A New Simple Method to Determine Crop Coefficients for Water Allocation Planning from Satellites: Results from Kenya

Crop coefficient Kc is defined as theratio of crop potential and grass referenceevapotranspiration. This paper discussesthe estimation of crop potentialevapotranspiration from remote sensing datato deduce spatial variable and regionalscale crop coefficients. Maps of cropcoefficients avoid the need to frequentlyuse remote sensing data because grassreference evapotranspiration obtained fromroutine weather data describe the shorterterm day-to-day variation. The new methodis based on the simplified Priestley &Taylor equation for cropevapotranspiration. It is argued that thisis justified for humid conditions and thecalculation procedure is validated withfield data collected from irrigatedvegetables in Kenya. The method isthereafter applied to Landsat-TM satellitedata covering the Lake Naivasha Basin,Kenya, and the spatially variable cropcoefficients are determined. This simpletechnique improves the planning ofirrigation water resources because Kcestimations based on satellite imagesreveal the real agricultural practicesbetter than expected acreage data andtabulated Kc-values published in theinternational literature.

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