Seasonality of recession time and lag response to precipitation of the Ankobra and Pra Rivers in Ghana, West Africa

The Guinea Coast region of West Africa experiences two wet seasons, the major season (Mar-Jul) and minor season (Aug-Nov). Although the majority of the seasonal precipitation occurs during the major season, peaking in June, the greater volume of streamflow occurs during the minor season. We seek to investigate the physical mechanisms responsible for this lag in the precipitation-streamflow relationship. Hydrograph recession time and lag response to a precipitation event indicate the relative importance of surface vs. sub-surface runoff in seasonal streamflow. We calculated the recession times and lag responses in two river basins, the Pra (29000 km^2) and Ankobra (9000 km^2) of southern Ghana. The recession time is shorter (i.e., 80% less than 7 days) during the major season and longer (i.e,. 60% more than 7 days) during the minor season. The Ankobra River had shorter recession times during both seasons compared to the Pra River. The lag response of these rivers has a range of 5-10 days during the major season, occurring with an average daily precipitation amount of 50-60 mm. For the minor season, the lag response is 2-3 days, with average daily precipitation amounts of 10-15 mm.

Although previous work in Guinea Coast hydrology has emphasized the role of purely surface mechanisms such as Hortonian runoff, our analysis indicates otherwise. For the minor season, the slower recession times and shorter lag responses vs. less precipitation per day indicates a dominant role for sub-surface mechanisms, both surface runoff due to saturated soil (saturation from below) and groundwater discharge. We hypothesize that the major season precipitation, particularly in the peak month of June, is critical for raising the water content of riparian soils. The saturation of the soil and the reduction in evapotranspiration during the minor season makes it more likely that even small amounts of precipitation will be transferred to streams.