PhD Thesis Emmanuel Obeng Bekoe - Cranfield University

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in the simulated streamflow of PBIAS of 70% was observed. However unlike in
the calibration phase where the entire overestimation is accounted for by the
dry period simulation, the overestimation of PBIAS of over 330% (108.6 mm)
observed in the dry period validation is less than that observed in the entire
validation period.
This shows that in terms of the PBIAS in the lumped mode simulation, the
overestimation of total flows during calibration is accounted for by the
overestimation of baseflow during the dry periods from November to February
suggesting that recharge to the groundwater store or the rate of groundwater
release may be a contributory cause. However this observation is not so for the
validation period suggesting that there might be other factors causing this
overestimation some of which are discussed in the proceeding sections.
6.1.2 Water Year Analysis
The daily performance statistics of the simulations for the entire validation
period (01/03/70-31/12/72) in Figure 5.10 and Table 5.3 are not good because
they all fell outside the acceptable values given by Gupta (1999) and Henrikson
(2003) but the weekly simulation had better values (Table 5.4) during the
calibration period (NSE= 0.83) and the periods 01/03/71-31/12/72 and 01/03/-
31/12/72. However, when the water year of 1970 was omitted from the
validation period and validation undertaken for 01/03/1971-31/12/72; there is a
significant improvement. Although the improvement in NSE from -0.51 to 0.28
for the daily simulation is still not acceptable according to Henrikson et al.,
(2003), the weekly simulation NSE is acceptable at 0.54 while the RMS error
also improved from 3.20 to 1.88 mm for the weekly and the PBIAS is also better
for the weekly than the daily with less overestimation.
From the validation simulation in both daily (Table 5.3) and weekly (Table 5.4)
timesteps, it is apparent that the year of 1972 has very good statistics with NSE
as high as 0.70 for the weekly as against 0.58 for the daily. However, the
statistics for the dry periods were poor in all respects for both calibration and
Emmanuel Obeng Bekoe PhD Thesis Chapter 6 Discussion of Results