Simulation of the effects of the alteration of the river basin land use on river water temperature using the multi-layer mesh-typed runoff model

A model displaying river water temperatures was established, and applied to a small river basin. Based on the results, the effects of the alteration of the river basin on the budget and river water temperature were discussed. The model was a multi-layer mesh-typed runoff model, and the behavior of water and heat transference was depicted. Also, in order to verify the model, the daily changes of streamflow and river water temperature were measured and compared with the model results. From the calculation, the flow rate and river water temperature profile agreed well with the measured ones along the streamline. Using the model, the effects of deforestation and air temperature rise on river water temperature were discussed. With deforestation, the temperature in summer was calculated to rise and fall in winter. This was explained by the change of flow pass of surface and subsurface. The air temperature was thereafter changed in the model. From the simulation under the air temperature rises, the daily air temperature was evenly changed through a year, and the ratio of the change of the river water temperature to the air was less than unity (i.e., >1 °Criver water/°Cair). By close investigation of the model calculation, the most influential factor was determined to be subsurface temperature. The rise of surface temperature was also less than unity, due to the enhancement of heat loss with the augmentation of evaporation at the surface, and because the subsurface temperature is calculated on the surface temperature as boundary condition, rise of subsurface temperature was also less than unity. Overall, the mechanism of the alteration of river water temperature was described as follows: with the change of river basin land use or meteorological conditions, the pass of subsurface flow is changed, and the streamflow and river water temperature are also changed.