Abstract
Terrestrial evapotranspiration (ET) is a central process in the climate system, is a major component in the terrestrial water budget, and is responsible for the distribution of water and energy on land surfaces especially in arid and semiarid areas. In order to inform water management decisions especially in scarce water environments, it is important to assess ET vegetation use by differentiating irrigated socio-economic areas and natural ecosystems. The global remote sensing ET product MOD16 has proven to underestimate ET in semiarid regions where ET is very sensitive to soil moisture. The objective of this research was to test whether a modified version of the remote sensing ET model PT-JPL, proven to perform well in drylands at Eddy Covariance flux sites using the land surface temperature as a proxy to the surface moisture status (PT-JPL-thermal), could be up-scaled at regional levels introducing also a new formulation for net radiation from various MODIS products. We applied three methods to track the spatial and temporal characteristics of ET in the World Heritage UNESCO Doñana region: (i) a locally calibrated hydrological model (WATEN), (ii) the PT-JPL-thermal, and (iii) the global remote sensing ET product MOD16. The PT-JPL-thermal showed strong agreement with the WATEN ET in-situ calibrated estimates (ρ = 0.78, ρ1month-lag = 0.94) even though the MOD16 product did not (ρ = 0.48). The PT-JPL-thermal approach has proven to be a robust remote sensing model for detecting ET at a regional level in Mediterranean environments and it requires only air temperature and incoming solar radiation from climatic databases apart from freely available satellite products.
Highlights
The global water cycle is changing due to the combined effects of climate change and human interventions during the 21st century [1]
The daily Priestley-Taylor Jet Propulsion Laboratory (PT-JPL)-thermal ET was aggregated into monthly estimates and compared against the in-situ calibrated hydrological model WATEN ET and MOD16 ET
We validated the ET estimates obtained from the modified PT-JPL model (PT-JPL-thermal) that introduces land surface temperature (LST) with a thermal inertia approach as a proxy to the soil moisture status against the on-site calibrated hydrological model WATEN and compared inter and intra-annual ET patterns with the globally available product MOD16 ET
Summary
The global water cycle is changing due to the combined effects of climate change and human interventions during the 21st century [1]. The Iberian Peninsula is predicted to be among the most affected areas by severe droughts by the end of the 21st century [8]. In this region, where irrigated agriculture represents over 80% of the total extracted water [9], land use shifts towards higher market-valued crops represent a major driver of change, which will markedly increase water withdrawals [10]. In the Guadalquivir basin in Spain, irrigation water requirements are expected to increase between 15% and 20% by 2050 [11] This may cause a redistribution of water between the surface and groundwater [12]. Monitoring the variations in the hydrological cycle components more closely, among them evapotranspiration (ET), is of major importance in countries facing intensified drought spills [13,14]
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