Abstract
Water loss is a crucial factor for vegetation in the semi-arid Sahel region of Africa. Global satellite-driven estimates of plant CO2 uptake (gross primary productivity, GPP) have been found to not accurately account for Sahelian conditions, particularly the impact of canopy water stress. Here, we identify the main biophysical limitations that induce canopy water stress in Sahelian vegetation and evaluate the relationships between field data and Earth observation-derived spectral products for up-scaling GPP. We find that plant-available water and vapor pressure deficit together control the GPP of Sahelian vegetation through their impact on the greening and browning phases. Our results show that a multiple linear regression (MLR) GPP model that combines the enhanced vegetation index, land surface temperature, and the short-wave infrared reflectance (Band 7, 2105–2155 nm) of the moderate-resolution imaging spectroradiometer satellite sensor was able to explain between 88% and 96% of the variability of eddy covariance flux tower GPP at three Sahelian sites (overall = 89%). The MLR GPP model presented here is potentially scalable at a relatively high spatial and temporal resolution. Given the scarcity of field data on CO2 fluxes in the Sahel, this scalability is important due to the low number of flux towers in the region.
Highlights
The Sahel is an arid and semi-arid region that stretches from the Atlantic Ocean in the west to the Red Sea in the east and separates the hyper-arid Sahara desert from the sub-humid and humid regions to the south (Figure 1)
Vapor pressure deficit (VPD) was calculated from field-measured air temperature (Ta, ◦ C) and relative humidity (RH, %) following the approach described in Ward et al [56]: es = 0.611 × exp 17.27 ×
Sjöström et al [21] and Ma et al [63] found that enhanced vegetation index (EVI) follows the seasonal dynamics of eddy covariance (EC) gross primary productivity (GPP) better than the moderate-resolution imaging spectroradiometer (MODIS)-derived GPP in Sahelian and Australian xeric savannas, respectively
Summary
The Sahel is an arid and semi-arid region that stretches from the Atlantic Ocean in the west to the Red Sea in the east and separates the hyper-arid Sahara desert from the sub-humid and humid regions to the south (Figure 1). The greening commences (Figure 2), and soil moisture increases humidity, which lowers the difference between the vapor pressure deficit isRainfall replenished, which impacts the amount of energy partitioned into evapotranspiration inside the leaf and that of the air. This difference in vapor pressure deficit was found to be a key Rainfall increases humidity, which lowers the difference between the vapor pressure factor deficit affecting of Sahelian plants [25,26].
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