ESTIMATING EVAPORATION FROM DAM RESERVOIRS BY DEVELOPMENT AND APPLICATION OF A NEW METHOD BASED ON ENERGY BALANCE CASE STUDY: DOOSTI DAM

Evaporation from bodies of water, like lakes and reservoirs among other natural evaporating surfaces like canopies and wet soils, constitutes a signicant source of water loss aecting global water balance and energy exchange between land and atmosphere. Hence, formulating lake evaporation rates has been of great interest for many years and is denitely a challenge due to its complicated nature. Evaporation from lakes and reservoirs in Iran is commonly estimated using pan evaporation, which is well known to have signicant uncertainty both in magnitude and timing. Reservoir operation and development of a new storage and water accounting strategies require more accurate evaporation estimates, especially for drinking water in arid conditions. In this study, an attempt has been made to estimate evaporation from a body of water using a new approach based on an energy balance model. For this purpose, a new energy balance method for two surfaces was established using water (evaporating surface) and bare soil (non-evaporating surface) as references. Considering similar conditions for two surfaces, an identical aerodynamic resistance ratio was assumed for both. With this assumption, a new form of energy balance was obtained which only depends on net radiation and temperature. The derived reference and water surface energy balance (RWEB) model were applied to estimate evaporation from the Doosti dam reservoir and was compared with other conventional methods. To evaluate the performance of the RWEB model, it was compared with the BREB and the pan methods. According to the evaluations, the RWEB results obtained for evaporation from 2011 to 2012 were satisfactory. The RMSD values for the RWEB and pan methods were obtained as 1.02 and 1.7, respectively. Therefore, the results indicate the good performance of the RWEB method. The RWEB sensitivity analysis showed that the model has the highest sensitivity to air and reference surface temperature and the least sensitivity to net radiation. Thus, evaporation from the body of water can be estimated accurately by precise measurements of air temperature and relatively reasonable estimations of the other parameters (reference and water temperature and net radiation).