of a large number of zerodrought years for low thresholds may significantly reduce the information content of the AMS, and in this case the PDS model is superior. The problem of minor droughts in the PDS was implicitly reduced by using the MA and SPA methods, and in this respect these methods have an important advantage as compared to the IC method.