Evaluation of methods for calculating potential evapotranspiration in climate change scenarios

Despite uncertainties involved by future scenarios, the acknowledgement of climate change problem (WMO 2019/1248 reinforces the past five years as the warmest in industrial records, part of the warmest decade on record 2010-2019, and the need for urgent mitigation and adaptation actions have only grown in recent years. In the European Territory (EEA 1/2017), a significant decrease in summer soil moisture content in the Mediterranean region, while increases in north-eastern countries are projected for the coming decades. The current temperature increase derived from the emission of gases to the atmosphere, in the range of 0.1-0.3 ºC per decade by the IPCC experts Special Report 2018, obliges a deep review of the agricultural productivity factors, according to the FAO-56 /2006.

Soil moisture content is thus approached as a dynamic variable, with changes in temperature as well as precipitation constantly affecting evapotranspiration and infiltration rates. In this paper, five computing methods for crop water evapotranspiration (Penman-Monteith proposed by FAO-56, Thornwaite, and three temperature-based methods: Hargreaves 1975, Hargreaves-Samani 1985, Samani 2000) are not only scientifically compared but also applied to a Spanish Study Case at Valencian Community in the Mediterranean Basin. Results are affected by local single crops coefficient (also proposed by FAO-56) for citrus trees in upper Palancia River catchment, representative of intensive agriculture in the area, and calculated under four future scenarios (from +1ºC to 4ºC of unitary temperature increase).

Analysed results by percentual comparison with Penman-Monteith estimation, demonstrate a similar application range (from -1% of variation in +1ºC scenario to -4% of variation in 4ºC scenario) for scarcer data-based methods (Hargreaves 1975, Hargreaves-Samani 1985 and Samani 2000) except Thornthwaite. Allowing to conclude that Thornthwaite projections in the Mediterranean Climate overestimate up to 3% (+1ºC scenario), 6% (+2ºC scenario), 11% (+3ºC scenario) and 16% (+4ºC scenario) the monthly values of crop evapotranspiration.