A novel approach for estimating the pan coefficient of irrigation water reservoirs

The evaporation rate (E) and pan coefficient (Kp) of agricultural water reservoirs for irrigation (AWRIs) were determined using a novel approach based on a reservoir energy balance model and measurements of pan evaporation. The model, previously developed and validated for an insulated Class-A pan, was upscaled to the case of AWRIs by introducing a new formulation of the wind function combining the Brutsaert and Harbeck formulae. The model was run for different combinations of area (S) and depth (D) at several locations of the Segura River Basin (South Eastern Spain) where routine pan evaporation measurements (Ep) were available. The monthly and annual Kp values were derived from the simulation runs as the ratio of the simulated AWRI evaporation to the measured pan evaporation, E/Ep. Results at a monthly scale indicated that D was the main factor affecting the evolution and the value of the surface temperature and evaporation rate, while S, for a given value of D, had only a small influence. Monthly Kp varied significantly through the year, the variation range being larger for deep AWRIs (0.5 < Kp < 1.5) than for shallow ones (0.8 < Kp < 1.2). At a yearly scale, Kp was found nearly constant for a given AWRI, with small inter-annual variations. Annual Kp varied in the range 0.76 (S = 50,000 m2) to 0.86 (S = 500 m2), depending primarily on S, and to a lesser extent, on the vapour pressure deficit measured at the location. Empirical formulae allowing to predict annual Kp as a function of S and vapour pressure deficit were proposed. It was concluded that the proposed modelling approach can be applied to accurately estimate values of Kp in AWRIs at regional scale, taking into account the influence of reservoir dimensions and local climate conditions. Its application under South Eastern Spain conditions supplied the monthly and annual values of Kp to be used under similar Mediterranean semi-arid climates.