Soil moisture sensor calibration, actual evapotranspiration, and crop coefficients for drip irrigated greenhouse chile peppers

Limited water supplies in arid regions put constraints on agriculture. In arid New Mexico, greenhouse chile pepper production has the potential for water and nutrient savings. The objectives of this study were to (1) compare two capacitance sensors - (Hydra probes and 5TM) and one TDR CS616 sensor, (2) compute actual evapotranspiration (ETa) for drip-irrigated chile peppers for three water treatments, and (3) develop new crop coefficients (Kc) for the three growing seasons in a greenhouse study. Three water treatments were (1) control where water was applied near the surface using two drip emitters, (2) partial root zone drying vertical (PRDv) where subsurface irrigation was applied at 20 cm depth from soil surface, and (3) partial root zone drying compartment (PRDc) where roots were divided into two compartments and irrigation were switched between compartments after 15 days. Sensor-generated volumetric water contents (θ) were correlated with the gravimetrically determined θ, and the new calibration coefficients improved the precision of θ estimates. From 2011 onward, irrigation amounts were adjusted to minimize deep percolation, and about 30% less water was applied in 2014 as compared to the 2011 growing season but no significant differences were observed in transpiration rate and leaf temperature. The ratio of intercellular to ambient CO2 concentrations (Ci/Ca) was significantly correlated to transpiration rate and vapor pressure deficit in 2014 (P < 0.05). ETa obtained from water balance and reference ET (ETr) from Penman-Monteith developed the Kc for drip-irrigated greenhouse chile peppers for three growing seasons. The maximum values of Kc were about 1.4 during 2013 and 1.2 during 2014. The 2011 growing season was shorter and the maximum Kc were closer to one. Crop coefficients for greenhouse grown chile peppers varied with growing seasons and irrigation treatment. Irrigation scheduling can be done based on the soil moisture or Kc for the known growing season. This study demonstrated the water saving potential of PRD.