Quantifying the effects of advection on single crop coefficients over a humid paddy field for sustainable irrigation

Abstract

Understanding the variation and magnitude of crop coefficient (Kc) is important to accurate determine crop evapotranspiration (ETc) and optimal irrigation scheduling. Sensible heat advection is expected to increase Kc by providing additional energy, while its effect on Kc over paddy field is rarely featured. A three-year experiment was conducted over a paddy field to develop local Kc using the eddy covariance technique (EC) in the Poyang River basin, southern China. The local Kc curve, the characteristics of advection and its contribution to Kc were investigated. The three-year average local Kc values during the initial, mid-season and late-season stages were 1.12, 1.29 and 1.13 for the early rice and 1.11, 1.39 and 1.02 for the late rice, respectively. The advection was more likely to occur during the late rice season than the early rice season. The three-year average advective days were 8.0 d for the early rice season and 29.7 d for the late rice season, respectively. The contribution of advection to daily Kc ranged from 0.6% to 13.2% for the early rice season and 1.1% to 37.7% for the late rice season, respectively. The upwind city was found to be a possible energy source for advection. The occurrence of advection was closely related to air temperature gradients (ΔT) between the city and paddy field and wind speed (u2). Our study indicated advection also existed in the humid paddy field and had considerable effects on Kc. These results are helpful for decision-makers to quantify crop water consumption and improve irrigation water efficiency.