DISTRIBUTED NUMERICAL MODELLING OF HYDROLOGICAL/HYDROGEOLOGICAL PROCESSES IN THE DENSU BASIN

ABSTRACT: The study was undertaken to develop a Densu basin integrated hydrological model capable of describing the major hydrological processes occurring in the land phase of the hydrological cycle and computation of water balance in the Densu river basin. Inadequate information on the hydrological processes as well as the regional water balance in the basin has adversely affected the sustainable development, assessment and efficient management of the basin’s water resources. A physically based numerical modeling system (MIKE SHE) was applied in an integrated approach. The MIKE SHE model provides a comprehensive framework for studying the interrelationship between upstream and downstream conditions and processes. The saturated and unsaturated zones flows were simulated using a fully implicit finite difference solution technique whereas the governing processes of the evapotranspiration were based on empirically derived equations. A conceptual model was developed using field data followed by model design, calibration and verification.

This study finds that actual evapotranspiration in savannah land cover in the basin ranges between 0.5 mm/day and 4.0 mm/day. In the forest land cover, actual evapotranspiration ranges between 0.4 mm/day and 5 mm/day. Actual evapotranspiration approaches zero during the dry periods in the case of savannah land cover whereas in the Forest land cover the minimum actual evapotranspiration in the dry periods is 0.4 mm/day

Results from the unsaturated zone study indicate that the Togo series soils provide a better medium for direct groundwater recharge with a maximum recharge rate of 0.7m/h and an average rate of about 0.3m/h. The recharge rate in the granite soils is an order of magnitude lower than that of the Togo series soils but longer recharge duration than in the Togo series soils. The Birimian soils are almost impermeable and therefore the recharge rate is as low as 0.003m/h. 90% of the rainfall in the Densu basin occurs within the duration of 2 to 4 hours. It was observed that the use of daily rainfall as inputs in hydrological models in tropical regions does not represent convective storm events with highly variable precipitation intensities. The use of hourly rainfall data is more appropriate if detailed description of hydrological processes is required. However for the purposes of annual water balance computations, daily rainfall data are equally appropriate.

Simulation of the annual water balance indicate that, 72% of annual rainfall is lost through evapotranspiration, 21% is routed to the Densu river and its tributaries and 8% is groundwater recharge. 0.0009% is base flow and soil moisture deficit amounts to O.O 16%. The application of the MIKE SHE model in the description and characterization of the flow processes in the basin was successful with the only limiting factor being lack of long time series data for model verification.