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Title:   Author  item
                                            WILUSZ, D.C. - Johns Hopkins Universityitem
                                            ZAITCHIK, B.F. - Johns Hopkins Universityitem
                                            Anderson, Marthaitem
                                            HAIN, C. - University Of Marylanditem
                                            YILMAZ, M.T. - Collaboratoritem
                                            MLADENOVA, I. - National Aeronautics And Space Administration (NASA)
  Submitted to: Remote Sensing of Environment
                                    Publication Type: Peer Reviewed Journal
                                    Publication Acceptance Date: 6/21/2016
                                    Publication  Date: 3/31/2017
                                    Citation: Wilusz, D., Zaitchik, B., Anderson, M.C., Hain, C., Yilmaz, M., Mladenova, I. 2017. Monthly flooded area classification using low resolution SAR imagery in the Sudd wetland from 2007-2011. Remote Sensing of Environment. 194:205-218.Interpretive Summary: The Sudd Wetland in South Sudan constitutes a major sink of water along the course of the White Nile River.  In order to better characterize the water budget in the transnational Nile River Basin, it is important to be able to quantify the evaporative losses from the Sudd, and to monitor temporal fluctuations in the flooded area associated with this wetland feature.  In this study, we use time series of radar images to map the flooded extent of the Sudd over time, and compare these areal estimates with variability in remote sensing estimates of other components of the water budget.  Radar wavelengths are strongly sensitive to surface water, including standing water under vegetation, making high resolution radar a valuable asset in this study.  Radar maps of flooded extent over a period of 5 years are compared to times eries of evapotranspiration and groundwater fluctuations averaged over the Sudd area, demonstrating good temporal correlation.  This work lays groundwork for water budget analyses in other major global river basins using satellite imaging.
                                Technical Abstract: The annual flood cycle of the Sudd wetland in South Sudan plays an important role in the Nile River Basin water balance. The wetland, however, is extensive and sparsely instrumented, which has inhibited credible understanding of regional flooding across space and time. Here we explore the potential to apply C-band ENVISAT Advanced SAR imagery for remote estimation of Sudd flooded area extent. Over a five year study period (2007-2011) the time-averaged flooded Sudd area was found to be 18,113 km2 with an average annual high of 29,772 km2 in late September and a low of 10,222 km2 in early May. Annual peak flood area ranges considerably from 19,373 km2 in 2009 to 36,708 km2 in 2007, but we found no systematic trend over the five year study period. Derived flood frequency maps identify areas of open water and permanent flooding (12% of total area), seasonal flooding (29%), and intermittent flooding (48%). To evaluate the certainty of our results, we consider their consistency with (1) prior studies, (2) evapotranspiration estimates from the Atmosphere-Land Exchange Inverse (ALEXI) surface energy balance algorithm, (3) watershed storage anomaly estimates from GRACE, (4) supervised classification of open water area using LANDSAT, and (5) a rough measure of water availability (upstream antecedent precipitation). The analyses show reasonable temporal and spatial consistency with available lines of evidence. We conclude that low resolution C-bandSAR imagery shows promise for study of Sudd wetland flood dynamics.


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