Dual Crop Coefficient of Watermelon under Deficit Irrigation, Mulching, and AquaCrop Model Performance in Arba Minch, Ethiopia

Single crop coefficient-based crop water estimation challenges watermelon irrigation agriculture in Arba Minch, Ethiopia. This study aimed to estimate the adjusted dual crop coefficient of watermelon under deficit irrigation with straw mulching, alongside validating the AquaCrop model for watermelon cultivation improvement. This experimental research followed a randomized complete block design with four treatments: full irrigation (T1), full irrigation with straw mulching (T2), 50% deficit irrigation (T3), and 50% deficit irrigation with straw mulching (T4). Average actual total water consumption of watermelon was 419.2 mm (T1), 338.4 mm (T2), 227.8 mm (T3), and 187.8 mm (T4), respectively. The straw mulching with full irrigation level (T2) of the present study reduced water consumption of watermelon by 20.1% compared with control treatment. Additionally, mulching decreased total water use by 17% under a 50% deficit compared with nonmulched treatment (T3). The average adjusted dual crop coefficient of watermelon varied from 0.17 (T4) to 0.42 (T1), 0.31 (T4) to 0.75 (T1), 0.41 (T4) to 1.04 (T1), and 0.34 (T4) to 0.84 (T1) in initial, development, mid, and late stages of watermelon growth, respectively. The average coefficient of variation for the dual crop coefficient was 9.45% (initial stage), 3.90% (development stage), 3.66% (mid-stage), and 4.72% (late stage), highlighting the variability within each growth stage. The AquaCrop model was evaluated for simulating evaporation, transpiration, and evapotranspiration, yielding a mean root mean square error (RMSE) of 1.6 mm. Similarly, evaporation, basal crop coefficient, and dual crop coefficient simulations also exhibited a mean RMSE of 1.6. The model demonstrated favorable performance with average R2 and Nash–Sutcliffe coefficient of efficiency (NSE) values of 0.84 and 0.74, respectively. This study demonstrates that combining modified dual crop coefficients with deficit irrigation and straw mulching can reduce watermelon water consumption in Arba Minch. The AquaCrop model, validated with high accuracy, offers a reliable tool to predict the dual crop coefficient of watermelon-irrigated agriculture with or without mulching field management practices.