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1 Pekel, J.-F.; Cottam, A.; Gorelick, N.; Belward, A. S. 2016. High-resolution mapping of global surface water and its long-term changes. Nature, 540(7633):418-422. [doi: https://doi.org/10.1038/nature20584]
Surface water ; Mapping ; Satellite imagery ; Landsat ; Earth observation satellites ; Water distribution ; Geographical distribution ; Seasonal variation ; Expert systems ; Climate change ; Hydrology ; Models ; Drought ; Evaporation ; Human behavior ; Lakes ; Plateaus / Central Asia / USA / Australia / Aral Sea / Tibetan plateau
(Location: IWMI HQ Call no: e-copy only Record No: H047905)
(8.75 MB)
The location and persistence of surface water (inland and coastal) is both affected by climate and human activity1 and affects climate2,3 , biological diversity4 and human wellbeing5,6 . Global data sets documenting surface water location and seasonality have been produced from inventories and national descriptions7 , statistical extrapolation of regional data8 and satellite imagery9–12, but measuring long-term changes at high resolution remains a challenge. Here, using three million Landsat satellite images13, we quantify changes in global surface water over the past 32 years at 30-metre resolution. We record the months and years when water was present, where occurrence changed and what form changes took in terms of seasonality and persistence. Between 1984 and 2015 permanent surface water has disappeared from an area of almost 90,000 square kilometres, roughly equivalent to that of Lake Superior, though new permanent bodies of surface water covering 184,000 square kilometres have formed elsewhere. All continental regions show a net increase in permanent water, except Oceania, which has a fractional (one per cent) net loss. Much of the increase is from reservoir filling, although climate change14 is also implicated. Loss is more geographically concentrated than gain. Over 70 per cent of global net permanent water loss occurred in the Middle East and Central Asia, linked to drought and human actions including river diversion or damming and unregulated withdrawal15,16. Losses in Australia17 and the USA18 linked to long-term droughts are also evident. This globally consistent, validated data set shows that impacts of climate change and climate oscillations on surface water occurrence can be measured and that evidence can be gathered to show how surface water is altered by human activities. We anticipate that this freely available data will improve the modelling of surface forcing, provide evidence of state and change in wetland ecotones (the transition areas between biomes), and inform water-management decision-making.
2 Donchyts, G.; Winsemius, H.; Baart, F.; Dahm, R.; Schellekens, J.; Gorelick, N.; Iceland, C.; Schmeier, S. 2022. High-resolution surface water dynamics in earth’s small and medium-sized reservoirs. Scientific Reports, 12:13776. [doi: https://doi.org/10.1038/s41598-022-17074-6]
Water reservoirs ; Surface water ; Water resources ; Dams ; Satellite imagery ; Monitoring ; Transboundary waters ; Conflicts ; Water levels ; Water scarcity ; Early warning systems ; Remote sensing ; Food insecurity
(Location: IWMI HQ Call no: e-copy only Record No: H051409)
(3.47 MB) (3.47 MB)
Small and medium-sized reservoirs play an important role in water systems that need to cope with climate variability and various other man-made and natural challenges. Although reservoirs and dams are criticized for their negative social and environmental impacts by reducing natural flow variability and obstructing river connections, they are also recognized as important for social and economic development and climate change adaptation. Multiple studies map large dams and analyze the dynamics of water stored in the reservoirs behind these dams, but very few studies focus on small and medium-sized reservoirs on a global scale. In this research, we use multi-annual multi-sensor satellite data, combined with cloud analytics, to monitor the state of small (10–100 ha) to medium-sized (> 100 ha, excluding 479 large ones) artificial water reservoirs globally for the first time. These reservoirs are of crucial importance to the well-being of many societies, but regular monitoring records of their water dynamics are mostly missing. We combine the results of multiple studies to identify 71,208 small to medium-sized reservoirs, followed by reconstructing surface water area changes from satellite data using a novel method introduced in this study. The dataset is validated using 768 daily in-situ water level and storage measurements (r2 > 0.7 for 67% of the reservoirs used for the validation) demonstrating that the surface water area dynamics can be used as a proxy for water storage dynamics in many cases. Our analysis shows that for small reservoirs, the inter-annual and intra-annual variability is much higher than for medium-sized reservoirs worldwide. This implies that the communities reliant on small reservoirs are more vulnerable to climate extremes, both short-term (within seasons) and longer-term (across seasons). Our findings show that the long-term inter-annual and intra-annual changes in these reservoirs are not equally distributed geographically. Through several cases, we demonstrate that this technology can help monitor water scarcity conditions and emerging food insecurity, and facilitate transboundary cooperation. It has the potential to provide operational information on conditions in ungauged or upstream riparian countries that do not share such data with neighboring countries. This may help to create a more level playing field in water resource information globally.
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