Precise soil water control using a negative pressure irrigation system to improve the water productivity of greenhouse watermelon

A negative pressure irrigation system (NPIS) was tested in 2016 and 2018 to determine the optimum soil water control conditions to improve water productivity (WP) and fruit quality for greenhouse-grown watermelon. Four levels of negative water pressure (-5, -10, -15 and -20 kPa) were imposed continuously to a clay-made emitter buried 5 cm under the soil surface during three major stages of greenhouse-grown watermelon (I-VE: vine extension, 32 days; II-FF: flowering and fruit setting, 16 days; III-FM: fruit expansion to maturity, 28 days), which created soil water conditions around the emitter being 96%, 80%, 72% and 63% of field capacity, respectively. Except for the testing stages with the four levels of water treatments, the water supply during other stages was all set with a constant -10 kPa water pressure. A traditional furrow irrigation treatment was included to assess the water-saving effects of the NPIS. The irrigation timing used for the furrow irrigation treatment was decided based on the soil water contents corresponding to the -15 kPa treatment under NPIS. The results indicated that with an increase in the negative pressure imposed on the emitters, the average daily evapotranspiration linearly decreased for all three stages, with the largest decrease occurring during III-FM, but the watermelon yield decreased more with the decline in the water supply during vine extension due to the after-effects of the reduced vine growth. Water stress improved watermelon quality by increasing sugar contents during II-FF and reduced acid contents during III-FM. Moderate water stress resulted in the highest water productivity (WP) as well as protein and total soluble solid contents in the melon during all three stages. Comparing the NPIS with furrow irrigation, the water use of the former was 18.3–31.2% less, the yield was 6.2–12.0% higher, and the WP was improved by 36.6–53.8%. The results indicated that NPIS is a water-saving irrigation system, and the water pressure of NPIS can be set at -5 to -10 kPa for vine extension and fruit expansion to maturity stages; and -10 to -15 kPa for flowering and fruit setting stage to achieve good quality, high WP and relatively stable yield for greenhouse-grown melon.