{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,4,8]],"date-time":"2026-04-08T23:11:23Z","timestamp":1775689883392,"version":"3.50.1"},"reference-count":30,"publisher":"MDPI AG","issue":"21","license":[{"start":{"date-parts":[[2024,10,29]],"date-time":"2024-10-29T00:00:00Z","timestamp":1730160000000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"DOI":"10.13039\/100018693","name":"Horizon Europe","doi-asserted-by":"publisher","award":["101081783"],"award-info":[{"award-number":["101081783"]}],"id":[{"id":"10.13039\/100018693","id-type":"DOI","asserted-by":"publisher"}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Remote Sensing"],"abstract":"<jats:p>The ICESat-2 and SWOT satellite earth observation missions have provided highly accurate water surface slope (WSS) observations in global rivers for the first time. While water surface slope is expected to remain constant in time for approximately uniform flow conditions, we observe time varying water surface slope in many river reaches around the globe in the ICESat-2 record. Here, we investigate the causes of time variability of WSSs using simplified river hydraulic models based on the theory of steady, gradually varied flow. We identify bed slope or cross section shape changes, river confluences, flood waves, and backwater effects from lakes, reservoirs, or the ocean as the main non-uniform hydraulic situations in natural rivers that cause time changes of WSSs. We illustrate these phenomena at selected river sites around the world, using ICESat-2 data and river discharge estimates. The analysis shows that WSS observations from space can provide new insights into river hydraulics and can enable the estimation of river discharge from combined observations of water surface elevation and WSSs at sites with complex hydraulic characteristics.<\/jats:p>","DOI":"10.3390\/rs16214010","type":"journal-article","created":{"date-parts":[[2024,10,29]],"date-time":"2024-10-29T04:51:33Z","timestamp":1730177493000},"page":"4010","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":2,"title":["Hydraulics of Time-Variable Water Surface Slope in Rivers Observed by Satellite Altimetry"],"prefix":"10.3390","volume":"16","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-9861-4240","authenticated-orcid":false,"given":"Peter","family":"Bauer-Gottwein","sequence":"first","affiliation":[{"name":"Department of Environmental and Resource Engineering, Technical University of Denmark, 2800 Kongens Lyngby, Denmark"},{"name":"Department of Geosciences and Natural Resource Management, University of Copenhagen, 1958 Frederiksberg, Denmark"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-2961-5925","authenticated-orcid":false,"given":"Linda","family":"Christoffersen","sequence":"additional","affiliation":[{"name":"National Space Center, Technical University of Denmark, 2800 Kongens Lyngby, Denmark"}]},{"ORCID":"https:\/\/orcid.org\/0009-0006-8869-1484","authenticated-orcid":false,"given":"Aske","family":"Musaeus","sequence":"additional","affiliation":[{"name":"Department of Environmental and Resource Engineering, Technical University of Denmark, 2800 Kongens Lyngby, Denmark"},{"name":"DHI, 2970 H\u00f8rsholm, Denmark"}]},{"given":"Monica Coppo","family":"Fr\u00edas","sequence":"additional","affiliation":[{"name":"Department of Environmental and Resource Engineering, Technical University of Denmark, 2800 Kongens Lyngby, Denmark"}]},{"given":"Karina","family":"Nielsen","sequence":"additional","affiliation":[{"name":"National Space Center, Technical University of Denmark, 2800 Kongens Lyngby, Denmark"}]}],"member":"1968","published-online":{"date-parts":[[2024,10,29]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"319","DOI":"10.1016\/j.asr.2021.01.022","article-title":"Altimetry for the Future: Building on 25 Years of Progress","volume":"68","author":"Abdalla","year":"2021","journal-title":"Adv. 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