A process-based typology of hydrological drought

Abstract. Hydrological drought events have very different causes and effects. Classifying these events into distinct types can be useful for both science and management. We propose a classification of hydrological drought types that is based on the governing drought propagation processes. In this classification six hydrological drought types are distinguished, i.e. (i) classical rainfall deficit drought, (ii) rain-to-snow-season drought, (iii) wet-to-dry-season drought, (iv) cold snow season drought, (v) warm snow season drought, and (vi) composite drought. The processes underlying these drought types are a result of the interplay of temperature and precipitation at catchment scale in different seasons. As a test case, about 125 groundwater droughts and about 210 discharge droughts in five contrasting headwater catchments in Europe have been classified. The most common drought type in all catchments is the classical rainfall deficit drought (almost 50% of all events), but in the selected catchments these are mostly minor events. If only the five most severe drought events of each catchment are considered, a shift towards more rain-to-snow-season droughts, warm snow season droughts, and composite droughts is found. The occurrence of hydrological drought types is determined by climate and catchment characteristics. The typology is transferable to other catchments, incl. outside Europe, because it is generic and based upon processes that occur around the world. A general framework is proposed to identify drought type occurrence in relation to climate and catchment characteristics.