Abstract
Quantitative estimates of forage availability at the end of the growing season in rangelands are helpful for pastoral livestock managers and for local, national and regional stakeholders in natural resource management. For this reason, remote sensing data such as the Fraction of Absorbed Photosynthetically Active Radiation (FAPAR) have been widely used to assess Sahelian plant productivity for about 40 years. This study combines traditional FAPAR-based assessments with agrometeorological variables computed by the geospatial water balance program, GeoWRSI, using rainfall and potential evapotranspiration satellite gridded data to estimate the annual herbaceous yield in the semi-arid areas of Senegal. It showed that a machine-learning model combining FAPAR seasonal metrics with various agrometeorological data provided better estimations of the in situ annual herbaceous yield (R2 = 0.69; RMSE = 483 kg·DM/ha) than models based exclusively on FAPAR metrics (R2 = 0.63; RMSE = 550 kg·DM/ha) or agrometeorological variables (R2 = 0.55; RMSE = 585 kg·DM/ha). All the models provided reasonable outputs and showed a decrease in the mean annual yield with increasing latitude, together with an increase in relative inter-annual variation. In particular, the additional use of agrometeorological information mitigated the saturation effects that characterize the plant indices of areas with high plant productivity. In addition, the date of the onset of the growing season derived from smoothed FAPAR seasonal dynamics showed no significant relationship (0.05 p-level) with the annual herbaceous yield across the whole studied area. The date of the onset of rainfall however, was significantly related to the herbaceous yield and its inclusion in fodder biomass models could constitute a significant improvement in forecasting risks of a mass herbaceous deficit at an early stage of the year.
Highlights
In the Sahel belt south of the Sahara desert, an arid to semi-arid region, the vegetation is composed of a herbaceous layer dominated by annual grasses and scattered woody plants including bushes, shrubs and small trees, among which are several thorny species [1,2]
Three models were developed for estimating herbaceous yields using: (a) only Fraction of Absorbed Photosynthetically Active Radiation (FAPAR) metrics (VI-model); (b) agrometeorological data (AGRO-model); and (c) both FAPAR metrics and agrometeorological data (VIAGRO-model)
AGRO and model had the weakest performance, which could be related thethree fact that all applied model had the weakest performance, which could be related to the fact that all applied variables variables are independent of internal factors whereas all FAPAR are independent internal (i.e.,are theinfluenced influence by of grazing) whereas metrics as well metrics asofwell as thefactors field data grazing over time. all the models showed a as the field datadistribution are influenced grazing over time
Summary
In the Sahel belt south of the Sahara desert, an arid to semi-arid region, the vegetation is composed of a herbaceous layer dominated by annual grasses and scattered woody plants including bushes, shrubs and small trees, among which are several thorny species [1,2]. These areas provide the bulk of pastoral livestock feeding [3] and contribute to carbon sequestration, nutrient uptake and cycling, soil fixation and soil biologic activity, as well as water cycle regulation. In the past two decades and with regard to the technological advances in sensor design for vegetation monitoring [7], new satellites such as the Satellite Pour l’Observation de la Terre-VEGETATION (SPOT-VGT) and the Moderate Resolution Imaging Spectroradiometer (MODIS)-TERRA/AQUA have been launched and more datasets have become available with higher spatial and denser temporal resolutions [8,9]
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