Evaporation from Savanna and Agriculture in Semi-Arid West Africa

Other literature type English OPEN
Ceperley, Natalie C. ; Mande, Theophile ; Giesen, Nick ; Tyler, Scott ; Yacouba, Hamma ; Parlange, Marc B. (2017)

Rain-fed farming is the primary livelihood of semi-arid West Africa. Changes in land cover have the potential to affect precipitation, the critical resource for production. Turbulent flux measurements from two eddy-covariance towers and additional observations from a dense network of small, wireless meteorological stations combine to relate land cover (savanna forest and agriculture) to evaporation in a small (3.5&thinsp;km<sup>2</sup>) catchment in Burkina Faso, West Africa. We observe larger sensible and latent heat fluxes over the savanna-forest in the headwater area relative to the agricultural section of the watershed. Fluxes above the savanna-forest are higher because of the greater number of exposed rocks and trees and the higher productivity of the forest compared to rainfed, hand-farmed agricultural fields. We deduce that there is a higher soil heat flux in the fields. Vegetation cover and soil moisture are found to be the primary controls of the evaporative fraction, defined as the latent heat over the available energy. Satellite derived vegetation index (NDVI) and soil moisture are determined to be good predictors of evaporative fraction. Our measurements provide an estimator that can be used to estimate evaporative fraction when only NDVI is available. Such large-scale estimates of evaporative fraction from remotely sensed data are valuable where ground-based measurements are lacking, which is the case across the African continent and many other semi-arid areas. Evaporative fraction estimates can be combined, for example, with sensible heat from measurements of temperature variance, to provide an estimate of evaporation when only minimal meteorological measurements are available in remote regions of the world. These findings reinforce local cultural beliefs of the importance of forest fragments for climate regulation and may provide support to local decision makers and rural farmers in the maintenance of the forest areas.
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