Abstract
A new approach is proposed to derive evapotranspiration (E) and irrigation requirements by implementing the combination equation models of Penman–Monteith and Shuttleworth and Wallace with surface parameters and resistances derived from Sentinel-2 data. Surface parameters are derived from Sentinel-2 and used as an input in these models; namely: the hemispherical shortwave albedo, leaf area index and water status of the soil and canopy ensemble evaluated by using a shortwave infrared-based index. The proposed approach has been validated with data acquired during the GRAPEX (Grape Remote-sensing Atmospheric Profile and Evapotranspiration eXperiment) in California irrigated vineyards. The E products obtained with the combination equation models are evaluated by using eddy covariance flux tower measurements and are additionally compared with surface energy balance models with Landsat-7 and -8 thermal infrared data. The Shuttleworth and Wallace (S-W S-2) model provides an accuracy comparable to thermal-based methods when using local meteorological data, with daily E errors < 1 mm/day, which increased from 1 to 1.5 mm/day using meteorological forcing data from atmospheric models. The advantage of using the S-W S-2 modeling approach for monitoring ET is the high temporal revisit time of the Sentinel-2 satellites and the finer pixel resolution. These results suggest that, by integrating the thermal-based data fusion approach with the S-W S-2 modeling scheme, there is the potential to increase the frequency and reliability of satellite-based daily evapotranspiration products.
Highlights
Evapotranspiration is the key variable for determining crop water requirements and is needed for the optimal allocation of water resources for irrigation
In order to determine the substrate and canopy resistances rs s and rs c by means of the approach described in the Sections 2.1 and 2.2, the NDVI-shortwave infrared transformed reflectance” (STR) domain has been plotted by considering the pixels included in the S-2 image subset covering a square area of approximately 10 × 10 km2 (~106 pixels) centered on the Ripperdan site
This does not represent a major drawback of the methodology, since the shape of the pixel distribution on the NDVI-STR space is identical for a given location, at any time, regardless of the surface and meteorological conditions
Summary
Evapotranspiration is the key variable for determining crop water requirements and is needed for the optimal allocation of water resources for irrigation. (c) reflectance and vegetation index (VI) based-methods: crop coefficient and canopy parameters, such as hemispherical albedo and leaf area index LAI, are obtained by means of different analytics from reflectance or vegetation indices These parameters are the basic inputs for the application of the widely used FAO-56 approach [3] for determining crop evapotranspiration, either by the direct calculation of the combination equation of Penman–Monteith or by using the crop coefficient and reference evapotranspiration. Thermal-based models have been intensively applied by using observation data from Landsat [13], which is, at the present moment, the only operational platform with medium resolution acquisitions in the thermal infrared (100 m), which are resampled to 30 m with a revisit time of 8 to 16 days depending on the site; EO-driven soil water balance models: these are simulation models of water balance using EO-based input data related to crop development [14,15]. These models produce a continuous spatially distributed output, the quality of which strongly depends on the availability of reliable soil physical and hydraulic properties, as well as precipitation/irrigation inputs [16]
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