Improving operational land surface model canopy evapotranspiration in Africa using a direct remote sensing approach

Abstract

Abstract. Climate change is expected to have the greatest impact on the world's economically poor. In the Sahel, a climatically sensitive region where rain-fed agriculture is the primary livelihood, expected decreases in water supply will increase food insecurity. Studies on climate change and the intensification of the water cycle in sub-Saharan Africa are few. This is due in part to poor calibration of modeled evapotranspiration (ET), a key input in continental-scale hydrologic models. In this study, a remote sensing model of transpiration (the primary component of ET), driven by a time series of vegetation indices, was used to substitute transpiration from the Global Land Data Assimilation System realization of the National Centers for Environmental Prediction, Oregon State University, Air Force, and Hydrology Research Laboratory at National Weather Service Land Surface Model (GNOAH) to improve total ET model estimates for monitoring purposes in sub-Saharan Africa. The performance of the hybrid model was compared against GNOAH ET and the remote sensing method using eight eddy flux towers representing major biomes of sub-Saharan Africa. The greatest improvements in model performance were at humid sites with dense vegetation, while performance at semi-arid sites was poor, but better than the models before hybridization. The reduction in errors using the hybrid model can be attributed to the integration of a simple canopy scheme that depends primarily on low bias surface climate reanalysis data and is driven primarily by a time series of vegetation indices.

Building a field- and model-based climatology of local water and energy cycles in the cultivated Sahel – annual budgets and seasonality

“…Energy balance closure ob- . Extracts from these eddy covariance data have been extensively analyzed in various -local, regional, methodological -studies (e.g., Boulain et al, 2009a;Merbold et al, 2009;Ramier et al, 2009;Tanguy et al, 2012;Verhoef et al, 2012;Marshall et al, 2013;Sjöstrom et al, 2011Sjöstrom et al, , 2013Lohou et al, 2014).…”

“…In this context, remote sensing estimations are particularly promising for this region. However, methods are still in development, and require context-specific field evaluation and calibration (e.g., Tanguy et al, 2012;Verhoef et al, 2012;Marshall et al, 2013). This is also true for model-derived estimates, as the ability of the current generation of land surface models (LSMs) to correctly reproduce dominant land processes in Africa is still largely in question (Boone et al, 2009a).…”

Characteristics of Dew Formation and Distribution, and Its Contribution to the Surface Water Budget in a Semi-arid Region in China

“…Non-rainfall processes such as dewfall have received little attention in the region. These non-rainfall processes are inadequate for use in numerical of the atmosphere models (Marshall et al 2013) and in the estimates of the land-surface water balance. In such model simulations, soils dry too rapidly, indicating unrealistic rates of evapotranspiration (Gao et al 2004).…”

Evaluation of geostationary satellite (COMS) based Priestley–Taylor evapotranspiration

“…The large discrepancy of these overestimation could be due to several reasons. Firstly, Marshall et al (2013) who found that the GLDAS R N , which is used exclusively in the GLDAS ET PT estimation, showed a large RMSE with an average of 88.30 W m −2 (42.99-118.46 W m −2 ) at five micrometeorological stations on subSaharan Africa. In addition, they reported that the energy balance closure problems are typically relevant with error in measuring turbulent fluxes, even though the observation errors in radiations measuring from the flux tower also remains as a possibility.…”