# Evaluation of Remote Sensing-Based Evapotranspiration
## 4 Results and Discussion
### Multi-Variable Calibration (Scheme II)
#### 4.3.1 Performance Analysis at Different Temporal Resolutions
The improvement shown in Figure 6 is consistent with the reduction in PBIAS in the validation period presented in Table 4. Overall, the three remote sensing-based ET datasets have the ability to successfully improve the hydrological model simulations at the daily temporal resolution, and PML shows the best improvement in hydrological model performance, followed by GLEAM and FLUXCOM. Regarding the monthly temporal resolution, KGE and PBIAS values for runoff simulation at different months from all three Scheme II variants are shown in Figure 7. From this figure, it can be observed that the majority of months show good performance, with high KGE and low PBIAS. In terms of KGE, around 58.3%, 58.3%, and 83.3% of months in the calibration period are higher than 0.5 based on Scheme II (FLUXCOM), Scheme II (GLEAM), and Scheme II (PML), respectively. In the validation period, the percentages of months with KGE higher than 0.5 are 83.3% for PML, followed by FLUXCOM (66.7%) and GLEAM (66.7%). As for PBIAS, the percentages of months with PBIAS lower than 25% are 75%, 75%, and 100% for FLUXCOM, GLEAM, and PML in the calibration period, \begin{table}
\begin{tabular}{l c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Periods**} & \multicolumn{2}{c}{**ET Datasets**} & \multicolumn{2}{c}{**KGE**} & \multicolumn{2}{c}{**PBIAS (\%)**} & \multicolumn{2}{c}{**RMSE**} & \multicolumn{2}{c}{**R**} & \multicolumn{2}{c}{**ME**} \\ \cline{2-11}  & & **ET** & **Runoff** & **ET** & **Runoff** & **ET** & **Runoff** & **ET** & **Runoff** & **ET** & **Runoff** \\ \hline \multirow{3}{*}{Calibration (2004–2008)} & PML-V2 & 0.77 & 0.87 & 0.84 & 1.46 & 0.60 & 74.72 & 0.85 & 0.89 & 0.02 & 1.56 \\  & FLUXCOM & 0.79 & 0.72 & 3.32 & 11.82 & 0.87 & 75.16 & 0.79 & 0.89 & \(-\)0.08 & \(-\)12.62 \\  & GLEAM & 0.84 & 0.75 & 5.24 & 9.34 & 0.77 & 76.10 & 0.85 & 0.89 & \(-\)0.12 & \(-\)9.98 \\ \hline \multirow{3}{*}{Validation (2009–2010)} & PML-V2 & 0.71 & 0.91 & 12.50 & 3.38 & 0.54 & 104.05 & 0.89 & 0.92 & 0.23 & 6.00 \\  & FLUXCOM & 0.80 & 0.85 & 3.37 & 2.58 & 0.82 & 108.42 & 0.81 & 0.92 & 0.08 & \(-\)4.58 \\ \cline{1-1}  & GLEAM & 0.86 & 0.86 & 0.79 & 0.33 & 0.74 & 110.36 & 0.86 & 0.92 & \(-\)0.02 & \(-\)0.58 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Evaluation indices of runoff and ET simulations based on Scheme II. respectively. For the validation period, the percentages are increased to 100%, 91.7%, and 100% for FLUXCOM, GLEAM, and PML, respectively. Figure 8 shows a temporal comparison of the simulated and observed monthly runoff for the calibration and validation periods based on Scheme I and the three variants of Scheme II. Overall, the DHSVM model calibrated based on Scheme I and all three variants of Scheme II can produce acceptable runoff simulations. Compared to Scheme I, the peak flow simulations based on Scheme II are much closer to the observation than those based on Scheme I in the calibration and validation periods. For example, the observed peak flow is 403 m\({}^{3}\)/s in June 2008, and the simulated peak flow based on Scheme I is 379 m\({}^{3}\)/s, while the multi-variable calibrated models generate simulated peak flows of 407 m\({}^{3}\)/s, 407 m\({}^{3}\)/s, and 412 m\({}^{3}\)/s based on Scheme II (PML), Scheme II (FLUXCOM), and Scheme II (GLEAM), respectively. Similar situations can be found in June 2006, March 2010, and July 2010, etc. Moreover, the KGE value is 0.96 in the validation period based on Scheme II (PML), which is higher than that based on Scheme I (KGE = 0.95). The values of PBIAS based on Scheme II are 3.57%, 2.39%, and 0.11% for PML, FLUXCOM, and GLEAM, respectively, which are lower than that based on Scheme I (PBIAS = 4.17%). Among the three datasets, PML slightly outperforms GLEAM and FLUXCOM at the monthly temporal resolution. Figure 6: Comparison of observed and simulated daily runoff in the calibration (2004) and validation periods (2009) based on Scheme I and Scheme II.