PhD Thesis Emmanuel Obeng Bekoe - Cranfield University

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The figures show that there is no apparent trend between the appraisal
statistics RMS, PBIAS and NSE and the annual mean daily flow in this study for
the years studied. However, this is not conclusive because of their relatively
short duration of observations (only five years) and more data would be
required before the variability can be properly explained.
6.7 Soil Parameters on ET
As discussed in Section 3.4.2.3, there are several relationships between soils
and evapotranspiration. In ACRU evapotranspiration is considered as soil
water evaporation (from the topsoil horizon only) and plant transpiration (from
all horizons in the root zone). This shows that soil parameters required for
evapotranspiration computation such as soil depth, field capacities, and
redistribution rates should be computed accurately for correct
evapotranspiration to be attained. However, as described in Section 4.2.5, soil
parameters were generated because direct data were not available. As such if
the generated data is not accurate the outcome of computed ET may be
different from actual ET computed. Therefore problems with soil parameters
could not be ruled out from the poor results seen in some of the simulations.
6.8 Manual Calibration
Hydrologic models are intended to be applicable to different watersheds and for
that reason they typically have parameters that must be adjusted to make the
model behaviour match the behaviour of the catchment of interest (Gupta et al.,
1999, Masden, 2000, Ajami et al., 2004). No mater how sophisticated the
model structure is if the parameters are poorly specified, the model simulated
fluxes (e.g. streamflows) can be quite different from those actually observed
(Gupta et al., 1999). Therefore the calibration procedure must be conducted
carefully to maximise the reliability of the model. ACRU model is described as a
multi parameter model (Schulze, 1995) and therefore has many parameters
such as COFRU, COAIM, BFRESP, CONST, STOIMP and FOREST which are
not directly observable and yet have to be estimated for the Densu catchment.
Emmanuel Obeng Bekoe Phd Thesis Chapter 6 Discussion