## 4 Discussion
### Uncertainties in Assessment Method  
EC is widely used to evaluate actual ET around the world. However, final flux data from EC may be not consistent with the actual condition at times. This gap may result from instrument sensitivity, weather conditions, and gap filling methods. Eddy covariance instruments generally have two types of analyzers: open-path CO\({}_{2}\)/H\({}_{2}\)O analyzers, and closed-path CO\({}_{2}\)/H\({}_{2}\)O analyzers. However, gaps exist in the observed data of the two analyzers, due to their different working mechanisms for calculating vapor and gas flux. Different types of EC instruments were installed at the flux sites in this study. For example, closed-path CO\({}_{2}\)/H\({}_{2}\)O analyzers (LI-6262, LI-COR) were installed in SKR and MKL, while open-path CO\({}_{2}\)/H\({}_{2}\)O analyzers (LI-7500) were installed in XSBNRa and QZ-SETORS. This may lead to uncertainties the assessment result. Unfavorable weather conditions such as snow, rain fall, and hurricanes can influence the stability of instruments and lead to wrong records. The advective condition should also be paid more attention, as it can destroy the energy balance by introducing extra energy [72; 73]. Wind speed has an influence on the source area of EC observation, and then brings errors for EC observation [74]. Different filling methods of EC gap data could also lead to discrepancy from actual ET [75]. Mean diurnal variation (MDV), look-up tables (LookUp), nonlinear regressions (Regr.) and artificial neural networks (ANN) are commonly used to gap-fill EC data [14; 76; 77]. However, previous studies have found that these methods can lead to quite large inconsistencies [75; 77]. [PERSON] et al. (2001) also suggested the need to standardize gap-filling methods to improve the comparability of flux data products, after comparison of seven gap-filling methods [75]. All of the sources of uncertainty further lead to a fundamental problem with EC; that is, the energy balances determined using EC are generally \"unclosed\", with the summary of sensible and latent heat fluxes often underestimating available energy by 20% or more [78]. Previous studies have shown that the energy balance closure ratio of forest stations in ChinaFLUX and ASIAFLUX is around 70-90% [79]. Thus, quite a lot of researchers have suggested that the closure issue of measured energy fluxes must be resolved before it is used to evaluate energy and water exchange products [73; 78; 80]. Currently, two main methods are commonly used to \"correct\" energy imbalance. The first is called the residual method, which assumes that sensible heat is correct, and that the residual from the subtraction of available energy from the summary of sensible heat and latent heat can be taken as the underestimation of latent heat [78]. The second is called the Bowen ratio method, which assumes that the EC technique provides correct estimates of the Bowen ratio (the ratio of sensible heat to latent heat), thus the residual can be proportionally assigned to original sensible heat and latent heat [81; 82; 83]. However, there is no consistent answer on which method is better to close the energy budget to date. Additionally, inconsistency in calculating the available energy can be also found in previous studies [9; 32; 34]. The main question is, under what conditions can soil heat flux and heat storage in vegetation be neglected in calculating the available energy? Some studies have found that latent heat underestimation results from neglecting soil heat, and that canopy heat storage could be rather large [84; 85]. An additional three points should be noted in terms of the uncertainties of product assessment using EC data. The first is the well-known scale mismatch problem. The footprints of EC water vapor exchange are often not well matched with corresponding remote sensing ET pixels. Thus, the site-to-pixel assessment method used in most studies may not be reasonable. In this study, we used a grid with a window size of \(2\times 2\) instead of just one pixel to match the flux site.