Research Article: Expanding the geography of evapotranspiration: An improved method to quantify land-to-air water fluxes in tropical and subtropical regions

Date Published: June 28, 2017

Publisher: Public Library of Science

Author(s): Daniela Jerszurki, Jorge L. M. Souza, Lucas C. R. Silva, Ben Bond-Lamberty.


The development of new reference evapotranspiration (ETo) methods hold significant promise for improving our quantitative understanding of climatic impacts on water loss from the land to the atmosphere. To address the challenge of estimating ETo in tropical and subtropical regions where direct measurements are scarce we tested a new method based on geographical patterns of extraterrestrial radiation (Ra) and atmospheric water potential (Ψair). Our approach consisted of generating daily estimates of ETo across several climate zones in Brazil–as a model system–which we compared with standard EToPM (Penman-Monteith) estimates. In contrast with EToPM, the simplified method (EToMJS) relies solely on Ψair calculated from widely available air temperature (oC) and relative humidity (%) data, which combined with Ra data resulted in reliable estimates of equivalent evaporation (Ee) and ETo. We used regression analyses of Ψairvs EToPM and Eevs EToPM to calibrate the EToMJS(Ψair) and EToMJS estimates from 2004 to 2014 and between seasons and climatic zone. Finally, we evaluated the performance of the new method based on the coefficient of determination (R2) and correlation (R), index of agreement “d”, mean absolute error (MAE) and mean reason (MR). This evaluation confirmed the suitability of the EToMJS method for application in tropical and subtropical regions, where the climatic information needed for the standard EToPM calculation is absent.

Partial Text

The amount of water that flows through the soil-plant-atmosphere continuum is a key factor to be considered in ecosystem conservation and management efforts. Estimates of water fluxes from land-to-air are needed, for example, for the introduction of new crops, prediction of migration of plant species, and improvement of soil and irrigation management under climate change [1–3]. Assessing water fluxes in situ can be costly and time consuming and, depending on the method used, such assessments are subject to large uncertainties [4]. Baseline estimates of water fluxes are missing in many parts of the world, including the tropical and subtropical regions [5], owing to limited measurements of reference evapotranspiration (ETo).

The basic principle that surrounds the notion of atmospheric water potential as a driving force of evapotranspiration, regardless of plant cover and soil properties, is rooted in the first and second laws of thermodynamics [57, 61]. Briefly, the balance of heat, mechanical work (W), and variation of internal energy (ΔU) of a system are considered to be in equilibrium at time zero:
where: Q is heat added to the system; W is the mechanical work; and, ΔU is the change in internal energy U of the system.

As expected, our observations showed large variability of all climatic parameters (Tmax, Tmin, RH, Rs, u2 and VPD) across the different climate zones sampled throughout Brazil. The results described here span ETo trends in humid subtropical, tropical with dry summers, and semi-arid regions (Table 3). In general, VPD was the most seasonally variable parameter in humid climatic zones, reaching its lowest values during wet summers. Across sites, high Tmax and Tmin and low RH in semi-arid climate resulted in the highest VPD and EToPM. These results reflect the geographical influence–governed by variation in atmospheric water potential and Rs–on ETo throughout the country.

Our results demonstrate that extraterrestrial radiation and atmospheric water potential can be used to reliably estimate ETo in tropical and subtropical regions. The influence of other climatic variables needed for the standard EToPM calculation, such as Rs and u2, was indirectly but sufficiently accounted for in the analysis of radiation and atmospheric water potential, as evidenced by the strong agreement identified between EToPM and EToMJS estimates. Previous studies have shown similar results in cold and wet climates [54–55]; however, in our analysis of subtropical climates, the response of ETo to changes in Ψair indicates lower sensitivity relative to that observed in warmer and drier climates. In colder and wetter climatic zones, where atmospheric water demand is low (Table 4), ETo estimates were most strongly associated with Rs and u2 [72].

In this study, we present a new model to estimate reference evapotranspiration in tropical and subtropical regions, where the climatic information needed for the standard ETo calculation is scarce or absent. We describe how geographical and seasonal variability in evapotranspiration can be accurately predicted based on radiation and atmospheric water potential estimates. The new simplified method is particularly robust in tropical and semi-arid climates, but can also be applied in subtropical and wet climates. In all cases, the new method has significant benefits with respect to accuracy and spatiotemporal scale of application relative to previous models. Continued measurements of air temperature and relative humidity (needed for Ψair modeling) across different land uses will improve the accuracy of land-to-air water flux estimates in future studies.