Propagation of drought through groundwater systems

Index words: drought, groundwater, simulation, synthetic data, extreme eventsThe transformation of droughts as a result of the propagation through groundwater systems is examined by comparing droughts in time series of groundwater recharge, levels and discharge. The groundwater system was simulated mostly as a reservoir, characterised by a reservoir coefficient j (d). Different aquifer characteristics were simulated using a range of j-values. A groundwater system causes attenuation and increasing persistence in the discharge. The attenuation decreases the drought severity, whereas the increasing persistence makes the cumulative probability distribution of both drought duration and severity steeper. This means that recharge and discharge from fast responding catchments (small j) have a large number of relativelyunseveredroughts, whereas slowly responding catchments have fewer droughts, but a larger probability of severe droughts. To evaluate the overall performance of the groundwater system with respect to drought, a new performance criterion has been introduced. This criterion shows that groundwater reservoirs with intermediately fast response (j approximately 200 d) have the lowest overall performance. The highest overall performance occurs for slowly responding systems. The influence of the characteristics of the recharge has been examined by using recharge from two catchments: the sub-humid Pangcatchment(UK) and the semi-arid Upper-Guadianacatchment(ES). The most important difference between the two catchments with respect to the propagation of drought is the difference in seasonality. The strong seasonality for the Pang prevents the pooling of droughts. For the semi-aridcatchment, the increase in the number of severe droughts as a result of the propagation through the groundwater system was much higher and thus the overall performance was much lower. For the Pangcatchment, a physically based groundwater flow model was used to examine the spatial distribution of drought. After normalising the groundwater levels with the standard deviation, the variations in the persistence dominate the drought behaviour in the groundwater levels. Thus the cumulative probabilitydistributionclose to the stream was less steep than at the interfluves. Finally, non-linearity in the groundwater level-discharge relationship increases the drought duration and, for most droughts, the severity.