FAO-56 methodology for the stress coefficient evaluation under saline environment conditions: Validation on potato and broad bean crops

This study aims at validating the methodology proposed by Allen et al. (1998) to calculate the stress coefficient Ks (ratio between actual and maximal evapotranspiration) under saline environment conditions not affected by soil water shortage. Validation was performed in Mediterranean region (Bari, southern Italy) on two crops: a winter crop (broad bean) and a spring crop (potato) grown in lysimeters, on clay and loam soils, having different levels of salinity. Preliminary observations were carried out to verify that the conditions established by Allen et al. (1998) for applying this Ks calculation methodology were fulfilled.The measured Ks values showed an evolution during the growing cycle. Ks values were close to 1 after emergence, and decreased at the end of the growing cycle. Contrarily, the calculated Ks values showed steady values during the whole crop cycle, being lower than the measured Ks. Only at the end of the crop cycle the calculated Ks values approached those measured. The various causes of differences between measured and calculated values of Ks are analyzed in this study.The observed differences between calculated and measured values of Ks led to an underestimation of calculated actual evapotranspiration (AET), at different stages in the crop growing cycle, by an average of 12%. The analysis of seasonal evapotranspiration as a function of soil salinity allows for a modulation of this mean value. The underestimation was quite negligible (close to 4%), if the average value of ECe during the crop cycle was close or lower than 3 dS m−1. On the contrary, the underestimation in evapotranspiration was close to 20% when the ECe raised up to 6 dS m−1.An underestimation of calculated AET in saline environment conditions, by the methodology proposed by Allen et al. (1998), causes the appearance of an additional water stress, mainly when soil salinity, increases due to the combined effect of soil water shortage and water quality. Different solutions are proposed to improve the calculation of AET in this condition.

Research highlights▶ The stress coefficient (Ks) is required to calculate crop AET in saline environments ▶ On two species Ks was calculated by FAO-56 method and measured in lysimeters having different saline soils. ▶ During the crop cycles calculated Ks is lower than measured Ks, mainly at early stages. ▶ Calculated seasonal AET underestimates measured seasonal AET from 4 to 20%. ▶ FAO-56 method to calculate Ks causes an additional water stress if soil salinity is high.