Integration of soil moisture in SEBS for improving evapotranspiration estimation under water stress conditions

Abstract

In this paper we integrate the information about water stress into SEBS, one of the surface energy balance models that employ remote sensing (RS) data. The level of water stress is taken into account in the calculation of sensible heat, through a modified definition of kB−1, the parameter that summarizes the excess aerodynamic resistance to heat transfer compared to momentum transfer. Surface energy balance models are employed to obtain evapotranspiration as the remainder of available energy minus sensible heat flux (H). These models assume that information on the ratio of actual to potential evaporation is implicitly embedded in the land surface temperature. This assumption is usually adequate where available energy is the limiting factor for evapotranspiration (ET), but there is a problem when water availability becomes limiting for ET. In this case, the daily evapotranspiration is often overestimated, in particular for sparsely vegetated semi-arid regions, because of an underestimation of sensible heat flux for these areas. Our method remedies this shortcoming by progressively decreasing kB−1 with increasing levels of water stress. This decreases aerodynamic resistance, and hence increases H, leading to lower estimates of ET. The decrease of kB−1 with a rise in plant water stress is based on general plant physiological observations related to vertical canopy stomatal conductance profiles, which affects the exchange of sensible and latent heat between the canopy and the atmosphere.The new approach was tested by comparing SEBS H outputs with field observations from Bowen ratio stations distributed over the Konya basin in Turkey, and the results indicate a large improvement when the soil moisture is integrated explicitly in the calculation of sensible heat flux by SEBS. More importantly, the new approach provides a considerable operational improvement for regional ET mapping through integrating microwave soil moisture measurements into SEBS, as illustrated by our findings for this semi-arid region. The daily ET map generated by the new SEBS model captured better the low ET values observed in the drylands, and also reflected better the overall contrast between irrigated fields and the extremely dry surrounding areas. Improved mapping of regional ET by soil moisture integrated SEBS offers the opportunity to provide more accurate estimation of energy and water fluxes in regions where plant water stress is a recurrent feature.