Enforcing energy balance closure by evaporation ratio method and its effect on evapotranspiration estimation of paddy fields

Given its importance as a hydrological variable for agricultural water management, Evapotranspiration (ET) should be accurately quantified. Turbulent flux is the key to the research on water and heat conditions of near surface layer and the basis for further calculation of actual ET. Evaporative fraction (EF, defined as the ratio between latent heat flux and available energy at the land surface), which is known to exhibit variation in response to changes in crop species, soil and meteorologic conditions, plays an important role in interpreting the components of energy balance budget and estimating ET, while such information is scarce for humid rice fields. In order to explore the influence of change characteristic of EF of Water-Saving Irrigated (WSI) paddies and energy balance closure on the calculation of ET, a field experiment was conducted at the experimental research base of the State Key laboratory from 2014 to 2016. Controlled irrigation practice was adopted in the monitoring paddy fields of observation sites from 2014 to 2016. The eddy covariance system was installed in the downwind direction of the test area to observe the Sensible Heat Flux (Hs), Latent Heat Flux (LE), net solar radiation, soil heat flux, air temperature, air relative humidity, atmospheric pressure, wind speed, wind direction and rainfall and so on. The study used the system to monitor the turbulent flux process of WSI paddies in 2014-2016. By means of forced closure of EF and filtering interpolation method, the data of day-time and night flux were corrected respectively, the seasonal variation characteristics of EF in paddy fields were analyzed, the proportion and process changes of turbulent flux before and after energy forced closure were compared, and the influence of energy forced closure on ET of WSI rice fields were explored. The results showed that EF in WSI paddies was larger than that in dry lands, and varied within a narrow range from 0.7 to 1.0. EF reached the maximum which was supposed to be 1.0 at the heading-flowering or milk maturity period. It quickly dropped below 0.8 at the last growth period. From 2014 to 2016, the average of EF was about 0.93. The data showed that latent heat flux accounted for the largest part of turbulent flux, and heat flux was absolutely the main energy consuming-term of water-saving irrigation paddy fields. The turbulent flux was increased significantly after enforced energy closure in the daytime and the diurnal peak value differentiated the most. The latent heat flux and sensible heat flux were increased by 32.1% and 19.8% on average respectively from 2014 to 2016. The latent heat flux after closure correction was much lower than that before correction at night, which indicated that the energy balance of paddies was in an over-closed state during day-night cycle. The sensible heat flux also was increased after correction, but the increment was smaller than that of latent heat flux. The ET calculated after enforced energy closure was increased evidently whether on an hourly or daily temporal scale. Rice ET in the WSI paddies was smaller than the result of during the summer growing season of flooded rice. The average daily ET calculated after enforced energy closure was about 3.85 mm/d from 2014 to 2016, which was 1.31 times of ET before energy forced closure. It indicated that the energy balance deficit resulted in underestimation of ET by eddy covariance system. The correction of the energy balance deficit is necessary and helpful for accurate estimation of crop evapotranspiration. The research results can provide important data support and methods for the accurate measurement of ET in paddies and the extensive research of ET model.