PhD Thesis Emmanuel Obeng Bekoe - Cranfield University

187
but slightly deteriorated to nearly 0.75 in the semi-distributed modes. The
PME’s for the daily and weekly calibrations failed (being negative) but passed
the criteria in the monthly simulations to 0.65. In the validation period the NSE
and PME failed (both having negative values) for all three categories of daily,
weekly and monthly results. However, breaking down the validation periods
into individual water years in both daily and weekly, it is realised that the year of
1972 has very good statistics with the NSE as high as 0.70 for the weekly as
against 0.58 for the daily and a PME of 0.42.
The statistics for the dry seasons were poor in all respects for both calibration
and validation. In the case of the PME appraisal statistic the simulation failed
because they were negative (Gupta et. al. 1999) for the daily calibration
simulation but was positive for two periods (01/03/1971 - 31/12/1972 and
01/03/1972 - 31-12/1972) in the validation period improving from 0.13 to 0.37
and 0.18 to 0.43 respectively when the daily simulations were aggregated into
weekly simulations.
In general, there was no significant improvement in model performance
between the lumped mode simulation and the semi distributed mode. However,
aggregating the daily streamflow output into weekly and monthly streamflow
significantly improved the performance. Overall the model performance was
erratic but the results suggested that the model was more suitable for long term
planning for irrigation and water supply.
The variation in the modelling results has been attributed to several causes
which are:
• The low density of raingauges which are unlikely to have adequately
captured the variability in rainfall within the catchment. Studies (e.g.
Shaw, 1994) suggest that a catchment of the size of the Densu should
have had five raingauge stations, instead of the two for which data were
available;
Emmanuel Obeng Bekoe Phd Thesis Chapter 7 Conclusions & Recommendations