Abstract:
Water resources management in watersheds has become increasingly important due to rapid
expansion of human settlements while pollution caused by industrial development has led to the
part of the available precious water resources unusable for consumption, thus aggravating scarcity
of fresh water resources. The impacts are further exacerbated due to global warming. The use of
the multi-parameter, distributed hydrologic models for water resources assessment in the local
basins are hindered due to scarcity of data and other resources. The lumped parameter rainfall
runoff hydrologic models are widely applied to predict watershed response of small watersheds
by simulating rainfall runoff generation and thus useful in water resource management in
ungauged basins. This study aims at identifying distinct characteristics of one such widely used
model, ABCD Water Balance Model, and studying its applicability to a selected sub basin in
Kelani River Basin for simulating catchment response in terms of rainfall runoff. The model was
subsequently applied to analyze surface and groundwater resources available in the basin,
targeting effective and sustainable water resources development and management.
The data required for the ABCD water balance model were precipitation, evapotranspiration,
average temperature and minimum and maximum temperatures. The model was developed in
Excel spread-sheet format focusing on the data period from 1994~2011 in the Kelani basin. For
model calibration, precipitation and potential evapotranspiration data during the period 1994 to
2001 were used. The generated model streamflow was compared with observed streamflow at
Glencorse station for the same period. For the validation of the model, the precipitation and
potential evapotranspiration data in the latter 10-year period were used. For estimating the
goodness-of-fit, Nash-Sutcliff efficiency coefficient method was used, while model response to
four distinct parameters were assessed based on sensitivity analysis and parameter optimization.
The calibrated model has shown that the model is less sensitive to parameters a (0.9) and b (20)
while on the other hand, the model was highly sensitive to parameter c (0.68) and d (0.01). It was
noted that even with the lesser amount of moisture infiltration from the upper soil zone, the aquifer
was able to produce runoff. Hence, it proved that in the wet zone, the propensity of the area to
produce runoff was largely independent of rainfall intensity. For the model calibration runs, the
correlation or coefficient of determination (R2) between model flow and observed flow was 0.77
with NASH coefficient value of 0.71 and MRAE of 0.27. The model produced a better response
to medium flows between 5% ~ 82% with NASH value of 0.78 and good response for high flows
below 5% of percent exceedance with acceptable results (NASH = 0.62). The model could not
response well for low flows (NASH = 0.45).
This model with four parameters could adequately simulate the rainfall runoff response of the
selected sub-watershed area in Kelani Basin (at Glencorse). Hence, this lumped parameter model
was deemed suitable for streamflow forecasting and water resources assessment in Kelani basin
and it can also be applied in areas elsewhere with similar hydrological characteristics.