Evaporative depletion assessments for irrigated watersheds in Sri Lanka
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Landsat Thematic Mapper (TM) measurements have been utilized to derive land-use patterns from a supervised multi-spectral classification. Independently, the water use of crops, perennial vegetation, surface water impoundments, and barren land has been assessed using a radiation and energy balance approach using the same Landsat images. The land-use classification had an overall accuracy of 85%, which is acceptable for a heterogeneous tropical watershed with abundant small-scale landscape features. Actual evaporation of the various land use types was verified on physical consistency by applying the inverse Penman-Monteith equation, taking the evaporation estimations from remote sensing as input. The surface resistance to evaporation was found to be similar to resistances experimentally obtained elsewhere, but in similar climatic conditions. A new calculation procedure for time integration of actual evaporation under conditions of very limited surface observations is outlined and applied to the 26,000-ha Kirindi Oya watershed in southern Sri Lanka. The annual watershed evaporation deviated 4% from evaporation derived as the residual water balance term. This new technique, based on Landsat measurements, helps in identifying the evaporative depletion of various land-use categories, which is an important input for water management evaluation procedures. The major finding is that a significant amount of irrigation water is used by the perennial vegetation in the vicinity of the irrigation systems.
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