Abstract
Abstract. Drought is understood as both a lack of water (i.e., a deficit compared to demand) and a temporal anomaly in one or more components of the hydrological cycle. Most drought indices, however, only consider the anomaly aspect, i.e., how unusual the condition is. In this paper, we present two drought hazard indices that reflect both the deficit and anomaly aspects. The soil moisture deficit anomaly index, SMDAI, is based on the drought severity index, DSI (Cammalleri et al., 2016), but is computed in a more straightforward way that does not require the definition of a mapping function. We propose a new indicator of drought hazard for water supply from rivers, the streamflow deficit anomaly index, QDAI, which takes into account the surface water demand of humans and freshwater biota. Both indices are computed and analyzed at the global scale, with a spatial resolution of roughly 50 km, for the period 1981–2010, using monthly time series of variables computed by the global water resources and the model WaterGAP 2.2d. We found that the SMDAI and QDAI values are broadly similar to values of purely anomaly-based indices. However, the deficit anomaly indices provide more differentiated spatial and temporal patterns that help to distinguish the degree and nature of the actual drought hazard to vegetation health or the water supply. QDAI can be made relevant for stakeholders with different perceptions about the importance of ecosystem protection, by adapting the approach for computing the amount of water that is required to remain in the river for the well-being of the river ecosystem. Both deficit anomaly indices are well suited for inclusion in local or global drought risk studies.
Highlights
According to the Australian Bureau of Meteorology, “drought is a prolonged, abnormally dry period when the amount of available water is insufficient to meet our normal use” (BoM, 2018)
soil moisture deficit anomaly index (SMDAI) appropriately indicates that anomalously low soil moisture during generally wet winter months is less of a hazard to vegetation than the same anomaly would be during generally dry summer months
We presented two drought hazard indices that combine the drought deficit and anomaly characteristics: one for soil moisture drought (SMDAI) and the other for streamflow drought (QDAI)
Summary
According to the Australian Bureau of Meteorology, “drought is a prolonged, abnormally dry period when the amount of available water is insufficient to meet our normal use” (BoM, 2018). Assuming that humans and other biota are accustomed to seasonal variations in water availability in the form of precipitation, soil moisture, streamflow or groundwater storage, droughts are mostly defined by the deviation of a water quantity at a specific point in time (e.g., precipitation in May 2005) from its long-term mean or median (e.g., of all May precipitation values during the reference period 1981–2010). It is further assumed for most drought hazard indicators that humans and other biota are used to interannual variability. Anomaly-based drought indicators that indicate less water than normal include the standardized precipitation index (SPI) (Mckee et al, 1993), the standardized precipitation evapotranspiration index (SPEI) (VicenteSerrano et al, 2010; Bergez et al, 2013), the China Z index (CZI) (Wu et al, 2001) and, for streamflow drought, the stan-
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