Assessment of the Effects of Climate and Land Use /Land Cover Change on Groundwater Resources in Koda Catchment, Mali, West Africa.

Abstract

Both Climate and Land Use/Land Cover (LULC) changes are the key factors that can modify water resources availability. Water resources which are a vital necessity for any existing life form are controlled by Climate Change and LULC changes. As Groundwater resources are considered to be more resilient to global changes and many studies have been conducted in order to assess the effects of CC and LULC changes on these resources. Groundwater is the permanent source of water in the Koda catchment, which occupies a surface area of 4921 km2. Surface water body abounds during the rainy season and dries a few months after this season. Groundwater resources are used to meet most water needs of the inhabitants of the Koda catchment, therefore a careful assessment of the effects of Climate and Land Use/Land Cover changes on groundwater resources is required to better manage these resources. To meet this objective scientific tools and models have been employed in this work to study the groundwater resources over Koda catchment. This study analyses, both at local and regional scales, the rainfall variability across Koda catchment over the period of 1986-2017. Rainfall data recorded from three meteorological stations (Bamako, Katibougou and Toubougou) were used. The standard precipitation index has been estimated in order to characterize theinterannual variability of rainfall. In addition, the non-parametrical Pettitt’s method (1979), U-statistic of Buishand (1982), Lee and Heghinian test (1977) and Hubert Segmentation (1989) have been evaluated through Khronostat software in order to detect the break point in the rainfall series. Furthermore, the study found that groundwater recharge is one of the most difficult fluxes to define, particularly in arid and semi-arid areas. To make a good estimation of the groundwater recharge which is the key parameter and essential for integrated water management and adaptation the Thornthwaite method, Water Table Fluctuation method and Gardenia model were used to estimate the recharge at different scales. In addition, in order to understand the behavior of the aquifer system, Watershed-scale groundwater flow models was developed. The groundwater flow model (Visual Modflow) was used to simulate the groundwater flow in the Koda catchment. The groundwater flow of the Koda catchment has been calibrated under steady state condition. To evaluate the dynamics of the LULC change over Koda catchment, the spatiotemporal variation of the different units of LULC present in the catchment has been examined. The Supervised Classification method, using Envi 4.5 Software coupled with ArcGIS, was applied to subset Landsat images from 1990 to 2016. Also,the projected Rainfall and Temperature derived from the outputs of three Regional Climate Models (RCMs) driven by three Global Climate Models GCMs under the Representative Concentration Pathways RCP 4.5 and RCP 8.5 Scenarios, were statistically downscaled and corrected using Multiscale Quantile Mapping bias correction method. Therefore,the projected trend of rainfall and temperature for the period 2021-2050 across Koda catchment has been determined. The hydrogeological modeling has been done using Gardenia model to assess the effects of Climate Change on groundwater. The statistical parameters between historical and observed data recorded at Katibougou station have been calculated and the RCA4 driven by two GCMs (IHEC-EC-EARTH and MPI-M-MPI-ESM-LR) have been shown the best correlation. Therefore, the projected temperature and rainfall patterns of these two GCM/RCM pairs, the PET values and Groundwater levels GWL in three piezometers have been used also as the inputs of the Gardenia model. The results show that, during the period of rainy season, July, August, September and October, the GWL decreases in the piezometers within the Koda Catchment for all the two RCM/ GCM pairs under RCP 4.5 compared to historical period. According to the GCM IHEC-EC-EARTH, the predicted decrease of GWL is up to 1.09 m for the RCP 4.5 and 1.26 m for the RCP8.5 within the Koda Catchment while the GCM MPI-M-MPI-ESM-LR showed the decrease of GWL during the rainy season from 0.62 m for the RCP 4.5 to 1.93 m for the RCP 85. The decrease is more significant for the RCP 8.5 than the RCP 4.5 except with one located near to the outlet of the studied catchment where the GCM MPI-M-MPI-ESM-LR projects an increase of 0.67 m of the observed GWL under the RCP 8.5. All the RCM/ GCM pairs project a decrease of Groundwater recharge over time. It is obvious that this decrease is more significant in RCP8.5 for all the piezometers. The results also show that the maximum recharge in the future is below the present dry conditions, which might lead to the drastic event (drought). The results show that recharge decreases from 1987 to 2050 and severe droughts occur from 2029 to 3039. Finally, the effects of LULC change on groundwater were assessed for the period 1987-2016. The decline of 8.4 % in groundwater recharge associated to the decrease of savannah and the increase of bare land and cultivated land might become so far obvious in the future if the current rate of deforestation continues in the Koda catchment. The general conclusion of this study is that the Climate and Land Use Land Cover changes are negatively threatened by the groundwater resources over the study area. The projected effects of climate change will be obviousin the 2030s (where the simulated droughts events are expected)on groundwater which is projected to be inadequately scarce. Therefore, it is necessary to develop a proper water management plan of these resources to offset these difficulties.