Propagation and spatial distribution of drought in a groundwater catchment

Abstract

Natural droughts are usually caused by a period of lower than average precipitation. The resulting shortage in precipitation propagates through the hydrological system causing drought in the different segments of the hydrological system (unsaturated zone, saturated groundwater, surface water). To analyse the propagation and spatial distribution of the drought in the groundwater system, drought is analysed simultaneously in simulated recharge, hydraulic heads and groundwater discharge for a groundwater catchment (Pang, UK). The recharge and hydraulic heads are simulated and analysed spatially. The groundwater discharge is analysed in four reaches along the stream. Comparison of droughts in the recharge and heads shows that the droughts are greatly attenuated and delayed in the groundwater system. Comparison of the droughts in the heads and the discharge shows the opposite effect, so that the droughts in the discharge resemble more the droughts in the recharge than those in the hydraulic heads. The faster response in the groundwater discharge compared to the hydraulic heads is probably caused by the non-linear relationship between groundwater storage and discharge. Thus in the recharge and in the discharge, there are many short droughts, whereas in the hydraulic head fewer, more severe droughts occur. It is likely that these results hold not only for the Pang catchment but also generally for catchments with a large groundwater system and a non-linear groundwater discharge relationship. The spatial distribution of the droughts in the hydraulic heads shows that short droughts are more severe near the streams, as they are attenuated further away. Long periods of below average recharge have relatively more effect near the groundwater divide. The spatial distribution of the droughts in the groundwater discharge is such that upstream the droughts are more attenuated than downstream.

Introduction

Drought is a recurring, natural phenomenon, which affects every part of the hydrological cycle. Droughts are mostly caused by periods of lower than average precipitation and propagate through the hydrological system. In each segment of the hydrological system (unsaturated zone, saturated groundwater and surface water), a drought can develop, depending on the intensity and duration of the shortage in precipitation. In natural systems, the droughts in the different segments of the hydrological system are related. A lack of precipitation causes low soil moisture content, which in turn causes low groundwater recharge. In groundwater systems, low groundwater recharge ultimately causes low groundwater levels and low groundwater discharge. This propagation of drought through groundwater has been studied by Eltahir and Yeh, 1999, Peters et al., 2003, Peters and van Lanen, 2003. Eltahir and Yeh (1999) show how the crossing characteristics (excursions above (floods) or below (droughts) a pre-set threshold level) change as a result of propagation through the groundwater system. Peters et al., 2003, Peters and van Lanen, 2003 compare drought characteristics in groundwater recharge and discharge and show how the droughts change from many, short droughts in the recharge to fewer, mostly longer droughts in the groundwater discharge.

However, droughts are not only different in the different segments of the hydrological system, but are also spatially variable. The spatial variability occurs not only in the precipitation, soil moisture and discharge, but also in groundwater, even if precipitation and soil moisture would be uniformly distributed in space. Whereas spatial variation of droughts in precipitation and discharge has been studied quite extensively (see for example, Demuth and Stahl, 2001), spatial variation in droughts in groundwater levels has hardly been studied. Yet, it is well known that there are clear spatial patterns in the temporal variation of phreatic groundwater levels (Gehrels et al., 1994, de Vries, 1995). These spatial patterns in the temporal characteristics of groundwater levels have clearly been shown by geostatistical methods for the interpolation of groundwater levels (van Geer, 1994, Stein, 1998, Kumar and Ahmed, 2003). Indeed, a study by Chang and Teoh (1995) showed a clear variation in droughts in a limited number of wells (13) throughout an aquifer.

In this paper, the propagation of drought under natural conditions through groundwater and the spatial variability of drought in groundwater will be analysed by studying the spatial variation of droughts in the groundwater recharge, groundwater levels and groundwater discharge in a groundwater catchment. The groundwater catchment for which these analyses will be done is the catchment of the Pang (UK), which is a tributary to the Thames. The development of droughts will be studied by using historical data. For these analyses, the groundwater recharge, levels and discharge need to be available with a high spatial and temporal resolution. Although in the Pang catchment, groundwater levels have been observed quite extensively, records with a sufficiently long, common time period are not available in sufficient spatial detail. Moreover, groundwater recharge has not been observed at all. Thus, groundwater recharge, levels and discharge had to be simulated. An additional reason for simulation of the groundwater levels is the fact that the purpose of this case study is not to analyse the specific flow conditions in the Pang catchment in every detail, but to analyse the spatial aspects of groundwater drought more generally. A secondary objective of this paper is to evaluate the drought event definition for groundwater levels, as little experience presently exists about the derivation of droughts from groundwater levels (Hisdal et al., 2004). Specifically the use of the threshold level approach for groundwater levels will be investigated.