Abstract
We present and validate a global parametric model of potential evapotranspiration (PET) with two parameters that are estimated through calibration, using as explanatory variables temperature and extraterrestrial radiation. The model is tested over the globe, taking advantage of the Food and Agriculture Organization (FAO CLIMWAT) database that provides monthly averaged values of meteorological inputs at 4300 locations worldwide. A preliminary analysis of these data allows for explaining the major drivers of PET over the globe and across seasons. The model calibration against the given Penman-Monteith values was carried out through an automatic optimization procedure. For the evaluation of the model, we present global maps of optimized model parameters and associated performance metrics, and also contrast its performance against the well-known Hargreaves-Samani method. Also, we use interpolated values of the optimized parameters to validate the predictive capacity of our model against monthly meteorological time series, at several stations worldwide. The results are very encouraging, since even with the use of abstract climatic information for model calibration and the use of interpolated parameters as local predictors, the model generally ensures reliable PET estimations. Exceptions are mainly attributed to irregular interactions between temperature and extraterrestrial radiation, as well as because the associated processes are influenced by additional drivers, e.g., relative humidity and wind speed. However, the analysis of the residuals shows that the model is consistent in terms of parameters estimation and model validation. The parameter maps allow for the direct use of the model wherever in the world, providing PET estimates in case of missing data, that can be further improved even with a short term acquisition of meteorological data.
Highlights
Evaporation, which is an overall term covering all processes in which liquid water is transferred as water vapour to the atmosphere—definition already provided by ancient Greek philosophers [1]—is crucial element of multiple disciplines and an essential input of hydrological modelling, water resources management, irrigation planning, and climatological studies
We emphasize the concept of potential evapotranspiration, PET, which is a theoretical quantity considered as “the rate at which evapotranspiration would occur from a large area completely and uniformly covered with growing vegetation, which has access to an unlimited supply of soil water, and without advection or heating effects” [4]
Since PET depends on soil properties, a better defined term is the so-called reference crop evapotranspiration, introduced by Doorenbos and Pruitt [5], and typically denoted as ET0, which refers to the evapotranspiration from a standardized vegetated surface
Summary
Evaporation, which is an overall term covering all processes in which liquid water is transferred as water vapour to the atmosphere—definition already provided by ancient Greek philosophers [1]—is crucial element of multiple disciplines and an essential input of hydrological modelling, water resources management, irrigation planning, and climatological studies. Numerous efforts are reported in the literature, presenting different expressions of evaporation (including actual, potential, reference crop, and pan evaporation), based on different types of data. We emphasize the concept of potential evapotranspiration, PET, which is a theoretical quantity considered as “the rate at which evapotranspiration would occur from a large area completely and uniformly covered with growing vegetation, which has access to an unlimited supply of soil water, and without advection or heating effects” [4]. Since PET depends on soil properties, a better defined term is the so-called reference crop evapotranspiration, introduced by Doorenbos and Pruitt [5], and typically denoted as ET0 , which refers to the evapotranspiration from a standardized vegetated surface (i.e., actively growing and completely shading grass of 0.12 m height, surface resistance s · m−1 , and albedo = 0.23). The globally accepted method for consistent estimation of PET is the Penman-Monteith ( referred to as PM) equation, as formalized by the Food and Agriculture
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