Building a field- and model-based climatology of surface energy and water cycles for dominant land cover types in the cultivated Sahel. Annual budgets and seasonality
Construction d'une climatologie fondés sur l'observation et la modélisation des bilans hydriques et énergétiques de deux types de cultures dominantes dans le Sahel cultivé. Bilans annuels et saisonnalité
Résumé
In the sub-Saharan Sahel, energy and water cycling
at the land surface is pivotal for the regional climate,
water resources and land productivity, yet it is still very
poorly documented. As a step towards a comprehensive climatological description of surface fluxes in this area, this
study provides estimates of long-term average annual budgets
and seasonal cycles for two main land use types of the
cultivated Sahelian belt: rainfed millet crop and fallow bush.
These estimates build on the combination of a 7-year field
data set from two typical plots in southwestern Niger with
detailed physically based soil-plant-atmosphere modeling,
yielding a continuous, comprehensive set of water and energy
flux and storage variables over this multiyear period. In
the present case in particular, blending field data with mechanistic modeling makes the best use of available data and
knowledge for the construction of the multivariate time series.
Rather than using the model only to gap-fill observations
into a composite series, model-data integration is generalized
homogeneously over time by generating the whole
series with the entire data-constrained model simulation. Climatological averages of all water and energy variables, withassociated sampling uncertainty, are derived at annual to subseasonal scales from the time series produced. Similarities
and differences in the two ecosystem behaviors are highlighted.
Mean annual evapotranspiration is found to represent
82-85% of rainfall for both systems, but with different
soil evaporation/plant transpiration partitioning and different
seasonal distribution. The remainder consists entirely
of runoff for the fallow, whereas drainage and runoff stand
in a 40-60% proportion for the millet field. These results
should provide a robust reference for the surface energy- and
water-related studies needed in this region. Their significance
and the benefits they gain from the innovative data-model
integration approach are thoroughly discussed. The model
developed in this context has the potential for reliable simulations outside the reported conditions, including changing
climate and land cover.
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