Decadal monitoring of the Niger Inner Delta flood dynamics using MODIS optical data

Abstract

International audienceWetlands provide a vital resource to ecosystem services and associated rural livelihoods but their extent, geomorphological heterogeneity and flat topography make the representation of their hydrological functioning complex. A semi automated method exploiting 526 MODIS (Moderate Resolution Imaging Spectroradiometer) 8-day 500 m resolution images was developed to study the spatial and temporal dynamics of the annual flood across the Niger Inner Delta over the period 2000–2011. A composite band ratio index exploiting the Modified Normalised Difference Water Index (MNDWI) and Normalised Difference Moisture Index (NDMI) with fixed thresholds provided the most accurate detection of flooded areas out of six commonly used band ratio indices. K-means classified Landsat images were used to calibrate the thresholds. Estimated flooded surface areas were evaluated against additional classified Landsat images, previous studies and field stage data for a range of hydrological units: river stretches, lakes, floodplains and irrigated areas. This method illustrated how large amounts of MODIS images may be exploited to monitor flood dynamics with adequate spatial and temporal resolution and good accuracy, except during the flood rise due to cloud presence. Previous correlations between flow levels and flooded areas were refined to account for the hysteresis as the flood recedes and for the varying amplitude of the flood. Peak flooded areas varied between 10 300 km2 and 20 000 km2, resulting in evaporation losses ranging between 12 km3 and 21 km3. Direct precipitation assessed over flooded areas refined the wetland’s water balance and infiltration estimates. The knowledge gained on the timing, duration and extent of the flood across the wetland and in lakes, floodplains and irrigated plots may assist farmers in agricultural water management. Furthermore insights provided on the wetland’s flood dynamics may be used to develop and calibrate a hydraulic model of the flood in the Niger Inner Delta

Thermal Imagery-Derived Surface Inundation Modeling to Assess Flood Risk in a Flood-Pulsed Savannah Watershed in Botswana and Namibia

“…During years with high discharge (2009)(2010)(2011)(2012), it is observed that the floodplains remain inundated for extended periods of time; long after the river discharge has peaked (Figure 15). Based on the ideas of Ogilvie et al [48], the floodwater recedes from the floodplains at a slower rate than water within the channels because it is trapped in topographic depressions [48]. This can be observed in both the Zambezi and Chobe Rivers in respect to their corresponding floodplains.…”

Characteristics and Factors Influencing the Hysteresis of Water Area–Stage Curves for Poyang Lake

“…Water 2017, 9, 938 2 of 20 Imaging Spectroradiometer (MODIS) optical data to research the spatiotemporal dynamics of the Niger Inner Delta flood [3]. In their study, water area-stage relationships were refined by accounting for hysteresis behavior.…”

“…Fortunately, as another feature of highly non-linear hydrologic systems of lake floodplains, research on hysteresis found between discharge levels and stages has been precise and long-standing and may provide a reference. Factors affecting stage-discharge relationships proposed by Boyer [16] are as follows: (1) the unsteadiness of river flows, which alter the slope of the water surface; (2) backwater effects that change downstream environments; (3) vegetation that affects bed roughness levels; (4) bed evolution as a result of dredging or sediment transport, etc.…”

“…Lake floodplains are vital for ecosystems and humanity, serving as important habitats for wetland biota and providing many services (e.g., flood protection and agriculture and fishing resources) [1][2][3]. As a key indicator used to evaluate a lake's ecosystem status and evolution, the lake inundation area has attracted much attention [4,5].…”

Big Data and Multiple Methods for Mapping Small Reservoirs: Comparing Accuracies for Applications in Agricultural Landscapes

“…Ground-based assessments of small reservoir locations, capacities, and seasonal volumes are time-consuming to conduct because of the spatial dispersion of these reservoirs and decentralized decision making regarding reservoir investments and maintenance [12]. Many previous studies have characterized inland surface water resources using free satellite imagery, such as from Landsat [13][14][15][16][17] and the Moderate Resolution Imaging Spectroradiometer (MODIS) [18][19][20][21][22]. Where the spatial and temporal resolution are sufficiently high, remotely sensed imagery provides a practical approach to small water body mapping and monitoring [23,24].…”