Comparison of different evaporation estimates over the African continent

Abstract

Abstract. Evaporation is a key process in the water cycle with implications ranging, inter alia, from water management to weather forecast and climate change assessments. The estimation of continental evaporation fluxes is complex and typically relies on continental-scale hydrological models or land-surface models. However, it appears that most global or continental-scale hydrological models underestimate evaporative fluxes in some regions of Africa, and as a result overestimate stream flow. Other studies suggest that landsurface models may overestimate evaporative fluxes. In this study, we computed actual evaporation for the African continent using a continental version of the global hydrological model PCR-GLOBWB, which is based on a water balance approach. Results are compared with other independently computed evaporation products: the evaporation results from the ECMWF reanalysis ERA-Interim and ERA-Land (both based on the energy balance approach), the MOD16 evaporation product, and the GLEAM product. Three other alternative versions of the PCR-GLOBWB hydrological model were also considered. This resulted in eight products of actual evaporation, which were compared in distinct regions of the African continent spanning different climatic regimes. Annual totals, spatial patterns and seasonality were studied and compared through visual inspection and statistical methods. The comparison shows that the representation of irrigation areas has an insignificant contribution to the actual evaporation at a continental scale with a 0.5 • spatial resolution when averaged over the defined regions. The choice of meteorological forcing data has a larger effect on the evaporation results, especially in the case of the precipitation input as different precipitation input resulted in significantly different evaporation in some of the studied regions. ERA-Interim evaporation is generally the highest of the selected products followed by ERA-Land evaporation. In some regions, the satellite-based products (GLEAM and MOD16) show a different seasonal behaviour compared to the other products. The results from this study contribute to a better understanding of the suitability and the differences between products in each climatic region. Through an improved understanding of the causes of differences between these products and their uncertainty, this study provides information to improve the quality of evaporation products for the African continent and, consequently, leads to improved water resources assessments at regional scale.

Impact of model structure on flow simulation and hydrological realism: from a lumped to a semi-distributed approach

“…Classical degreeday formulation for snow melt. 11 free parameters (see Valéry et al, 2014b Trambauer et al, 2014;Hu et al, 2015;Miralles et al, 2016). Two types of snow data are used for model validation: fractional snow cover and snow water equivalent.…”

Using satellite-based evapotranspiration estimates to improve the structure of a simple conceptual rainfall–runoff model

“…While previous studies have used SET estimates to constrain the parameters of hydrologic models (Winsemius et al, 2008;van Emmerik et al, 2015), the recent interest in diagnostic improvements to model structure Gupta and Nearing, 2014) suggests that it would be potentially more valuable to use the ET data to actually improve the model structure when possible. This study attempts to explore this possibility in the context of using satellite-based data to drive a streamflow simulation model for a poorly gauged basin in Africa.…”

“…The quality of a model-derived estimate of ET depends on various sources of uncertainty (inputs, parameters, process representation, structure, etc.) inherent to the model-based scheme used, and common problems include both over-and underestimation of evaporative fluxes (Trambauer et al, 2014). Recently, methods that use satellite-based remotely sensed climatic and environmental observations have provided an alternative approach to the estimation of ET (e.g., Bastiaanssen et al, 1998;Arboleda et al, 2005).…”

“…Several studies have advocated and/or implemented the idea of using physically consistent estimates for the parameters of hydrologic models (Pokhrel et al, 2008(Pokhrel et al, , 2012Savenije, 2010;Schaefli et al, 2011;Kumar et al, 2013;Troch et al, 2015, and references therein). However, in catchment-scale modeling, it is a common practice to use parameter estimates that are calibrated by adjusting the simulated streamflows to try to match the observed data.…”

“…Such information can take various forms and be incorporated in different ways. Evapotranspiration (ET) can be used to constrain model parameters that are sensitive to the ET process (Winsemius et al, 2008;van Emmerik et al, 2015). Alternatively, ET can be used as a calibration target along with streamflow within a multi-objective setting (Zhang et al, 2009).…”