The Bowen ration energy balance (BREB) is considered as a standard method for estimating lake evaporation. The BREB method however requires numerous input data which may not be readily available especially in developing countries. This limitation could be solved by using methods with fewer data requirements. Evaporation from lakes and reservoirs in Iran is commonly estimated using pan evaporation because there have not been a consensus on which methods are most applicable under the limited data condition and arid climate. Therefore, the objective of this research was to determine the most appropriate evaporation methods over Doosti dam reservoir in Iran. Eighteen existing methods were tested and ranked based on the BREB method. The Jensen-Haise, Makkink, Penman and deBruin methods were among the most consistent methods with BREB in which the RMSD values were obtained 1.2, 1.34, 1.62 and 1.65 mm d −1 , respectively. Finally, we concluded that methods which rely only on air temperature, or air temperature combined with sunshine data (e.g., Jensen-Haise and Makkink), were relatively cost-effective options for estimating evaporation at the study area due to their simplicity, least sensitivity and high accuracy.Nomenclature E the evaporation rate (mm d −1 ) R n the net radiation (W m −2 ) N the change in the energy storage in the water (W m −2 ) λ the latent heat of vaporization c the specific heat of water (J kg −1 ) F in and F out the heat fluxes from water flows in and out of the water body (W m −2 ) F P the heat inflow from precipitation (W m −2 ) Water Resour Manage G the heat conducted from the lake to the sediments (W m −2 ) β Bowen ratio (dimensionless) P the atmospheric pressure (kPa) c B the specific heat of air at constant pressure (0.61°C −1 ) T a the air temperature (°C) (°F for the Blaney-Criddle Jensen-Haise and Stephens-Stewart equations) T w the water surface temperature (°C) e s the saturation vapor pressure at the water surface temperature (Pa) e a the atmospheric vapor pressure (Pa) e a * the saturated vapor pressure at temperature of the air (mb) Δ the slope of the saturated vapor pressure-temperature curve (Pa°C −1 ) γ the psychometric coefficient (Pa°C −1 ) u the wind speed (m s −1 ) α Priestley-Taylor empirically derived constant (dimensionless) R s the incoming solar radiation (W m −2 ) C the mass-transfer coefficient (dimensionless) A s the area of the water body (hec) D the hours of daylight D TA the total annual hours of daylight SVD the saturated vapor density at mean air temperature (g m −3 ) e * a,max the saturated vapor pressures at maximum air temperature (Pa) e * a,min the saturated vapor pressures at minimum air temperature (Pa) E BREB the estimated evaporation values using BREB method (mm d −1 ) E eq the estimated evaporation values obtained by any methods (mm d −1 )
Numerous equations exist for estimating reference evapotranspiration (ETo). Relationships were often subject to rigorous local calibration, hence having limited global validity. The Penman-Monteith (P − M) equation is widely perceived as the best equation for estimating daily and monthly ETo in all climates. The main shortcoming of the P − M equation is that it requires numerous weather data that may not always be available. This study evaluates the methods to estimate missing data in the context of their influence on the performance of the ETo equations. The performance of other ETo equations under missing data are also compared. ETo equations are ranked individually in semi − humid and semi − arid climates based on their accuracy. Results indicate that the P − M equation is more sensitive in semi − arid climate than semi − humid climate under missing data conditions. The accuracy of the P − M equation under these conditions increases remarkably if any available relationships between dew point and minimum temperatures and also long-term average wind speed for each station are exploited. Finally, the minimum data requirements necessary for adequate performance of the P − M equation are air temperature for semi − humid climates, air temperature and wind speed for semi − arid climates, and the availability of a relationship between dew point and minimum temperature, especially for semi − arid climate. In absence of the satisfaction of such minimum requirements, the Hargreaves-Samani equation is preferable for semi − humid climates and the Hargreaves equation modified by Droogers and Allen (2002) for semi − arid climates.
An attempt has been made to estimate evaporation from a water body by developing a new approach based on the energy balance model. For this purpose, a new energy balance method for two surfaces was established: water (evaporating surface) and dry bare soil (non-evaporating surface as reference). An identical aerodynamic resistance ratio was assumed for both surfaces due to their similar conditions. 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) method was applied to estimate evaporation from Doosti dam reservoir in Iran. In order to evaluate the performance of the RWEB, comparison was performed with Bowen ratio energy balance (BREB) method as well as some conventional methods. According to the evaluations, the evaporation results of RWEB from 2011 to 2012 were satisfactory with RMSD value of 1.026 mm month−1 and R2 = 0.937. Furthermore, the RWEB sensitivity analysis showed the highest sensitivity to air temperature and the lower sensitivity to net radiation. Thus, evaporation from a water body can be estimated accurately by precise measurements of air temperature and relatively reasonable estimations of other parameters (reference, water temperature and net radiation).
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