# Flow Duration Curves from Surface Reflectance in the Near Infrared Band
## 5 Conclusions  
The scarce availability of long and continuous time series of ground discharges brings the community to enlarge the sources of monitoring englobing the use of satellite remote sensing information. The number of the current satellite missions and their extended spatial and temporal coverage represents a unique opportunity to the global freshwater monitoring of the hydraulic regime, even in poorly gauged areas. The flow duration curves (FDCs) describe the hydraulic regime of the river at specific sites well, providing a cumulative distribution of the flows in terms of percentage of exceedance. Here, we investigated the possibility to describe the hydraulic regime through the reconstruction of FDCs with the use of reflectance ratios from near infrared images, which recently have demonstrated their capability to describe the variability of the river flows with good accuracy [11; 12; 13; 25]. The analysis presented in this work refers to the main course of the Mississippi River, from the upstream section of St. Cloud to the downstream section of Vicksburg, analyzing a total of 13 stations where records of river discharge are available almost for all the analyzed period (2003-2019). We reconstructed the FDCs for the first 13 years and compared them against the reflectance duration curves, RDCs, built from the NIR images of the MODIS product. We validated the FDC for the rest of the four years in terms of temporal river discharges, demonstrating that in most of the cases, the reflectance ratio is able to describe the flow regime at different locations. The good description of the mean river flow for most of the sites highlights the large potential of the NIR sensor to provide precious information at local scale. Higher errors are expected at FDC tails, where very low or high flows cannot be observed by this kind of sensor, due to its passive nature and its medium resolution. Future applications will investigate the use of multi-mission sensors, perhaps referring to additional NIR sensors and active sensors (i.e., radar altimeters) to ensure a denser temporal frequency and a more reliable and accurate estimation of the extreme (especially high) flows. If we merge the observations retrieved from several NIR sensors (e.g., Landsat, MODIS/Terra and MODIS/Aqua, OLCI/Sentinel-3, and MSI/Sentinel-2 currently available), we have the possibility to densify the time series reaching a frequency almost daily (depending on the clouds). In addition, with the use of radar altimetry, it is possible to overcome the problem of the clouds and obtain a dense time series of water level with the use of multiple tracks and different missions (SARAL/AltiKa, Sentinel-3, Sentinel-6, Jason-3, CryoSat-2, and the future SWOT). The joint use of both the sensors (NIR and radar altimeter) is expected to advance our capability to record high and low discharge peaks, which are fundamental to prevent and forecast extreme events of floods and droughs. Moreover, the extended applications to other rivers characterized by different hydrological regimes (i.e., big rivers with strong seasonality) could demonstrate a drastic improvement of the results. Indeed, the attempt shown here represents just one example of how the satellite can be used also for ungauged river basin, providing continuous, reliable, and updated information for the management of the flows. The following are available online at [[https://www.mdpi.com/article/10_3390/app11083458/s1](https://www.mdpi.com/article/10_3390/app11083458/s1)]([https://www.mdpi.com/article/10_3390/app11083458/s1](https://www.mdpi.com/article/10_3390/app11083458/s1)). Conceptualization, A.D.; methodology, A.T.; software, A.T.; validation, A.T. and A.D.; formal analysis, A.T.; investigation, A.T. and A.D.; resources, A.T.; data curation, A.T.; writing--original draft preparation, A.T.; writing--review and editing, A.D.; visualization, A.T.; supervision, A.D. All authors have read and agreed to the published version of the manuscript. This research received no external funding. Not applicable.