Application of CRE hypothesis in Yongdam Dam basin using flux evapotranspiration measurements

Evapotranspiration in water cycle process is an essential factor to be considered in the water supply plan for multi-purpose dams. Since evapotranspiration is caused by complex actions such as meteorological factors and evaporative surface characteristics, it is not easy to quantitatively estimate actual evapotranspiration (AET). There are technical limitations in observing evapotranspiration of basin in the context of hydrological view. On the other hand, the complementary relationship of evapotranspiration (CRE) hypothesis enables the estimation of AET without complex hydrologic modelling. This study verifies the CRE in Yongdam Dam basin using the pan evaporation (Epan) as potential evapotranspiration (PET) and the AET from the flux tower. Since the Epan can be overestimated due to the weather and surrounding environment, a correction factor of 0.77, the calculated using the FAO Penman-Monteith equation, was used. WET is defined as evapotranspiration under an unlimited supply of moisture and is calculated using the Priestley-Taylor equation. AET and PET are expressed as AET+ and PET+, respectively, through a scaling process using WET, and moisture index (MI) is a relative ratio between AET and PET. CRE refers to the relationship between AET+ and PET+ according to MI, and in the observed data-based CRE, the relationship is verified by adjusting the Priestley-Taylor coefficient. The coefficient was adjusted to 0.99 in consideration of weather conditions such as wind speed in the Yongdam Dam basin instead of the original value of 1.26. The RMSE for CRE improved from 0.685 to 0.075 for AET+ and from 0.437 to 0.315 for PET+, suggesting that the observational data on evapotranspiration form a complementary relationship.