Estimating Evapotranspiration of Greenhouse Tomato under Different Irrigation Levels Using a Modified Dual Crop Coefficient Model in Northeast China

Accurate quantification of evapotranspiration (ETc) and its components are critical for enhancing water use efficiency and implementing precision irrigation. A two-year experiment was conducted for greenhouse-grown tomatoes under mulched drip irrigation with three irrigation treatments during 2020-2021 in Northeast China. Three different irrigation treatments were applied by setting upper and lower soil moisture irrigation thresholds (i.e., W1, 65%theta FC-75%theta FC, W2, 75%theta FC-85%theta FC, W3, 85%theta FC-95%theta FC, respectively, where theta FC is field capacity). In this study, a modified dual crop coefficient (Kc) model was proposed to simulate daily ETc, plant transpiration (Tr) and soil evaporation (Es). The simulations of the model were validated against observed data from the sap flow system combined with the soil water balance method. The controlling factors on the variations of evapotranspiration and its components were also identified by using the path analysis method. Results showed that the modified dual Kc model can accurately simulate daily ETc, Es, and Tr for the greenhouse tomato under different irrigation conditions, with the coefficients of determination ranging from 0.88 to 0.98 and the index of agreement higher than 0.90. The seasonal cumulative ETc of tomato for W1-W3 were 138.5-194.4 mm, of which 9.5-15.8% was consumed by Es. Path analysis showed that the net radiation (Rn) was the dominant factor controlling the variations of Tr and ETc during the growing seasons. The canopy coverage degree (Kcc) was the dominant controlling factor of Es, while the temperature (Ta) was the primary limiting factor affecting Es. This study can provide reference information for developing proper irrigation management in a greenhouse-grown tomato in the north cold climate regions.