Surface-Energy Partitioning in a Rice Paddy Field.

Understanding energy and water balances in rice paddy fields provides us knowledge not only for irrigation planning, but also for estimating changes in water balance caused by global warming. Rice paddy fields are considered as a major source of methane, a greenhouse gas, in the agricultural section. Methane emission may be related to the surface energy and water balances in rice paddy fields. However, factors that determine energy partitioning remain unknown. We measured surface energy balance in a rice paddy field to investigate a relationship between energy partitioning and surface and meteorological conditions. Field observation was carried out in a rice paddy field from June to August 2007. Energy balance was measured with the Bowen ratio method. Ponding water depth was measured using time domain reflectometry. The surface-energy balance was expressed as: Q_RN = Q_G + Q_SW + Q_E + Q_H, where Q_RN, Q_G, Q_SW, Q_E, and Q_H are net radiation, soil heat flux, heat storage in ponding water, latent heat flux, and sensible heat flux (W m^-2). Bowen ratio ranged between 0.3 and 0.6 in daytime on sunny days, indicating that more than half of available energy at the surface (Q_RN - Q_G - Q_SW) was distributed to latent heat flux. Energy partitioning varied depending on growth stages of rice plants: in an early-growing stage, net radiation was mostly partitioned to heat storage in the surface water in early morning and to latent heat flux after 10:00 am., while the after the early-growing stage, most of net radiation was partitioned to latent heat flux in all the daylight hours. This variations of surface-energy partitioning was resulted from surface-water exposure to the sun, and low water temperature and low wind velocity in the morning in the early-growing stage. Surface energy partitioning and evapotranspiration rate varied depending on surface and meteorological conditions.