Abstract:
In a period where water resources are becoming scarce due to increased population and
human activities, it is very important to have appropriate models and objective functions
for water resources management especially in rural contexts. Therefore, the selection of
appropriate model and objective function and to ascertain their suitability on a rural
watershed is necessary. Preliminary screening of hydrological models was carried out
based on the application availabilities and modelling purpose. Five models namely
HEC-HMS, SWAT, TOPMODEL, MIKE SHE and SWMM were shortlisted. The
shortlisted models were reviewed under several criteria such temporal scale, spatial
scale, hydrological processes, documentation, resources requirement, user interface and
model acquisition cost. Similarly, objective functions recommended on ‘Guide for
hydro-meteorological practices’ by WMO namely NSE, RMSE, RAEM and MRAE
were reviewed. Review of the objective functions was based on criteria such as
mathematical implications, flow regimes and modelling purpose. The review of
hydrological models and objective function suggested the Storm Water Management
Model (SWMM) and Mean Ratio of Absolute Error (MRAE) as an appropriate model
and objective function respectively for water resources modelling in rural watersheds.
Accordingly, the SWMM was applied to the Ellagawa (1342 km2) and Ratnapura (653
km2) watersheds in the Kalu river basin of Sri Lanka using observed rainfall and
streamflow from 2006-2014. In the present work, the SWMM model was calibrated and
validated while investigating the effect of layout modifications to carry out continuous
simulation of streamflow. Initially, two lumped models were developed for Ellagawa
and Ratnapura watershed. Then a semi-distributed model with three sub-watersheds was
developed for Ellagawa watershed. Model calibration was done for 2006-2010, and
verification was carried out for the period 2011-2014. High, medium and low flow in the
flow duration curve and the annual water balance were also observed during the
calibration and validation. Ellagawa and Ratnapura lumped were calibrated with MRAE
0.3634 and 0.4531 respectively and validated with MRAE 0.5865 and 0.7843
respectively. Annual water balance errors of Ellagawa and Ratnapura lumped model
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were 38% and 31% respectively during calibration and 10.25% and 11% respectively
during validation. Ellagawa and Ratnapura lumped models calibrated intermediate flow
with MRAE 0.40 and 0.37 respectively. Manning’s roughness coefficient for pervious
layer, depression storage for pervious layer, saturated hydraulic conductivity and initial
defect, lateral discharge coefficient and deep percolation coefficient were the main
parameters to be calibrated. Manning’s roughness coefficient of pervious layer (npervious)
was optimized in the range (0.02-0.028), depression storage of pervious layer
(d-store pervious) was optimized in the range of (1.2mm-2.5mm). Similarly, saturated
hydraulic conductivity (Ksat) was optimized in the range of (0.3mm/hr.-0.67mm/hr.).
Furthermore, the initial moisture deficit (Θ) was optimized in the range of (0.2-0.5).
Ellagawa semi-distributed model showed some improvement in overall and intermediate
flow compared to Ellagawa lumped model. MRAE for overall hydrograph was reduced
by 19% and MRAE for intermediate flow was reduced by 24%. However, Ellagawa
semi-distributed model showed a poor estimation of annual, seasonal and monthly
streamflow compared to Ellagawa lumped model. Hence, the semi-distributed model
with single gauging cannot be considered as a better and meaningful modelling option in
SWMM with certainty. This study recommends more application of SWMM for
continuous modelling of streamflow in monsoon regions and more research on
automatic optimization, objective function and groundwater.