Rivers originating in High Mountain Asia are crucial lifelines for one-third of the world’s population. These fragile headwaters are now experiencing amplified climate change, glacier melt, and ...permafrost thaw. Observational data from 28 headwater basins demonstrate substantial increases in both annual runoff and annual sediment fluxes across the past six decades. The increases are accelerating from the mid-1990s in response to a warmer and wetter climate. The total sediment flux from High Mountain Asia is projected to more than double by 2050 under an extreme climate change scenario. These findings have far-reaching implications for the region’s hydropower, food, and environmental security.
In the event of a flood disaster, first response agencies need inundation maps produced in near real time (NRT). Such maps can be generated using satellite-based information. In this study, we ...developed mapping techniques that rely on synthetic aperture radar (SAR) on-board earth-orbiting platforms. SAR provides valid ground surface measurements through cloud cover with high resolution and sampling frequency that has recently increased through multiple missions. Despite numerous efforts, automatic processing of SAR data to derive accurate inundation maps still poses challenges. To address them, we have developed an NRT system named RAdar-Produced Inundation Diary (RAPID). RAPID integrates four processing steps: classification based on statistics, morphological processing, multi-threshold-based compensation, and machine-learning correction. Besides SAR data, the system integrates multisource remote-sensing data products, including land cover classification, water occurrence, hydrographical, water type, and river width products. In comparison to expert handmade flood maps, the fully-automated RAPID system exhibited “overall,” “producer,” and “user” accuracies of 93%, 77%, and 75%, respectively. RAPID accommodates commonly encountered over- and under-detections caused by noise-like speckle, water-like radar response areas, strong scatterers, and isolated inundation areas—errors that are in common practice to ignore, mask out, or be filtered out by coarsening the effective resolution. RAPID can serve as the kernel algorithm to derive flood inundation products from satellites—both existing and to be launched—equipped with high-resolution SAR sensors, including Envisat, Radarsat, NISAR, Advanced Land Observation Satellite (ALOS)-1/2, Sentinel-1, and TerraSAR-X.
•We built a fully automated system to retrieve inundation maps from high resolution SAR images.•Statistical, morphological and machine learning approaches are integrated.•The system extensively uses multi-source remote sensing ancillary data.•The system can detect both connected and isolated open flood areas.•The system shown reliable performance in state scale during multiple flood events.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Approximately 40% of the Tibetan Plateau (TP) is underlain by continuous permafrost, yet its impact on fluvial water and sediment dynamics remains poorly investigated. Here we show that water and ...sediment dynamics in the permafrost‐dominated Tuotuohe basin on the TP are driven by air temperature and permafrost thaw, based on 33‐year daily in situ observations (1985–2017). Air temperature regulates the seasonal patterns of discharge and suspended sediment concentration (SSC) by controlling the changes in active contributing drainage area (ACDA, the unfrozen erodible landscape that contributes hydrogeomorphic processes within a catchment) and governing multiple thermal processes such as glacier‐snow melt and permafrost thaw. Rainstorms determine the short‐lived fluvial extreme events by intensifying slope processes and channel erosion and likely also by enhancing thaw slumps. Furthermore, the SSCs at equal levels of discharges are lower in autumn (September–October) than in spring (May–June) and summer (July–August). This reduced sediment availability in autumn can possibly be attributed to the increased supra‐permafrost groundwater runoff and the reduced surface runoff and erosion. Due to rapid climate warming, the ACDA has increased significantly from 1985 to 2017, implying expanding erodible landscapes for hydrogeomorphic processes. As a result, the fluvial water and sediment fluxes have substantially increased. In a warmer and wetter future for the TP, the fluvial sediment fluxes of similar permafrost‐underlain basins will continue to increase with expanding erodible landscapes and intensifying thermal and pluvial‐driven geomorphic processes. Thus, permafrost thaw should be considered as an important driver of past and future water and sediment changes for the TP.
Key Points
Climate warming expands the erodible landscape and increases fluvial water and sediment fluxes on the Tibetan Plateau
Air temperature regulates seasonal discharge and sediment dynamics by controlling glacier‐snow melt and permafrost processes
Increasing rainstorms results in more frequent fluvial extreme events
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Here we provide global estimates of the seasonal flux of sediment, on a river-by-river basis, under modern and prehuman conditions. Humans have simultaneously increased the sediment transport by ...global rivers through soil erosion (by 2.3 ± 0.6 billion metric tons per year), yet reduced the flux of sediment reaching the world's coasts (by 1.4 ± 0.3 billion metric tons per year) because of retention within reservoirs. Over 100 billion metric tons of sediment and 1 to 3 billion metric tons of carbon are now sequestered in reservoirs constructed largely within the past 50 years. African and Asian rivers carry a greatly reduced sediment load; Indonesian rivers deliver much more sediment to coastal areas.
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BFBNIB, NMLJ, NUK, PNG, SAZU, UL, UM, UPUK
Accelerated glacier‐snow‐permafrost erosion due to global warming amplifies the sediment availability in cold environments and affects the time‐varying suspended sediment concentration (SSC) and ...discharge (Q) relationship. Here, the sediment‐availability‐transport (SAT) model is proposed to simulate dynamic SSC‐Q relationships by integrating the sediment availability coupled by thermal processes, fluvial processes and long‐term storage exhaustion into a sediment rating curve (SSC = a × Qb with a and b as fitting parameters). In the SAT‐model, increased sediment sources from glacier‐snow‐permafrost erosion are captured by changes in basin temperature, showing an exponential amplification of SSC when basin temperature increases. Enhanced fluvial erosion by the elevated water supply from rainfall and meltwater is captured by the factor of runoff surge, which results in a linear amplification of SSC. The SAT‐model is validated for the permafrost‐dominated Tuotuohe basin on Tibetan Plateau utilizing multi‐decadal daily SSC/Q in‐situ observations (1985–2017). Results show that sediment rating curves for Tuotuohe display significant inter‐annual variations. The higher parameter‐b in a warmer and wetter climate confirms the increased sediment availability due to the expanded erodible landscapes and gullying‐enhanced connectivity between channels and slopes. Through capturing such time‐varying sediment availability, the SAT‐model can robustly reproduce the long‐term evolution, seasonality, and various event‐scale hysteresis of SSC, including clockwise, counter‐clockwise, figure‐eight, counter‐figure‐eight, and more complex hysteresis loops. Overall, the SAT‐model can explain over 75% of long‐term SSC variance with stable performance under hydroclimate abrupt changes, outperforming the conventional and static sediment rating curve approach by 20%. The SAT‐model not only advances understanding of sediment transport mechanisms by integrating thermal‐ and fluvial‐erosion processes, but also provides a model framework to simulate and project future sediment loads in other cold basins.
Key Points
Glacier‐snow‐permafrost melting elevates sediment availability by enlarging erodible landscapes and enhancing channel‐slope connectivity
A sediment‐availability‐transport (SAT)‐model is developed to simulate dynamic suspended sediment concentration (SSC)‐discharge relations by integrating sediment availability controlled by thermal/fluvial processes
The SAT‐model can robustly reproduce the long‐term evolution, seasonality and various event‐scale hysteresis of SSC for cold basins
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Floods are among the top-ranking natural disasters in terms of annual cost in insured and uninsured losses. Since high-impact events often cover spatial scales that are beyond traditional regional ...monitoring operations, remote sensing, in particular from satellites, presents an attractive approach. Since the 1970s, there have been many studies in the scientific literature about mapping and monitoring of floods using data from various sensors onboard different satellites. The field has now matured and hence there is a general consensus among space agencies, numerous organizations, scientists, and end-users to strengthen the support that satellite missions can offer, particularly in assisting flood disaster response activities. This has stimulated more research in this area, and significant progress has been achieved in recent years in fostering our understanding of the ways in which remote sensing can support flood monitoring and assist emergency response activities. This paper reviews the products and services that currently exist to deliver actionable information about an ongoing flood disaster to emergency response operations. It also critically discusses requirements, challenges and perspectives for improving operational assistance during flood disaster using satellite remote sensing products.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
We analyze 4000-year flood history of the lower Yellow River and the history of agricultural development in the middle river by investigating historical writings and quantitative time series data of ...environmental changes in the river basin. Flood dynamics are characterized by positive feedback loops, critical thresholds of natural processes, and abrupt transitions caused by socio-economic factors. Technological and organizational innovations were dominant driving forces of the flood history. The popularization of iron plows and embankment of the lower river in the 4th century BC initiated a positive feedback loop on levee breaches. The strength of the feedback loop was enhanced by farming of coarse-sediment producing areas, steep hillslope cultivation, and a new river management paradigm, and finally pushed the flood frequency to its climax in the seventeenth century. The co-evolution of river dynamics and Chinese society is remarkable, especially farming and soil erosion in the middle river, and central authority and river management in the lower river.
Establishing a quantitative description of global riverine fluxes is one of the main goals of contemporary hydrology and geomorphology. Here we study changes in global riverine water discharge and ...suspended sediment flux over a 50-year period, 1960–2010, applying a new version of the WBMsed (WBMsed v.2.0) global hydrological water balance model. A new floodplain component is introduced to better represent water and sediment dynamics during periods of overbank discharge. Validated against data from 16 globally distributed stations, WBMsed v.2.0 simulation results show considerable improvement over the original model. Normalized departure from an annual mean is used to quantify spatial and temporal dynamics in both water discharge and sediment flux. Considerable intra-basin variability in both water and sediment discharge is observed for the first time in different regions of the world. Continental-scale analysis shows considerable variability in water and sediment discharge fluctuations both in time and between continents. A correlation analysis between predicted continental suspended sediment and water discharge shows strong correspondence in Australia and Africa (R2 of 0.93 and 0.87 respectively), moderate correlation in North and South America (R2 of 0.64 and 0.73 respectively) and weak correlation in Asia and Europe (R2 of 0.35 and 0.24 respectively). We propose that yearly changes in intra-basin precipitation dynamics explain most of these differences in continental water discharge and suspended sediment correlation. The mechanism proposed and demonstrated here (for the Ganges, Danube and Amazon Rivers) is that regions with high relief and soft lithology will amplify the effect of higher than average precipitation by producing an increase in sediment yield that greatly exceeds increase in water discharge.
•New version of the WBMsed global hydrology model introduces a floodplain component.•Considerable improvement in daily suspended sediment flux predictions is reported.•Intra-basin precip. dynamics explain differences between sediment and water discharge.•Intra-basin lithology and relief distribution can enhance or dampen precip. dynamics.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Recent discharge observations are lacking for most rivers globally. Discharge can be estimated from remotely sensed floodplain and channel inundation area, but there is currently no method that can ...be automatically extended to many rivers. We examined whether automated monitoring is feasible by statistically relating inundation estimates from moderate to coarse (>0.05°) resolution remote sensing to monthly station discharge records. Inundation extents were derived from optical MODIS data and passive microwave sensors, and compared to monthly discharge records from over 8000 gauging stations and satellite altimetry observations for 442 reaches of large rivers. An automated statistical method selected grid cells to construct “satellite gauging reaches” (SGRs). MODIS SGRs were generally more accurate than passive microwave SGRs, but there were complementary strengths. The rivers widely varied in size, regime, and morphology. As expected performance was low (R < 0.7) for many (86%), often small or regulated, rivers, but 1263 successful SGRs remained. High monthly discharge variability enhanced performance: a standard deviation of 100–1000 m3 s−1 yielded ca. 50% chance of R > 0.6. The best results (R > 0.9) were obtained for large unregulated lowland rivers, particularly in tropical and boreal regions. Relatively poor results were obtained in arid regions, where flow pulses are few and recede rapidly, and in temperate regions, where many rivers are modified and contained. Provided discharge variations produce clear changes in inundated area and gauge records are available for part of the satellite record, SGRs can retrieve monthly river discharge values back to around 1998 and up to present.
Key Points
Automated method to locate and construct satellite gauging reaches
Valuable to gap‐fill and extend monthly station discharge records
Satellite river discharge gauging is currently feasible for many rivers globally
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Remote sensing analysis is routinely used to map flooding
extent either retrospectively or in near-real time. For flood emergency
response, remote-sensing-based flood mapping is highly valuable as it ...can
offer continued observational information about the flood extent over large
geographical domains. Information about the floodwater depth across the
inundated domain is important for damage assessment, rescue, and
prioritizing of relief resource allocation, but cannot be readily estimated from
remote sensing analysis. The Floodwater Depth Estimation Tool (FwDET) was
developed to augment remote sensing analysis by calculating water depth
based solely on an inundation map with an associated digital elevation model
(DEM). The tool was shown to be accurate and was used in flood response
activations by the Global Flood Partnership. Here we present a new version
of the tool, FwDET v2.0, which enables water depth estimation for coastal
flooding. FwDET v2.0 features a new flood boundary identification scheme
which accounts for the lack of confinement of coastal flood domains at the
shoreline. A new algorithm is used to calculate the local floodwater
elevation for each cell, which improves the tool's runtime by a factor of 15
and alleviates inaccurate local boundary assignment across permanent water
bodies. FwDET v2.0 is evaluated against physically based hydrodynamic
simulations in both riverine and coastal case studies. The results show good
correspondence, with an average difference of 0.18 and 0.31 m for the
coastal (using a 1 m DEM) and riverine (using a 10 m DEM) case studies,
respectively. A FwDET v2.0 application of using remote-sensing-derived flood
maps is presented for three case studies. These case studies showcase FwDET v2.0 ability to efficiently provide a synoptic assessment of floodwater.
Limitations include challenges in obtaining high-resolution DEMs and
increases in uncertainty when applied for highly fragmented flood inundation
domains.
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