PhD Thesis Emmanuel Obeng Bekoe - Cranfield University

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• The aggregated daily into monthly simulations were superior over the
aggregated weekly and also the daily in as far as the appraisal statistics are
concerned, making the ACRU model more suitable for long term planning for
irrigation and water supply;
• The lumped simulations performed better than the semi-distributed mode
simulations.
Weak Points:
• Much overestimation is noticed of simulated flows against observed during
the rainy season. This overestimation is very pronounced in 1968 which
was an exceptionally wet year on record (Meteorological Services
Department of Ghana, 2004);
• The model performance in the drier periods is poor, which is the time in
which water availability is most critical for the intake points of the water
supply system of Koforidua.
The variability in modelling results described in a study in River Niger (Conway
& Mahé, 2005), also in West Africa, interestingly shows the same performance
variation observed with the modelling of the Densu Basin as discussed
previously in Section 6.1. Conway & Mahé, (2005) attributed the variability in
model results to failure to represent additional sources of water loss in the
catchments such as percolation to deep aquifers, evaporation losses from
surface waters and an overall underestimate of soil moisture storage. However,
Conway & Mahé’s, (2005) losses were errors across all years suggesting failure
of the model to represent processes adequately whereas in this modelling of
Densu catchment the errors are prominent in particular years suggesting
problems with data.
From the points listed above, it is considered that reporting on the modelling for
the Densu Basin at a daily time step to show, for example, individual peaks and
floods is not appropriate because of the performance statistics. However, when
the daily results are aggregated into monthly average flows, the improved
Emmanuel Obeng Bekoe Phd Thesis Chapter 7 Conclusions & Recommendations