Coupling land surface and crop growth models to estimate the effects of changes in the growing season on energy balance and water use of rice paddies

A crop growth model was coupled to a simple land surface model to estimate the effect of changes in the growing season for rice (Oryza sativa L.) on the energy balance and water use of rice paddy fields. The crop growth model consisted of calculations of phenological development (Ps), growth of leaf area index (LAI) and canopy height (h). The land surface model consisted of calculations of energy balance, radiation transport and stomatal movements using the output of the crop growth model (Ps, LAI, h). Using a coupled model, the energy fluxes and water/canopy temperatures of rice paddies can be calculated from climatic data only. The model was evaluated using whole season observations at three experimental sites with different growing seasons in a humid temperate climate. The seasonal variations in calculated energy fluxes showed good agreement with observed values with a root mean square error of 10–20 Wm2.To understand how the change in growing season affects energy balance and water use of rice paddies, variations in water/canopy temperatures and evapotranspiration were estimated using the model for five different transplanting times based on climatic data for the Miyazaki Plain in Japan. A common trend in energy balance was obtained for all growing seasons (i.e., water temperature was higher than air temperature, whereas canopy temperature was lower than air temperature after transplanting, and these differences decreases with rice growth) under normal climatic conditions. Mean values of the ratios of total transpiration to total evapotranspiration (T/ET) during the rice growth period based on climatic data from 20 years (1981–2000) were 0.40, 0.48, 0.48, 0.46 and 0.36 for transplanting at March 1, April 1, May 1, June 1 and July 1, respectively. This suggests that the water use efficiency based on total evapotranspiration would be higher for transplanting during mid-season. Using the proposed model, the water use by rice paddies can be estimated for given climatic conditions and growing seasons, which makes it possible to modify the growing season and water management for climate change.