Evapotranspiration and crop coefficient for watermelon grown under plastic mulched conditions in sub-tropical Florida

• Lysimeter-based watermelon crop ET (ETc) with plastic mulch in Florida was 278 mm.
• Crop coefficients (Kc) were higher than literature and reduced the ETc error by 30%.
• Initial stage ETc and Kc was higher than literature due to wet row-middle bare soil.
• A multivariate model predicted initial Kc as a function of rainfall and humidity.
• Results will improve watermelon ETc estimates for sub-tropical environments.

Evapotranspiration (ETc) fluxes from plastic mulch production systems are different than open field production systems and require quantification of ETc for plastic mulch environment. A 3-year study was conducted to quantify bi-weekly ETc and develop a crop coefficient (Kc) for drip-irrigated watermelon grown with plastic mulch in sub-tropical Florida using four large drainage lysimeters. The average seasonal ETc was 278 mm (min = 244 mm and max = 344 mm). Variability in ETc was mainly influenced by rainfall, especially during the initial growth period when it was high. The initial, mid-season, and late season Kc values were 0.65, 1.01, and 0.71, respectively. The initial Kc was considerably higher than literature values. This finding was mainly due to high soil moisture at the beginning of the growing season resulting from surface application of water for plastic mulched raised bed preparation and additional wetting from rainfall that increased evaporative flux. The Kc values were statistically higher (p = 0.008) than FAO-56, which underestimated ETc by 30%. A polynomial model was developed to predict Kc as a function of days after transplanting. To improve the ETc estimates for the first two bi-weekly periods when the large area of bare and wet soil results in significantly higher evaporation, a multivariate model (r2 = 0.78) was developed to predict the Kc as a function of rainfall and relative humidity. The model can be used to adjust Kc, and therefore ETc, for the initial stages when evaporation accounts for most of ETc and is sensitive to frequency of wetting. Use of Kc values from this study will help improve the accuracy of ETc estimates for drip-irrigated watermelon in subtropical Florida and elsewhere with similar environmental condition.