Regional Assessment and Projection of Hydrologic Water Balance Due to Climate Change in the Context of Complementary Relationships

In ecohydrologic projection, the evapotranspiration is the key component in hydrologic water balance cycle. The complementary relationship was established based on the physics of energy and mass transfer at land surface—atmosphere interface and defines the large-scale interaction between actual (ET_a) and potential (ET_p) evapotranspiration with respect to moisture availability. The change of regional complementary relationship due to climate change is paramount importance in understanding and predicting the future ecohydrologic impacts. In this paper, the complementary relationship hypothesis was validated in Soyang-gang dam basin with 2,783 km² of basin area along with the other major dam basins in Korea. Then, the representative raw and downscaled GCM (global climate model) outputs with RCP (representative concentration pathways) 4.5 and 8.5 scenarios were validated in the context of the complementary relationship hypothesis established using the observation data. To calculate the complementary relationship, the moisture availability was defined as annual precipitation and ET_p from GCM and observation data were estimation by applying the Hargreaves et al. (1985) equation that uses a minimum of meteorological data. ET_a was estimation by applying each of the GCM data with attributes of evapotranspiration and the water balance method using observation data. The bias correction process for raw GCM output is essential for properly use for projecting the future regional hydro-environmental impacts. The projection of the river flow regime considering the increase of actual evapotranspiration shows the changing (increasing) rates of the river flows from high to low flow range are attenuated in comparison with relatively high increasing rate of precipitation. The increase of actual evapotranspiration and consequent decrease of effective precipitation may possibly be the cause.