Evaporation from cultivated and semi-wild Sudanian Savanna in west Africa

Natalie C Ceperley,Marc B Parlange,Scott Tyler,Theophile Mande,Hamma Yacouba,Nick Van De Giesen

doi:10.5194/hess-21-4149-2017

Abstract

Abstract. 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 km2) 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 all year. Higher fluxes above the savanna forest are attributed to the greater number of exposed rocks and trees and the higher productivity of the forest compared to rain-fed, hand-farmed agricultural fields. Vegetation cover and soil moisture are found to be primary controls of the evaporative fraction. Satellite-derived vegetation index (NDVI) and soil moisture are determined to be good predictors of evaporative fraction, as indicators of the physical basis of evaporation. Our measurements provide an estimator that can be used to derive 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.

Highlights

The Sudanian savanna in southeastern Burkina Faso is a patchwork of savanna, forest, and scrubland with some patches more representative of the drier Sahel and others more representative of the more humid Guinean forests
Ceperley et al.: Evaporation from cultivated and semi-wild Sudanian Savanna agriculture is the dominant livelihood in large parts of west Africa, despite its high level of dependence on seasonally controlled hydrology
Separation between the lower and upper parts of the catchment is apparent in the Normalized difference vegetation index (NDVI) time series, where the savanna forest consistently stays more green, with the field only surpassing it due to its delayed senescence in September 2010

Summary

Introduction

The Sudanian savanna in southeastern Burkina Faso is a patchwork of savanna, forest, and scrubland with some patches more representative of the drier Sahel and others more representative of the more humid Guinean forests. Vegetation is mainly deciduous according to seasonal moisture availability, but spatial variations in topography, water availability, and plant communities result in some variation in greenness and some evergreen species, for example, near the springs. People in this region rely on a mix of hunting and gathering, small-scale agriculture, and pastoralism. As land claims and regulations have changed, communities have been forced to rely more on agricultural production as a primary source of food and income, resulting in land conversion for agriculture. Ceperley et al.: Evaporation from cultivated and semi-wild Sudanian Savanna agriculture is the dominant livelihood in large parts of west Africa, despite its high level of dependence on seasonally controlled hydrology. Transformation of forest land to agriculture has been shown by model simulations to alter global circulation, hydrology, and biogeochemistry both in the present and in predictions of the future (Abiodun et al, 2008; Feddema et al, 2005; Mande et al, 2011; Steiner et al, 2009; Sylla et al, 2015; Vitousek, 1997)

Results

Discussion

Conclusion