Using satellite remote sensing to assess evapotranspiration: Case study of the upper Ewaso Ng’iro North Basin, Kenya

Actual evapotranspiration (ETa) is one of the most useful indicators to explain whether the water is being used as “intended”. ETa variations, both in space and time and for different land use types are seen to be highly indicative for the adequacy, reliability and equity in water use; the knowledge of these conditions is essential for judicious water resources management. Unfortunately, ETa estimation under actual field conditions is still a big challenge for both scientists and water managers. The complexity associated with the estimation of ETa has led to the development of various methodological approaches for estimating ETa over time.During the past two to three decades, significant progress has been made in estimating actual evapotranspiration using satellite remote sensing. These methods provide powerful means for computing ETa from a pixel scale to that of an entire basin. In this study, surface energy balance algorithm for land (SEBAL) was used to compute a complete radiation and energy balance along with the resistances for momentum, heat and water vapor transport for each pixel in the Upper Ewaso Ng’iro North River Basin, in Kenya. This was then applied to assess the spatio-temporal distribution of ETa in the basin. The mean annual ETa estimated from SEBAL for 2000, 2003 and 2006 were compared with the mean annual ETa calculated from water balance method for the same periods and a good correlation of about 70% was observed.It was further observed that ETa increased gradually from 2000 to 2006 with an annual rate of about 15%. The estimated daily, monthly and annually ETa distribution for the period of study were used to analyze water use patterns across the basin thus giving more insights into the underlying factors impacting on the water resources which could be used to facilitate the formulation of appropriate water resources management strategies for the basin.