Abstract:
Increasing population, varying climatic condition, and water crisis necessitate water resource
planning and development. Incorporating the subwatershed management approach leads to
viable options when making decisions on water resources planning and development.
Meanwhile, the approach needs streamflow management by incorporating spatial variability
of characteristics which requires a hydrologic model and estimation of parameters with gauged
data. However, due to non-availability of gauged streamflow at the subwatershed level hinders
the estimation of model parameters for each subwatershed. Therefore, it's necessary to identify
a suitable lumped model and evaluate the transferability of model parameters for streamflow
estimation, which could provide important insights to support the planning and development
of watershed management in a distributed manner.
In this study, first, an extensive review was conducted on lumped conceptual hydrologic
models to support and facilitate the information for choosing a suitable lumped model for the
study. Secondly, the model calibrated and validated using eight years of daily input data from
2009/2010 to 2016/2017, using a semi-automatic calibration method for Ellagawa main and
Ratnapura subwatershed of Ellagawa. Hereafter, the calibrated and validated parameters for
both watersheds were compared to find out the variation and similarities of parameters. Then,
the calibrated parameters for Ellagawa main watershed transferred to Ratnapura and from
Ratnapura to Ellagwa watershed, and the applicability of the parameters transfer evaluated in
the main and subwatershed for streamflow estimation.
Attribute ranks, of the assessment, revealed that the Tank model with having 4.2 scores got
high ranks in the shortlisted 4 models. The model successfully calibrated with the MRAE
values of 0.450 for Ellagawa and 0.415 for Ratnapura watershed. The calibrated parameters
verified with the MRAE values of 0.452 and 0.361 for Ellagawa and Ratnapura watershed
respectively. When the calibrated parameters for Ellagawa transferred to Ratnapura and from
Ratnapura to Ellagawa for the data period from 2009/2010 to 2016/2017 which was same for
both watersheds, the transferred parameters to Ratnapura simulated the overall streamflow of
the watershed with significant MRAE value of 0.445, while transferred parameters of
Ratnapura to Ellagawa, showed a decline in the model performance with the MRAE value of
0.551.
Findings of the study revealed that the Tank model is the right lumped conceptual model for
the transferability of parameters across the scale in a watershed. In the first case of parameter
transfer of the model from main to subwatershed the transferred parameters estimated the
subwatershed response with a high level of accuracy. Similarly, the calibrated parameters of
the subwatershed estimated the behavior of the main watershed satisfactorily, while, compared
to the first case the model showed a significant decline in the performance. This indicates the
applicability of a calibrated lumped model of the main or subwatershed to other ungauged sub
or main watershed for streamflow estimation to achieve the objective of subwatershed
management and accurately make the decisions on water resources planning and management.