# Flow Duration Curves from Surface Reflectance in the Near Infrared Band
## 4 Discussion  
By the analysis of the temporal series of the observed discharges, the k-s test and the plots of Figure 4 underlined that the eight-day sampling guaranteed by MODIS can replace the daily time series without a strong effect on the potential evaluation of FDCs, with only exception for very high flows, as depicted at some stations. In fact, during major events, the dynamic of the river is fast enough to attenuate flow peaks, with the possibility of not being caught having an eight-day sampling. About the choice of the calibration and the validation phases, the distribution of the flood events is quite uniform in time and no substantial differences are found between the calibration (2003-2015) and the total period (2003-2019) of analysis. However, the k-s test  
Figure 9: Errors between estimated and observed discharges in function of the duration. does not allow the FDCs built in the calibration period to be assumed valid for all the periods. It is expected that in the validation phase, the error due to new intense flood events is not completely negligible. The extensive data availability of MODIS appears sufficient to detect the range of variability of the river flows for most of the sites, even though extreme events (e.g., very high or low flows) might never be observed by the satellite due to a number of causes. First, the MYD09A1 product provides an estimate of the surface spectral reflectance selected from all the acquisitions within the eight-day composite on the basis of observation coverage, the absence of clouds or cloud shadow, and solar zenith. This product can be less sensitive to the high/low flows because of the \"average\" value over the eight-day period. It is expected that a daily and higher resolution product can be beneficial for detecting the low flows even though more investigations are necessary for testing different regime conditions and climatic areas. Second, during drought period, the spatial resolution of the satellite product can limit the capability to detect a sensitive variation of the river flows. The stations selected for the study have widths ranging from 200 m to 1 km, and during the period of low flows, the extension of the water surface into the pixel can provide a limited variation of radiance, hence of reflectance. Third, the cloud coverage during the flood events is an obstacle to the monitoring capacity of the satellite. Consequently, the high flows can be rarely observed and as is known, the underestimation of the flows is expected. However, even if not fully exhaustive during the flood events, the wetted conditions of the areas after a flooded event can last more than one day and, perhaps, it can be detected by other missions orbiting in the same area in a successive period. A multi mission approach can be a solution for a more robust and reliable detection of intense events. A larger availability of multispectral products from several satellite platforms, i.e., Envisat MERIS, Sentinel-3 OLCI, Sentinel-2 MSI, and Landsat family, is expected to increase the frequency of the sampling and, hence, the possibility to monitor even extreme events [25]. Concerning the middle flows, the procedure enables a reliable estimation of the hydraulic regime of the river in the area of interest and may provide useful information for the management of the flows. Indeed, as demonstrated by Figures 6 and 7, the procedure is able to cover periods of ground missing data through the exploitation of satellite information. The overall accuracy on the estimation of the river flows is considered sufficient, even if the evaluation of reliable extreme events is probably the most relevant open issue to deal with. The results of Figure 9 show the good performances for the evaluation of the medium flows and this characteristic encourages the use of the RDC for the management of the water resources.