# Flow Duration Curves from Surface Reflectance in the Near Infrared Band
## 3 Results
### FDCs and RDCs Definition Based on Available Datasets  
Since the FDCs are strongly sensitive to extreme events, i.e., periods of very high (floods) or very low (droughts) values of river discharge, they can vary significantly in relation to the periods considered for their construction. To test if such an aspect may affect the results of our investigation, FDCs estimated referring to the period of calibration (13 years; 2003-2015) are compared with those retrieved adopting all available data (17 years; 2003-2019). In addition, a further analysis is carried out to check the consistency of the datasets, based on two sampling options: Daily or eight-day interval, which is compatible with the frequency of the satellite product. The results of both the analyses are illustrated in Figure 4. In the first (4 a) and third column (4 c), Figure 4 shows the FDCs built for (i) the entire observation period (tot), from 2003 to 2019, (ii) for the selected calibration period (cal), from 2003 to 2015 and (iii) for the calibration period, but with an eight-day sampling interval. Because the comparison carried out over the FDCs is not sufficient to gather the discrepancies among the configurations, the second (4 b) and the fourth (4 d) column show the differences in discharge between the FDCs built considering the calibration and the total period, and between the FDCs built in the calibration period, with daily and eight-day sampling.  
Although FDCs are quite similar in shape, significant differences emerge in relation to the sampling period (i.e., cal vs. tot), especially for medium to high flows. Here, the k-s test rejects the null hypothesis for all the sites, confirming that the two distributions cannot be considered coming from the same distribution. Generally, looking at the highest discharge values (i.e., low durations), the largest errors are between the 13% and 26%, with the exceptions of McGregor, where the periods are coincident due to the missing data and Chester, for which the errors are higher (up to 32%). At Memphis, the use of one year for the calibration is not sufficient to describe a reliable FDC and the error overcomes the 45%. For low discharge values, the differences between the FDCs are minimal, tending to 0, thus the selection of the period has low impact. Concerning the sampling frequency (every eight days or daily), FDCs are pretty close to each other, with negligible differences (under 11%). The higher error values are obtained at St. Claud and Keokuk stations, with 16% and 14%, respectively. For all sites, the k-s test fails to reject the null hypothesis and therefore the eight-day sampled data can be used instead of the daily series without affecting the distribution of FDCs.  
Similarly, Figure 5 represents the RDCs built for the total period (2003-2019) and for the calibration period (2003-2015), at every site. For the reflectance ratio C/M, the differences are more evident in the high values range (duration lower than 50%), tending towards 0 for lower values. As expected, for extreme high events, the RDCs built in the calibration period provide lower values than considering the total period, with the highest errors greater than 20% at Prescott and McGregor, between 10% and 16% at Clinton, St. Louis, Thebes, and Vicksburg, whereas they are less than 10% for the rest of the sites. Despite the differences, the k-s test still fails to reject the null-hypothesis and hence, no big differences are found in the two distributions.  
Figure 4: (**a,c**) show flow duration curves (FDCs) calculated for (i) the entire period (tot), from 2003 to 2019, (ii) for the selected calibration period (cal), from 2003 to 2015, and (iii) for the calibration period adopting an eight-day sampling interval for all the sites analyzed. (**b,d**) columns show differences in discharge between the FDCs built referring to the calibration and total period, and between the FDCs built in the calibration period, but in case of considering daily or eight-day sampling intervals.