A generic method for hydrological drought identification across different climate regions

Abstract

Abstract. The identification of hydrological drought at global scale has received considerable attention during the last decade. However, climate-induced variation in runoff across the world makes such analyses rather complicated. This especially holds for the drier regions of the world (both cold and warm), where, for a considerable period of time, zero runoff can be observed. In the current paper, we present a method that enables to identify drought at global scale across climate regimes in a consistent manner. The method combines the characteristics of the classical variable threshold level method that is best applicable in regions with nonzero runoff most of the time, and the consecutive dry days (period) method that is better suited for areas where zero runoff occurs. The newly presented method allows a drought in periods with runoff to continue in the following period without runoff. The method is demonstrated by identifying droughts from discharge observations of four rivers situated within different climate regimes, as well as from simulated runoff data at global scale obtained from an ensemble of five different land surface models. The identified drought events obtained by the new approach are compared to those resulting from application of the variable threshold level method or the consecutive dry period method separately. Results show that, in general, for drier regions, the threshold level method overestimates drought duration, because zero runoff periods are included in a drought, according to the definition used within this method. The consecutive dry period method underestimates drought occurrence, since it cannot identify droughts for periods with runoff. The developed method especially shows its relevance in transitional areas, because, in wetter regions, results are identical to the classical threshold level method. By combining both methods, the new method is able to identify single drought events that occur during positive and zero runoff periods, leading to a more realistic global drought characterization, especially within drier environments.

Evaluation of drought propagation in an ensemble mean of large-scale hydrological models

“…tropical and arid regions in Africa and Asia). The analysis of drought characteristics can be done on a high number of grid cells with different climate using the method of Van Huijgevoort et al (2012b). The analysis of drought propagation features and the classification of hydrological droughts into types requires visual inspection and expert knowledge.…”

“…These case study areas correspond to natural headwater catchments in Europe with contrasting climate and catchment characteristics (Table 2). A short description of the case study areas is given in this subsection; more detailed descriptions can be found in Van Lanen et al (2008) and .…”

“…Van Lanen et al, 2004Tallaksen et al, 2009;Hannaford et al, 2011;Van Loon et al, 2011a;. Therefore, the correct simulation of storage is important if large-scale models are to be used in hydrological drought analysis.…”

“…Gudmundsson et al, 2012;Stahl et al, 2011b). Therefore, in this study, we investigated a multi-model ensemble, as was previously done in some other drought studies (Wang et al, 2009(Wang et al, , 2011Gudmundsson et al, 2012;Stahl et al, 2012;Van Huijgevoort et al, 2012a). The aim of this paper is explicitly not to compare individual models or model approaches, but to see whether large-scale models in general can reproduce drought propagation.…”

“…Changnon Jr., 1987;Tallaksen and Van Lanen, 2004;Sheffield and Wood, 2011, however without making a distinction between rain and snow seasons in cold climates). Despite these different ways that a hydrological drought can develop from the meteorological situation, some drought propagation features are common to all hydrological droughts (Eltahir and Yeh, 1999;Van Lanen et al, 2004;Van Loon et al, 2011b;:…”