Malawi Hydrogeological and Water Quality Mapping: Assessing Groundwater Resources Under Extreme Weather

The distributed recharge model developed by Scheidegger et al. (2015) is used to estimate the recharge values under extreme weather events. Synthetic extreme dry and wet rainfall and evaporation time series are produced by repeating a dry or a wet year within the historical rainfall and evaporation time series. The Standardised Precipitation Index (SPI) method is used to identify the most wet and most dry years. Heat maps showing the severity of drought or wet periods across the country are used. These maps show inconsistencies of the calculated indices across the country, with oddities observed in the north part of the country. Six scenarios are considered in which, the wet year is repeated once, twice, and three times and then the dry year is repeated in the same fashion. The estimated long term average recharge values are compared to the historical ones. On average, the groundwater system is expected to be in shortage of 9% of historical long term average recharge values calculated for the country when four successive years of drought years are considered. The groundwater system contains approximately 11 % more resources than that is calculated historically when four successive wet years are considered. AquiMod lumped groundwater model is used to estimate representative transmissivity and storage coefficient values for three catchments. Groundwater levels recorded at the boreholes in Chitipa, Endongolweni School, and Namwera are used for this purpose. The numerical model produces acceptable groundwater time series for the first two boreholes but fails to produce the groundwater level fluctuations at the Namwera borehole. It is believed that inconsistencies between the calculated recharge and the groundwater level time series are the reason for this failure. The optimised hydrogeological parameters lead to transmissivity values varied between 20 and 1500 m2/day. Storage coefficient (specific yield) on the other hand varied between 0.02 and 0.3. The AquiMod models were run using the synthetic meteorological extreme scenarios and the groundwater level fluctuations are compared to those produced using the historical recharge values. The uncertainties associated with the determination of extreme weather periods in the northern Malawi are propagated in this modelling exercise. Whereas the higher extreme weather signals in the south lead to the determination of clearly identifiable extreme weather events, the less clear signals in the north induce the production of incorrect synthetic wet scenarios for this region

Malawi Hydrogeological and Water Quality Mapping: National Scale Recharge Estimation

A toolbox of different recharge values and a distributed recharge model have been applied to estimate the recharge values over Malawi. The toolbox is prepared within Microsoft Excel and coded using Visual Basics. The distributed recharge calculation is undertaken using the BGS ZOODRM model. The model uses gridded daily rainfall and potential evaporation data as well as gridded landuse, topography, soil, and river data to calculate recharge. The distributed recharge model is calibrated by matching the simulated overland flows to the observed ones at selected gauging stations. However, difficulties were encountered during the calibration of the recharge model due to: (i) the resolution of the model grid being relatively coarse so that the topographical characteristics could not be fully captured, (ii) the number of runoff zones specified in the model not being enough to represent the characteristics of the study area, and (iii) there being a need to improve the representation of land cover in the model since the land cover affects the estimated recharge values. The estimated recharge values presented in this study are highly affected by the quality of data used in the distributed recharge model. Comparing the recharge values estimated from the recharge model and averaged over the district areas to the recharge values calculated using the recharge toolbox, it was clear that the former agree with the values of at least one analytical method included in the toolbox. However, there was no consistency of agreement, i.e. the recharge values produced by the distributed model did not agree with one particular method. The sensitivity analysis results indicate that the recharge values are highly affected by the soil type parameter values specified in the model and by the definition of spatial distribution of land cover. To improve the accuracy of recharge calculations using the distributed recharge model, it is recommended that maps with a better representation of these features are included in the model. In addition, further model calibration runs are needed to improve the quality of the estimated recharge values. This can be only achieved by obtaining better field data

Flash flood mitigation as a positive consequence of anthropogenic forcing on the groundwater resource in a karst catchment

The Mediterranean coastal region is prone to high-intensity rainfall events that are frequently associated with devastating flash floods. This paper discusses the role of a karst aquifer system in the flash floods of a Mediterranean river, the Lez river. Most of the Lez river watershed is located on karst terrains where interactions between surface water and groundwater take place. During extreme rainfall events, the presence of fractures and well-developed karst features in carbonate terrains enhances the infiltration processes and involves the concentration of the recharge into highly organized and permeable flow paths. The groundwater, therefore, quickly moves towards the natural outlets of the karst system. The influence of the Lez karst aquifer system on the associated river floods dynamics is analysed while considering the spatially distributed rainfall, as well as the time series of the groundwater level within the aquifer and of the Lez river discharge measured at various gauging stations. Special attention is given to the relative importance of the surface and underground processes involved in flash flood genesis. It is shown that the karst groundwater contributes to flash floods under certain conditions, while high-rate pumping within the karst aquifer, which generates significant drawdown, may mitigate flash floods under other conditions