PhD Thesis Emmanuel Obeng Bekoe - Cranfield University

171
intermediate groundwater store and coefficient of baseflow release from the
intermediate store into the stream (Schulze, 1995).
The earlier analysis of the delayed initial wetting up of the catchment following
the 1967 dry season (in comparison to the observed) which the calibration could
not correct and the generally poor simulation during the dry seasons suggests
weaknesses in the recharge and baseflow generation model components.
Everson (2001) and New (2002) also found some limitations of ACRU in relation
to groundwater simulation.
According to Everson (2001) a limitation observed during a relatively wet year
simulation was the inability of ACRU to account for lateral subsurface soil water
flow during a water balance study in South Africa, where analysis of the
simulated unsaturated flow from the B horizon to the groundwater zone and
saturated drainage could not account for the predominance of subsurface flow
found in the catchment. The soils were reported as having organic matter
contents of 6-10%, and high water holding capacities but the subsoils had a
very high clay content and poor infiltration leading to soil water logging and
lateral interflow within the more permeable upper soil layer. These mechanisms
could occur in Densu Basin because of the heterogeneity of soils within this
large catchment with water holding capacities and high clay contents, between
35-55%, in the subsoils (Adu and Asiama, 1992).
New (2002) observed two shortcomings of the ACRU model which affect
baseflow simulation which are 1) the inability of the model to simulate the
wetting-up of the catchments at the beginning of the wet season, which resulted
in over prediction of streamflow and 2) the tendency of the ACRU model to
underestimate dry season baseflow. In this modelling with ACRU for the Densu
Basin, the dry season produced the most overestimation which may be as a
result of reasons deduced in the earlier Sections such as “poor” input data, and
weaknesses in the groundwater routine in ACRU where the delay in the initial
wetting up of the catchment in 1968 (in comparison to the observed) could not
Emmanuel Obeng Bekoe Phd Thesis Chapter 6 Discussion