Satellite measurements supplemented with meteorological data to operationally estimate evaporation in Sri Lanka

Ten day estimates of actual evaporation in Sri Lanka have been made for the period June 1999–2000, using AVHRR radiances from day-time ascending mode passes of the NOAA 14 meteorological satellite, supplemented with meteorological data measured simultaneously at ground level. During the 12-month period of the study, a total of 88 images have been processed using a residual energy budget and temporally constant evaporative fraction approach to yield 36 estimates of 10-day evaporation for the entire land area of Sri Lanka at 1 km2 resolution. The evaporation estimates, when compared with data from a large aperture scintillometer, showed errors of 17 and 1% in the case of 10-day and monthly evaporation respectively. The comparison with available independent evaporation estimates for specific catchments showed errors ranging from 2 to 8%. These estimates together with images of soil moisture status estimated using remote sensing techniques and rainfall from rain gauge network data were incorporated in a geographic information system (GIS). This provides a solid basis for understanding water consumed by agriculture and environment, in addition to providing a solid foundation for understanding the efficiency of major irrigation systems. The availability of data for the whole country and in a GIS environment with the facility for conducting studies at various spatial scales increases their usefulness for hydrological research in Sri Lanka.