Estimation of Cucumber Evapotranspiration in Solar Greenhouse in Northeast China

The solar greenhouse without heating system has been widely used for vegetable production in Northeast China since the 1980s. The experiment was conducted in the solar greenhouse to study the relationship between evapotranspiration (ET) of cucumber and climatic factors. Reference evapotranspiration (ET0) and pan evaporation (Ep) were applied to determine cucumber evapotranspiration by regression methods. Results showed that the ET of cucumber increased with the increasing of solar radiation, air temperature and vapor pressure. The solar radiation was the most important factor that influences ET among environmental parameters. The determination coefficient (R2) was 0.865 between ET and Ep, higher than that between ET and ET0 (application of a constant vegetation coefficient, R2 = 0.46). The pan coefficient (kp, determined from the ratio of ET to Ep) and the crop coefficients (kc, determined from the ratio of ET to ET0) showed considerable variability and no obvious systematic pattern could be described throughout the study period. Ep was linearly related to ET and 20 cm pan can be well used to determine the water requirement of cucumber under subsurface drip irrigation in this type of solar greenhouse.