Abstract:
This thesis reports on the suitability of using a hydrological model to predict the
impact of future catchment changes on the surface water resources of tropical
West African catchments, as a basis for sustainable watershed management
strategies. The economically-important but data-poor Densu Basin (2100km2)
in Ghana is used as a case study. Prior to the identification of an appropriate
model, a range of model selection criteria to achieve the research objectives
were developed, which included minimum data requirements. Following a
review of available models, the ACRU hydrological model was chosen using
these selection criteria. As this is the first use of the ACRU model in the
Tropics, a sensitivity analysis was performed to gain a better understanding of
the correspondence between the data and the physical processes being
modelled and to inform future data collection priorities. The most sensitive
parameters identified were rainfall; soil water content at field capacity and
monthly crop coefficients. The sensitive parameters were different from those
previously identified for semi arid regions, thereby improving the wider
understanding of the behaviour of the ACRU model in a new climatic region of
Africa for future studies. Although the ACRU model performed well during
calibration (e.g. Nash and Sutcliff Efficiency > 0.8), it performed poorly during
the validation period (e.g. NSE < 0.5). Aggregation of the daily output to
monthly averages improved the performance appraisal statistics to a level
where the model is appropriate for longer term water management issues such
as irrigation planning and water supply planning. However, the prediction of the
hydrological consequences of future catchment changes using the model could
not be performed due to the variable model performance. A detailed analysis of
input data quality and model performance identified a number of contributory
reasons which included the poor distribution of available data from rain gauges
and evaporation stations, poor data management and problems with
groundwater processes within ACRU. Recommendations to the Densu River
Basin Management Board to address the main problem area of data quality
include ensuring that all data (new as well as existing) is subject to QC/QA
procedures, the development of data archiving / back-up systems, making
historical data available to the wider hydrological community; and guidance on
deriving model inputs from available data for future modellers. Finally
recommendations on which areas need further study during future research
using the ACRU model are given.
