Abstract
Remote sensing allows regional evapotranspiration (ET) values to be obtained. Surface conductance is a key variable in estimating ET and controls surface flux interactions between the underlying surface and atmosphere. Limited by the influence of clouds, ET can only be estimated on cloud-free days. In this study, a gap-filling method is proposed to acquire daily surface conductance, which was coupled into a Penman-Monteith (P-M) equation, to estimate the regional daily ET over the Hai River Basin. The gap-filling method is coupled with the canopy conductance, surface conductance and a simple time extension method, which provides more mechanisms and is more comprehensive. Field observations, including eddy covariance (EC) fluxes and meteorological elements from automatic weather station (AWS), were collected from two sites for calibration and validation. One site is located in Guantao County, which is cropped in a circular pattern with winter wheat and summer maize. The other site is located in Miyun County, which has orchard and summer maize crops. The P-M equation was inverted to the computed surface conductance at the field scale, and latent heat fluxes from EC were processed and converted to daily ET. The results show that the surface conductance model used in the gap-filling method performs well compared with the inverted surface conductance, which suggests that the model used here is reasonable. In addition, the relationship between the results estimated from the gap-filling method and EC measurements is more pronounced than that between the other method and the EC measurements. The R 2 values improve from 0.68 to 0.75 at the Guantao site and from 0.79 to 0.88 at the Miyun site. The improvement mainly occurs during the growing crop season, according to the temporal variations in the results.
Highlights
Energy and water flux exchange is an important physical process in the research field of hydrometeorology and global change
The sample numbers indicate that both satellite data and eddy covariance (EC) measurements are of good quality
This study proposed a gap-filling method to acquire daily surface conductance from clear day surface conductance estimation
Summary
Energy and water flux exchange is an important physical process in the research field of hydrometeorology and global change. Quantitative research on surface heat flux contributes significantly to water cycle and climatic change interpretations. Ground observations and ET computations are highly precise in estimating individual sites or at local scales and include observations based on pan evaporation, lysimeter, Bowen ratio, eddy covariance (EC) and large aperture scintillometer (LAS) [4,5,6,7,8,9,10]. EC can provide continuously precise measurements of water and heat flux over a homogeneous surface, revealing the character of mass and energy exchange between specific surface features and the atmosphere. Regarding the underlying surface heterogeneity and turbulence complexity, regional interpolation of ET from sparse measurements is unreliable [11]
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