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

Abstract

ABSTRACT:Actual Evapotranspiration (ET a ) is one of the most useful indicators to explain whether the water is being used as "intended". ET a 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, ET a estimation under actual field conditions is still a big challenge to both scientists and water managers. The complexity associated with the estimation of ET a 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 ET a 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 ET a 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 ET a for the same periods and a significant correlation was established. The ET a 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.

Evapotranspiration Mapping in a Heterogeneous Landscape Using Remote Sensing and Global Weather Datasets: Application to the Mara Basin, East Africa

“…Markedly, we observe weak linear association of NDVI and ET at 16-day and monthly temporal resolution in humid highland regions and semi-arid areas with continuous moisture supply such as wetlands. This result is in agreement with Mutiga et al [64] who noted a weak correlation between daily NDVI and SEBAL ET for elevations beyond 2100 m.a.s.l. in the upper Ewaso Ng'iro North Basin, Kenya.…”

Remote sensing techniques for predicting evapotranspiration from mixed vegetated surfaces

“…Despite the advantages of using RS techniques to measure ET, several disadvantages have also been reported (Rana and Katerji 2000, Courault et al 2005, Allen et al 2011b. These include the time period between satellite captures Wood 2006, Jiang et al 2009), the high costs associated with obtaining high resolution images particularly airborne images (Stisen et al 2008, Mutiga et al 2010, the uncertainty in estimating aerodynamic components (Min and Lin 2006), errors in spatial datasets (Chen et al 2005, Wu et al 2010 and some errors in measuring narrow vegetation areas such as riparian zones (Nagler et al 2005a(Nagler et al , 2005b(Nagler et al , 2009a(Nagler et al , 2009b. It should also be noted that temporal differences between satellite/airborne images can result not only from spectral changes but also from daily, monthly and yearly changes in solar position which directly affect vegetation density (Weng et al 2004).…”

Water Allocation as a Planning Tool to Minimise Water Use Conflicts in the Upper Ewaso Ng’iro North Basin, Kenya

“…Other sources of water in the basin include groundwater (boreholes and shallow wells) and flood water usually stored in small dug water pans. Due to the arid nature of most parts of the basin, the atmospheric demand for water is very high with daily potential evaporation varying from 3 to 8 mm/day (Mutiga et al 2010). Gichuki et al (1998) observed that only 3% of the basin (mountain moorlands) receives a water surplus while 50% experiences water deficits of more than 1,000 mm/year.…”

“…Gichuki et al (1998) observed that only 3% of the basin (mountain moorlands) receives a water surplus while 50% experiences water deficits of more than 1,000 mm/year. This is because potential evapotranspiration in the basin increases with decreasing altitude while precipitation decreases with decreasing altitude (Wiesmann et al 2000;Mutiga et al 2010) implying that water deficits increase with decreasing altitude away from the mountain. Moreover, if the amount of water abstracted for various uses is considered, the situation becomes worse thus water use conflicts become more critical downstream of the basin.…”