Tomato Evapotranspiration, Crop Coefficient and Irrigation Water Use Efficiency in the Winter Period in a Sunken Chinese Solar Greenhouse

Abstract

In the Northern China Plain (NCP), sunken solar greenhouses (SSG) are now increasingly being used for vegetable cultivation in winter due to low winter temperatures. Investigating plant transpiration and determining crop coefficients are helpful when developing irrigation scheduling and improving crop growth. In this study, a three-season experiment was conducted in a commercial tomato SSG to investigate changes in microclimate, sap flow (SF), photosynthesis traits and plant physiological parameters, and to calculate the crop coefficient and evaluate the irrigation efficiency using current irrigation management practices. Results show that the average transmissivity from top plastic covers was 0.69, and the inside temperature increased by approximately 10 °C in November and 15–18 °C in December, which guaranteed the growth of tomatoes in winter. The leaf photosynthesis rate (Pn) is linearly related to radiation, however, a concave quadratic function is a better fit for Pn and VPD, with the highest Pn at approximately 1.0 kPa VPD; leaf transpiration is positively and linearly related to both radiation and VPD. Therefore, increasing greenhouse transmissivity and maintaining an internal VPD of approximately 1 kPa could produce a high leaf Pn and low transpiration concurrently. Daily total SF was linearly correlated with solar radiation, VPD and temperature with determination coefficients of 0.87–0.96, 0.89–0.91 and 0.62–0.84, respectively. Correcting the slope of SF to radiation with VPD (SF = (0.12 + 0.14VPD)Rs), R2 increased by 0.08, and the root mean square error and relative error decreased by 0.047 mm day−1 and 6.53%, respectively. Therefore, this integrated equation is recommended to estimate daily tomato transpiration when plant height is approximately 1.5 m, and the leaf area index (LAI) is between 2 and 2.5. During the fruit expansion and ripening period, the average basal crop coefficients (Kcb) for greenhouse tomatoes in winter was between 0.99 and 1.11. The irrigation efficiency increased from 0.3 in the first season to 0.6–0.69 in the second and third seasons when the tensiometer method was used. Therefore, using the tensiometer method to guide tomato irrigation could markedly improve irrigation efficiency in greenhouses.