Abstract:
Hydrological prediction, when data is available, is relatively easily
achieved albeit subject to significant uncertainties. However, the problem of ungauged
basins presents considerable difficulties. Over recent decades, there has
been a continual decline in hydrological gauging networks, reducing the accuracy
of hydrological predictions whilst increasing the uncertainty associated with
predictions and management of water quantity and quality. Most of the developed
countries usually use a number of models to estimate hydrological parameters. Soil
Water Assessment Tool (SWAT) is a commonly used hydrological model to
estimate surface runoff in the sub basin level and SWAT model interface with GIS.
Kalaoya river basin is situated in the Western part of Sri Lanka covering four
administrative districts. It falls about 76% within dry zone and about 24% in the
intermediate zone. It is approximately between latitude 7° 69' N to 8° 42'N and
longitude 79° 83' E to 80° 77' E. Basin area is about 2870 sq km. The objective of
this study was to apply SWAT to estimate the flow partem at the major outlet in
the Kalaoya sub catchments.
Preparation of basic data in this model needed land use map, digital
elevation model (DEM), soil map, stream network and meteorological data. The
land use map and the digital elevation model covering the study area were derived
with the help of Geographical Information System and digital elevation model was
created by using SRTM data. The physical soil properties were found in the
literature. According to the availability of stream flow data, finally the model was
calibrated using monthly stream flow from 1985-1991 period to Dambulu Oya sub
catchments. Manual calibration was done by changing carve number (CN) and
groundwater dearly (GW delay) in the model. Carve number changed up to 75 and
GW delay (days) changed up to 10. According to this change, final manual
calibration shows good result. Arrangement between observed and simulated
stream flow data under the above changes, model was validated by comparing the
observed and simulated stream flow data at the basin out let for the period of 1992-
1998 and showed a good arrangement. Therefore, SWAT model can be used to
predict surface runoff in Dambulu oya sub catchments in the future.
