Energy balance and evaporation loss of an agricultural reservoir in a semi-arid climate (south-eastern Spain)

A typical agricultural water reservoir (AWR) of 2400 m2 area and 5 m depth, located in a semi-arid area (southern Spain), was surveyed on a daily basis for 1 year. The annual evaporation flux was 102.7 W m-2, equivalent to an evaporated water depth of 1310 mm year-1. The heat storage rate G exhibited a clear annual cycle with a peak gain in April (G ~ 45 W m-2) and a peak loss in November (G ~ 40 W m-2), leading to a marked annual hysteretic trend when evaporation (¿E) was related to net radiation (Rn). ¿E was strongly correlated with the available energy A, representing 91% of the annual AWR energy loss. The sensible heat flux H accounted for the remaining 9%, leading to an annual Bowen ratio in the order of 0.10. The equilibrium and advective evaporation terms of the Penman formula represented 76 and 24%, respectively, of the total evaporation, corresponding to a annual value of the Priestley-Taylor (P-T) coefficient (¿) of 1.32. The P-T coefficient presented a clear seasonal pattern, with a minimum of 1.23 (July) and a maximum of 1.65 (December), indicating that, during periods of limited available energy, AWR evaporation increased above the potential evaporation as a result of the advection process. Overall, the results stressed that accurate prediction of monthly evaporation by means of the P-T formula requires accounting for both the annual cycle of storage and the advective component. Some alternative approaches to estimating Rn, G and ¿ are proposed and discussed. © 2009 John Wiley & Sons, Ltd.