Automatic measurement of greenhouse tomato evapotranspiration based on negative pressure irrigation system

Accurately measuring crop evapotranspiration is important for developing suitable irrigation schedule and improving crop water use efficiency. In order to obtain the crop evapotranspiration conveniently, rapidly and accurately, the negative pressure irrigation (NI) system based on negative pressure device was established in this study. In combination with negative pressure device, we increased the liquid level detection device, which could collect the liquid level information real-timely, and the evapotranspiration was calculated by formula with system controller. The new method of crop evapotranspiration measurement could avoid many problems, i.e. high cost on measurement, high-intensity work and poor accuracy and so on. To test and verify the accuracy of calculation results, 2-seasons (early-spring season and autumn-winter season) field experiments were carried out at the National Experiment Station for Precision Agriculture (40°10′43″N, 116°26′39″W), Xiaotanshan Beijing, China. The NI measured results were compared with those with the water balance method; (CK). The soil water content in surface (0-20 cm), soil water storage (0-100 cm) and tomato yield were measured in the experiment during the whole growing season. The dynamic change of soil water content in greenhouse was studied and the tomato evapotranspiration and water use efficiency were compared. The results showed that: the variation of surface soil water content (0-20 cm) and soil water storage (0-100 cm) were stable under the negative pressure irrigation condition and the annual variation range were 21.4%-23.8% and 322.2-333.3 mm, respectively. The seasonal variation regularity of tomato evapotranspiration could be obtained with the method of negative pressure irrigation. The variation of early-spring tomato evapotranspiration in greenhouse showed a single peak curve under the negative pressure irrigation condition. The seasonal change ranged from 0.46 mm to 5.68 mm, and the peak appeared at May 20th. The autumn-winter tomato evapotranspiration had a seasonal change of 0.56-3.43 mm, which was less than the early-spring season, and the peak appeared at October 12th. Compared with the water balance method, the tomato evapotranspiration of the negative pressure irrigation system had taken on the same change rule during the growth period, and the maximum water consumption rate all appeared at tomato fruit-set period. The annual tomato evapotranspiration was 533.4 mm based on the method of negative pressure irrigation with no significant difference with that of the water balance method (541.6 mm) (P>0.05). The tomato evapotranspiration that measured by the 2 methods (NI and CK) had showed a extremely significantly linear positive correlation (R2=0.971, P0.05). The method based on negative pressure irrigation system can make the calculation of crop evapotranspiration conveniently, which not only had no significant difference with those of the water balance method, but also simply, low cost, rapidly and with high accuracy. This method provides an effective technology to measure crop evapotranspiration in solar greenhouse. © 2017, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.