Browsing by Author "Mussa, Kassim"

Now showing 1 - 2 of 2

Characterization and simulation of nested groundwater flow systems in basins with contrasting climate and geology
(NM-AIST, 2022-12) Mussa, Kassim
The pattern, organization and hierarchy of nested groundwater flow systems, albeit complex, their understanding is very crucial for informed groundwater resources development and management. This study sought to characterize and simulate the impacts of varying climate and anthropogenic influences on the nested groundwater flow systems and the spatio-temporal association of nested groundwater flow systems, utilizing real field test cases of Singida semi- arid fractured aquifer and the Kimbiji coastal, humid sedimentary aquifer in Tanzania. Groundwater flow modelling and simulation of nested groundwater flow systems was carried out using the USGS finite difference modelling code (MODFLOW 6), utilizing ModelMuse version 5 as the Graphical User Interface (GUI). Hydrogeochemical and isotopic signature analyses complemented the modelling approach. The comparison of isotopic composition from borehole, rivers, lakes and rainfall showed that generally, boreholes in the Kimbiji aquifer and some in the Singida aquifer had depleted isotopic values and enriched isotopic values were in samples from open water bodies (e.g., rivers and lakes). The depletion was prominently so in the deep boreholes, indicating a limited influence of evaporation during groundwater recharge. Regional flow fluxes make up 74% of the total inflows into the Kimbiji coastal Neogene aquifer, and only 26% of the inflows are made up of the local flow systems. In the Singida aquifer, regional flow accounts for 56% of the total flux in the lower aquifer and makes 94% contribution to the total amount of groundwater inflow in the Singida aquifer. Only 6% of the groundwater storages comes from local recharge and other sources like lakes. The effect of land cover change dynamics on groundwater recharge has been more prominent in the Kimbiji aquifer, while the effect of climate varying (rainfall and temperature) featured more prominently in the Singida semi-arid aquifer. Dependence of local recharge on heavy rainfalls is one of the key features of the Singida aquifer, while local recharge in the Kimbiji aquifer is quasi-uniform, occurring at more or less similar rate, albeit decreasing with decreasing rainfall amounts and increasing surface temperature, and thus evapotranspiration. Local flow system fluxes were observed to be the main feeders for the upper unconfined aquifers in the two study areas, while regional flow systems are for the deep semi-confined aquifers, with appreciable exchanges of water as revealed by nested groundwater flow simulation. The study findings will contribute to various global, regional and local technical and policy-based efforts towards sustainable groundwater development and management, considering climate variability and non-climatic factors. This includes the contribution to Sustainable Development Goals (SDGs), particularly SDG on access to clean water and sanitation
Natural Groundwater Recharge Response to Climate Variability and Land Cover Change Perturbations in Basins with Contrasting Climate and Geology in Tanzania
(MDPI, 2021-08-30) Mussa, Kassim; Mjemah, Ibrahimu; Machunda, Revocatus
The response of aquifers with contrasting climate and geology to climate and land cover change perturbations through natural groundwater recharge remains inadequately understood. In Tanzania and elsewhere in the world, studies have been conducted to assess the impact of climate change and variability, and land use/cover changes on stream flow using different models, but similar studies on groundwater dynamics are inadequate. This study, therefore, examined the influence of land use/cover and climate dynamics on natural groundwater recharge in basins with contrasting climate and geology in Tanzania, applying the modified soil moisture balance method, coupled with the curve number (CN). The method hinges on the balance between the incoming water from precipitation and the outflow of water by evapotranspiration. The different parameters in the soil moisture balance method were computed using the Thornthwaite Water Balance software. The potential evapotranspiration (PET) was calculated using the daily maximum and minimum temperatures, utilizing two-temperature-based PET methods, Penman–Monteith (PM) and Hargreaves–Samani (HS). The rainfall data were obtained from the gauging stations under the Tanzania Meteorological Agency and some additional data were acquired from climate observatories management by water basins. The results show that there has been a quasi-stable CN in the Singida semi-arid, fractured crystalline basement aquifer (74.2 in 1997, 73.64 in 2005, and 73.87 in 2018). In the Kimbiji, humid, Neogene sedimentary aquifer, the CN has been steadily increasing (66.69 in 1997, 69.08 in 2008, and 71.42 in 2016), indicating the rapid land cover changes in the Kimbiji aquifer as compared to the Singida aquifer. For the Kimbiji humid aquifer, the PET calculated using the Penman– Monteith (PM) method for the 1996/1997, 2007/2008, and 2015/2016 hydrological years were 1156.5, 1079.5, and 1143.9 mm/year, respectively, while for the Hargreaves–Samani (HS) method, the PET was found to be 1046.1, 1138.3, and 1204.4 mm/year for the 1996/1997, 2007/2008, and 2015/2016 hydrological years, respectively. For the Singida semi-arid aquifer, the PM PET method resulted in 2083.3, 2053.6, and 1875.4 mm/year for the 1996/1997, 2004/2005, and 2017/2018 hydrological years, respectively. The HS method produced relatively lower PET values for the semi-arid area (1839.4, 1814.7, and 1710.2 mm/year) for the 1996/1997, 2004/2005, and 2017/2018 hydrological years, respectively. It was equally revealed that the recharge and aridity indices correspond with the PET calculated using two temperature-dependent methods. The decline of certain land covers (forests) and increase in others (built-up areas) have contributed to the increase in surface runoff in each study area, possibly resulting in the decreasing trend of groundwater recharge. An overestimation of the PET using the HS method in the Kimbiji humid aquifer was observed, which was relatively smaller than the overestimation of the PET using the PM method in the Singida semi-arid aquifer. Despite the difference in climate and geology, the response of the two aquifers to rainfall is similar. The combined influence of climate and land cover changes on natural groundwater recharge was observed to be prominent in the Kimbiji aquifer, while only climate variability appreciably influences natural groundwater recharge in the Singida semi-arid aquifer. El Nino and the Southern Oscillation as part of the climate variability phenomenon dwarfed the time lags between rainfall and recharge in the two basins, regardless of their difference in climate and geology