Revisiting the crop coefficient–reference evapotranspiration procedure for improving irrigation management

The consumptive use of water by irrigated crops is typically quantified using the crop coefficient–reference evapotranspiration (Kc–ETo) procedure; yet, recent results showed that Kc might change with ETo, in response to high atmospheric demand. It is not known if the reduced Kc at high ETo applies to other crops with different aerodynamic features of the canopies. This paper seeks to provide the Kc values for soybean, wheat and potato, and propose an adaptation to the Kc–ETo procedure, hypothesising that the inverse relation between Kc and ETo would be general for all types of crops. Our results showed average Kc values of 0.90, 1.18 and 1.28, respectively, for soybean, wheat and potato cropping systems in the Brazilian cropping systems. However, Kc decreased as ETo increased, for all crops considered in this study, because of the increase of internal plant resistances to vapour diffusion from the leaves to the atmosphere. When ETo was above > 4 mm d−1, the water use by such crops was lower than that prescribed by Allen et al. (JAMA, 1998). The time-based Kc curves in Allen et al. (JAMA, 1998) are inappropriate for the studied crops under high demanding conditions and, besides the considerations suggested by Allen et al. (JAMA, 1998) (i.e., crop development stage, presence or absence of weeds), Kc recommendations for practical irrigation management should be based on the average ETo values of the previous days of the irrigation procedure.