Abstract
Surface energy distribution in paddy fields and the ratio of latent heat flux (LE) to available energy, termed as the evaporative fraction (EF), are essential for an understanding of water and energy processes. They are expected to vary in different ways in response to changes in the soil moisture condition under water-saving irrigation practice. In this study, the diurnal and seasonal variations in energy distribution were examined based on the data measured by the eddy covariance system and corrected with enforcing energy balance closure by the EF method in water-saving irrigated rice paddies in 2015 and 2016. Soil heat flux (G) values were similar in magnitude to sensible heat flux (Hs) values, with both accounting for approximately 5% of the energy input. Both magnitudes of G and Hs were significantly lower than that of LE. Generally, EF in water-saving irrigated rice paddies was larger than that of other ecosystems, and varied within a narrow range from 0.7 to 1.0. Diurnally, EF decreased till noon and then increased slowly in the afternoon till sunset. It was found be less varied between 10:00 and 14:00. Seasonally, the alternative drying-wetting soil water conditions in water-saving irrigated rice paddies resulted in a change in the variation of the EF. The LE flux is the largest component of available energy, with EF being mostly higher than 0.9. EF, increasing consistently till the tillering stage, remaining high from the late tillering to milk stage, and then following a declining trend. The maximum EF (approaching 1.0) was found in the milk stage. The results of EF in water-saving irrigated rice paddies will be helpful for estimating daily or long temporal scale evapotranspiration (ET) by the EF method based on satellite-derived ET.
Highlights
Surface energy budgets and partitioning in different terrestrial ecosystems are essential prerequisites for understanding the regional hydro-thermal cycle and climate change
For flux data collected following the procedure of quality control, gap filling, and soil heat storage correction, the underestimation of turbulent fluxes was found to be a major contributor to the energy closure problem [12,13,14,15,16,17,18]
This indicated that Hs was more sensitive to available energy than latent heat flux (LE) during the diurnal variation
Summary
Surface energy budgets and partitioning in different terrestrial ecosystems are essential prerequisites for understanding the regional hydro-thermal cycle and climate change. The implementation of water-saving irrigation (WSI) in paddy rice led to alternative drying-wetting conditions and change in phenological characteristics, which is quite different from upland (either irrigated or non-irrigated) crop ecosystems or flooding rice paddies This results in changes in energy partitioning, soil moisture content, crop growth, and heat and vapor flux transfer between rice paddies and atmosphere [43,44,45]. The main objectives of this study included: documenting energy balance partitioning and EF variation at hourly, daily, and seasonal scales along rice growth under the WSI condition in east China; and discussing the effect of soil moisture on EF, which is of great theoretical and practical significance in simulating the hydrothermal conversion further and estimating the daily or long temporal scale ET by the remote sensing method
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