# Evaluation of Remote Sensing-Based Evapotranspiration
## 4 Results and Discussion
### Multi-Variable Calibration (Scheme II)
#### 4.3.2 Hydrological Signature Analysis  
Hydrological signatures refer to specific characteristics of the hydrograph [50]. The application of hydrological signatures can aid in interpreting the links between models and the underlying hydrological processes, and it provides more information about hydrological model performance [51; 52]. Hydrological signatures mainly include rising limb density, peak distribution, runoff ratio, different quantile flows, and flow duration curves (PDCs), etc. Among various hydrological signatures, the FDC is widely used to assess hydrological model performance [21; 30]. Therefore, besides regular performance evaluation indices, shown in Section 3.3, the FDC is partitioned into three segments, including low flow (0.7-1.0 flow exceedance probabilities), intermediate flow (0.2-0.7 flow exceedance probabilities), and high flow (0-0.2 flow exceedance probabilities), which are used in this study. Figure 10 compares the three segments of observed and simulated runoff based on Scheme I and Scheme II. In this figure, it is obvious that the simulated intermediate flow and high flow based on Scheme I and the three variants of Scheme II are much closer to the observation than the low flow. In the calibration period, the simulated low flow and intermediate flow under all calibration schemes tend to be underestimated, and a similar conclusion can be obtained for the validation period. Thus, it is essential to improve the performance of low flow simulations under Scheme I and Scheme II. Compared to Scheme I, the simulated low flow located in the range of exceedance probability 0.97-1.0 (namely, extremely low flows) based on the three variants of Scheme II performs better in the calibration period, and the better performance in the validation period occurs in the range of exceedance probability 0.99-1.0. Furthermore, the simulated intermediate flow located in the range of exceedance probability 0.23-0.26 under Scheme II (PML) is more consistent with the observations in the calibration period. A similar conclusion can be obtained for the validation period when the exceedance probability is within 0.30-0.39. Moreover, the better performance of intermediation flow simulations based on Scheme II (FLUXCOM) and Scheme II (GLEAM) also can be found in the validation period, when the  
Figure 9: Comparison of KGE and PBIAS values for runoff simulations at different seasons from Scheme II. ranges of exceedance probability are 0.22-0.24 and 0.25-0.27 for Scheme II (FLUXCOM) and Scheme II (GLEAM), respectively. The improvement in hydrological model performance based on the three variants of Scheme II is indeed confirmed by their high flow simulations. When the exceedance probability is within 0-0.012--namely, extremely high flows--the runoff simulations based on the three variants of Scheme II outperform that based on Scheme I in the calibration period. In the validation period, a similar improvement can be observed clearly in Figure 10.  
Figure 10: Comparison of three segments of FDC of observed and simulated daily runoff in the calibration and validation periods based on Scheme I and Scheme II. Above all, we can conclude that all three variants of Scheme II improve the performance of extreme flow simulations (including extremely low flow and high flow) compared to Scheme I. Low flow and intermediate flow simulations under Scheme II (PML) are more consistent with the observations than those under Scheme II (FLUXCOM) and Scheme II (GLEAM). For high flow, the three variants of Scheme II have comparably good performance in the calibration and validation periods. However, the improvements in extremely high flow simulations based on Scheme II (FLUXCOM) and Scheme II (GLEAM) are almost similar, and it is better than those based on Scheme II (PML).