A CONCEPTUAL MODEL FOR SURFACE-GROUNDWATER INTERACTIONS WITHIN A REGIONAL-SCALE FRACTURED ROCK AQUIFER

More than 72% of sub-saharan Africa land surface is comprised of hard rock with fractured rock aquifers supplying water to an estimated 25% of the rural population. Given low porosity and storativity, fractured rock aquifers are particularly vulnerable to stresses such as projected population growth and climate variability. General circulation models of sub-saharan Africa predict increases in temperature and in occurrences of extreme precipitation trends, such as flooding and drought. Adaptation strategies that promote optimal uses of water resources have emerged, although, most focus exclusively on surface water resources. This project aims at developing a conceptual model of the Koumfab watershed, in northern Togo, that will serve as a basis for an integrated model that simulates SW-GW interactions and their responses to different climate scenarios. The Koumfab watershed (83 km²) is underlain by crystalline rocks of the West African Craton and consolidated sedimentary rocks of the Volta basin. Climate data relies on a single, within-watershed weather station that records precipitation, humidity, and temperature data. These data are used to compute the average potential evapotranspiration over a period of 1992-2016 (1853.3 mm) which exceeds the average annual precipitation over the same period (1104.4mm). Surface water data within the watershed consists of limited records of reservoir stage and stream discharge. The ground water flow system within this watershed is characterized using a total of 37 wells to determine depth to water and hydraulic gradient, estimate hydraulic parameters from pumping test responses, and lithologic unit descriptions. Analysis of well logs suggest that the Koumfab watershed is underlain by a semi-confined aquifer; it consists of a weathered horizon (25 m average thickness) overlying a fissured horizon where most of the productive fractures occur (20 – 40m). Characteristic of fractured rock aquifers, transmissivity values are highly variable and range from 2.3x10^-6 – 3.1x10^-4 m²/s in the sandstone unit, and 5.8x10^-6 – 7.3x10^-4 m²/s in the migmatite/gneiss unit. Planned drone surveys of surface outcrops will be used to relate fracture patterns of the two different units to hydraulic properties, such as fracture transmissivity and porosity.