Real-time precise irrigation scheduling and control system in solar greenhouse based on ET and water balance

Abstract: China is experiencing critical water scarce in agricultural production, and to improve irrigation water use efficiency has been the priority of agriculture development for years. Real-time irrigation scheduling, as well as its integration with irrigation control system, should be paid more attention other than the solely use of sprinkler and drip irrigation systems to improve irrigation water use efficiency in Chinese irrigation community. The evapotranspiration(ET) and water balance (ET-WB) method was applied in this paper to schedule irrigations in a solar greenhouse for cherry tomato in Jurong City in Jiangsu Province. A modified reference ET equation for solar greenhouse was used, and FAO56 suggested initial parameters were referenced to calculate crop evapotranspiration (ETc). Irrigations were triggered in a daily scale when accumulated crop evapotranspiration exceeded readily available water (RAW) across the root depth, which was defined by management allowable depletion (MAD) suggested by Huffman et al.(2013). Then an irrigation event was applied to replenish the soil to field capacity. A user-friendly irrigation scheduling software, namely ETSch, was developed using Java on a laptop, to facilitate managers with calculations involved in ET-WB method. The ETSch calculated ET using meteorological data measured from a field station in greenhouse, and outputed the accumulated ETc and RAW, as well as the irrigation decision on that day. Once an irrigation was triggered, ETSch would send a control signal through a serial connection to the irrigation control system, which was designed using a single-chip microcomputer (SCM) MSP430F169. The SCM system controlled the irrigation amount of the drip irrigation system arranged for each irrigation treatment. Once received an irrigation command, the SCM system would open the corresponding valves and read the flow from a flowmeter. The total water amount would be calculated in the SCM through an integration of flow and stops irrigation when it reached the scheduled amount. Both hardware and software of the SCM control system were detailly designed and developed. The experiment result showed that the irrigation control system worked well after flow correction, with an average error of only 1.1% between set and actual irrigation amount . The scheduled irrigations avoided the occurrence of over-high or low soil water content, and maintained crop water requirement over the crop season, though with an averagely 9.1% underestimate of soil moisture. The estimated soil moisture under ET-WB method showed a consistent change with measured values generally, which proved the efficiency of the developed irrigation scheduling and control system. The underestimate of soil moisture would probably be caused by the error of soil property settings and lateral flow from outside greenhouse with much rainfall. To conclude, a real-time precise irrigation scheduling and control system was developed for greenhouse planted cherry tomato based on ET-WB method, including a laptop-based scheduling software and an SCM-based precise controller. The framework of irrigation scheduling and control system could be an example of further smart irrigation systems, and the data collected could be used for agricultural models calibration and benefit the improvement of irrigation scheduling efficiency. To improve the feasibility of such real-time precise irrigation scheduling and control system for field-scale applications, remote connections should be built for data collection system, irrigation scheduling system and irrigation control system.