Optimizing Irrigation Amount for Greenhouse Lettuce Production Based on Pan-measured Evaporation

Abstract

【Objective】 Optimizing irrigation is critical to sustaining agricultural production in arid and semi-arid regions. The purpose of this paper is to investigate the feasibility of using pan-measured evaporation (Ep) to optimize irrigation for greenhouse lettuce production. 【Method】 The experiment was conducted using weighing-lysimeters installed in a greenhouse. We compared five treatments by irrigating the crop at 30% (I1), 50%, (I2), 70% (I3), 90% (I4) and 110% (I5) of Ep measured from a standard evaporation pan with diameter of 20 cm, respectively. In each treatment, we measured the evapotranspiration, above-ground biomass, yield and water use efficiency, from which we calculated the optimal irrigation amount and its associated pan coefficient (Kp). We rotated the lettuce after harvesting the first one. 【Result】 It was found that Ep increased as time elapsed, and the cumulative Ep of the rotated crop was 96.0 mm, increasing 33.4% compared to the first harvest. Solar radiation affected Ep most at significant level (P<0.01). The cumulative evapotranspiration in the first and second harvests varied from 43.4 mm to 70.4 mm, and 58.3 to 73.6 mm, respectively, both increasing with the irrigation amount. The cumulative pan-evaporation in the rotated crop under I1, I2 and I5 was 58.3 mm, 67.0 mm and 73.6 mm, respectively. With the increase in irrigation amount, the aboveground biomass increased firstly followed by a falling. Compared with I1 and I5, I3 increased the aboveground biomass by 17.5%~38.2% (P<0.05) and 4.2%~13.2% (P<0.05), respectively. Optimizing the irrigation significantly improved the yield and water use efficiency. Compared with I1 and I5, I3 increased the yield by 36.6%~37.3% (P<0.05) and 6.1%~23.7% (P<0.05), water use efficiency by 9.9%~15.2% (P<0.05) and 12.6%~61.4% (P<0.05), respectively. There was a quadratic relationship between the yield and the pan irrigation coefficient (Kp=0.77) with P<0.01. 【Conclusion】 Among all treatments we compared, irrigating 70% of evaporation measured from the 20 cm standard pan was optimal for greenhouse lettuce production with a pan irrigation coefficient of 0.77.

Keywords

• greenhouse lettuce
• weighing lysimeter
• water surface evaporation
• yield
• water use efficiency