The comparison of the KNMI index with indices based on hydrological variables in order to see if the KNMI index can capture the propagation of meteorological drought into hydrological drought.
Comparison of drought indices for the province of Gelderland, The Netherlands.
Even though The Netherlands is generally seen as a wet country, droughts can cause severe problems. Significant drought conditions are observed in the index based on precipitation and potential evaporation by the Royal Dutch Meteorological Institute (KNMI). In this study the spatial and temporal variability of meteorological and hydrological drought indices has been investigated. We were interested to see if the KNMI can capture the propagation of meteorological drought into hydrological drought. The Standardized Precipitation Index (SPI), the Standardized Precipitation Evapotranspiration Index (SPEI), the Standardized Runoff index (SRI), the Standardized Water Level Index (SWI, groundwater) and the Soil Moisture Anomalies (SMA) were used to quantify drought in the province of Gelderland, The Netherlands, at multiple time scales within the period 1988-2013. The results indicate that drought indices based on different variables show the same major drought events. Drought indices based on precipitation and potential evaporation are more variable in time while drought indices based on discharge and groundwater have a smoothed signal. Spatial distribution of meteorological drought is uniform over Gelderland, whereas large spatial variation exists in hydrological drought (groundwater). Areas with deep groundwater tables experience still wet conditions during selected drought events, caused by the slow response to changes in precipitation. Runoff measurements are less useful for spatial analysis of drought due to limited spatial coverage and short length of the available data. Due to differences in temporal and spatial variation, meteorological drought indices (like the index used by KNMI) cannot simply be used to represent hydrological drought conditions.