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Spatialization of energy and water fluxes : combination of surface-atmosphere exchange modelling and optical, thermal and microwave remote sensing

Abstract : A precise estimate of evapotranspiration (ET) at the landscape scale remains a priority to understand land-atmosphere-interactions, especially over semi-arid lands. Regarding data availability over large areas and at multiple scales, remote sensing observations provide very relevant information to feed ET models. Commonly, there are three main variables, derived from remote sensing, that can be used to determine the spatial distribution of ET: the surface (0-5 cm) soil moisture (SM) derived from microwave data, the land surface temperature (LST) derived from thermal infrared radiances and vegetation indices (or fractional vegetation cover fc) derived from visible/near infrared reflectances. However, very few studies have attempted to combine all three variables within a single ET model. In this context, the main objective of this thesis is to improve the estimation of ET by combining multi-resolution optical/microwave remote sensing and surface-atmosphere exchange modelling. In the first part, the thermal-based two-source energy balance (TSEB) model based on LST, fc and the Priestley Taylor (PT) coefficient (αPT) relating ET to the net radiation is tested over an heterogeneous watershed in Niamey, Niger (Wankama catchment). The model predictions of area-averaged latent (LE) and sensible (H) heat fluxes are compared to data acquired by a Large Aperture Scintillometer (LAS) set up over a transect about 3.2 km-long and spanning three vegetation types (millet, fallow and degraded shrubs). The results obtained for H and LE are relevant. However, an overestimation of simulated fluxes is recorded at the end of the season. This is mainly due to the fixed maximum value for αPT (generally set to 1.26). In the second part, a new model named TSEB-SM derived from the TSEB formalism is developed by using, in addition to LST and fc data, the near-surface SM as an extra constraint on soil evaporation. An innovative calibration procedure is proposed to retrieve three key parameters: the Priestley Taylor coefficient (αPT) and the parameters (arss and, brss) of a soil resistance formulation. In practice, arss and brss are retrieved at the seasonal time scale from SM and LST data with fc lower than a given threshold fc,thres(fc,thres is set to 0.5), while αPT is retrieved at the daily time scale from SM and LST data for fc> fc,thres. TSEB-SM model is tested over 1 flood- and 2 drip-irrigated wheat fields using in situ data collected during two field experiments in 2002-2003 and 2016-2017 in the Tensift watershed, central Morocco. The coupling of the soil resistance formulation with the TSEB formalism improves the estimation of soil evaporation, and consequently, improves the partitioning of ET. Analysis of the retrieved time series indicates that the daily αPT mainly follows the phenology of winter wheat crop with a maximum value coincident with the full development of green biomass and a minimum value reached at harvest. Finally, TSEB-SM is applied in real-life using 1 km resolution MODIS LST and fc data and the 1 km resolution SM data disaggregated from SMOS (Soil Moisture and Ocean Salinity) observations by using a disaggregation algorithm (DisPATCh). The approach is validated during a four-year period (2014-2018) over a rainfed wheat field in the Tensift basin, central Morocco. The field was seeded for the 2014-2015 (S1), 2016-2017 (S2) and 2017-2018 (S3) agricultural season, while it remained under bare soil conditions during the 2015-2016 (B1) wheat seasons. The constraint applied on the soil evaporation by using the SM derived from SMOS data is one of the main controlling factors of the evaporative fraction, which helps determine with more accuracy the LE/H partitioning. Moreover, the retrieved αPT increases after rainfall events, suggesting a relationship with the soil water availability in the root zone.
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Bouchra Ait Hssaine. Spatialization of energy and water fluxes : combination of surface-atmosphere exchange modelling and optical, thermal and microwave remote sensing. Hydrology. Université Paul Sabatier - Toulouse III, 2019. English. ⟨NNT : 2019TOU30093⟩. ⟨tel-02956251⟩

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