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 from different land use types are seen to be highly indicative for the adequacy, reliability and equity in water use; the knowledge of these terms is essential for judicious water resources management. Unfortunately, ETa estimation under actual field conditions is still a big challenge to both scientists and water managers. The complexity associated with the estimation of ETa has lead to the development of various methodological approaches for estimating it over time. During the last two to three decades, significant progress has been made to estimate actual evapotranspiration using satellite remote sensing These methods provide a powerful means to compute ETa from pixel to the entire basin scale. 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 vapour transport for each pixel and then applied to assess the ETa at the basin scale and subsequently carryout a simple water balance for the basin. This was done for the Upper Ewaso Ng’iro North River Basin, in Kenya. Landcover/ landuse for 2000, 2003 and 2006 were assessed and compared with ETa for the same periods and a significant correlation was established. The ETa values obtained for the three time periods was used to analyse water use patterns across the basin thus giving insights on the underlying factors impacting on the water resources of the basin and hence facilitating the formulation of appropriate water resources management strategies for the basin. *Corresponding Author Email: [email protected] 1. . INTRODUCTION Since water resources management strategies are usually formulated on basin scale, understanding of the hydrological processes at this scale is a pre-requisite for implementation of these strategies. Increasing pressure on water resources requires a sound knowledge of where, when and how much water is used. Evapotranspiration (ET) is an important factor for evaluating water productivity and monitoring of irrigation performance. Evapotranspiration is a process governed by the energy and heat exchanges at the land surface, with the upper bound being constrained and controlled by the amount of available energy and water respectively (Hemukamara et al., 2003). However, direct measurement of actual evapotranspiration is difficult and most cases only provides point values. Quantitative analysis of various components of the hydrological cycle within a river basin is essential for systematic planning and management of the water resources. Although ET is one of the most important components of the hydrological cycle, it is one of the most difficult to measure especially under composite terrains (Wu et al, 2006). Traditionally, total evaporation (ET) from agricultural fields has been estimated by multiplying the potential evapotranspiration (ETo) by crop coefficients (Kc) determined according to the crop type and the crop growth stage as highlighted in Allen et al., 1998. Actual Evapotranspiration (ETa) is one of the most useful indicators to explain whether the water is being used as “intended” or not. ETa variations, both in space and time and from different land use classes (particularly agricultural lands) are thought to be highly indicative for the adequacy, reliability and equity in water use; the knowledge of these terms is essential for judicious water resources management. Unfortunately, ETa estimation under actual field conditions is still a very challenging task for scientists and water managers. The complexity associated with the estimation of ET has lead to the development of various methods for estimating this parameter over time Allen et al., 1998. Using recent developments in the field of remote sensing application in water management, this paper shows that remote sensing tools can help in improving water management by providing information on the existing patterns of water use; identifying the weaknesses in the approach to water management; and assisting in identifying the potential areas where there are opportunities for improving water use efficiency. In a semi-arid environment, ET from the land surface plays a critical role not only in water circulation within that surface, but also in many associated physical and ecological variables, such as atmospheric dynamics, subsurface water storage, and vegetation growth. Spatial variability in landuse, landcover, soil properties and water flows, most hydro-meteorological parameters exhibit an evident spatial variation, which cannot be obtained from a limited number of synoptic observations. This problem however, can be overcome by applying a physically based multi-step SEBAL formulated by Bastiaanssen, 1995. This study attempted to estimate the temporal change and spatial distribution of evapotranspiration and other components of the surface heat budget in the Ewaso Ng’iro North River Basin in Kenya during a rainy season from 2000 to 2006. Evapotranspiration data enhances the understanding underlying factors that impact on water use and subsequently taking