Abstract:
Actual Evapotranspiration (AET) is a major component of the river basin hydrological cycle
over land surface and energy balances. More than 60% of input water on land is returned to the
atmosphere through evapotranspiration and it greatly influences the water availability on the land
surface. Estimation of AET is an essential part in various fields. Thus, this study mainly aims at
assessing the impact of urbanization-associated land use changes on actual evapotranspiration and
water balance in the Kelani River Basin, Sri Lanka. Thronthwaite equation and land use conditions are
mainly considered in this study to estimate AET. Average monthly temperature data has been
obtained from NASA MOD1C3 and data has been validated using observed temperature data
processed by the Meteorological Department in Sri Lanka. Potential Evapotranspiration (PET) was
calculated using monthly average temperature and fractional vegetation cover was calculated using
Landsat images (TM, ETM and OLI) to identify the land use and land cover changes from year 2000 to
2018. Both potential evapotranspiration and fractional vegetation cover are used to estimate AET.
TRRM data was used to get Digital Elevation Model (DEM) and Landsat images were used to
calculate Normalized Differential Vegetation Index (NDVI), Normalized Differential Building Index
(NDBI). All the analysis used in this study have been carried out using raster calculator, zonal statistics
and pivot tools in ArcGIS 10.1 software. The study revealed that land use and land cover is a major
fact to determine AET. Upper catchment of Kelani River has obtained high AET values due to the
vegetation cover and the elevation. Lower part of the catchment is associated with the low values due
to build - up areas. AET has decreased by 2018 and urbanization is the main reason for it. Rainy
seasons reduce the AET since high humidity and reducing AET can be highlighted when considering
the temporal changes of AET in Kelani river basin. Evapotranspiration as a major component of water
cycle should be considered because it can be a significant fact to reduce precipitation.
