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Forecasting flood and drought events requires accurate modeling tools. Hydraulic river models are based on estimates of riverbed geometry which are traditionally collected in situ. The novel Ice, Cloud and Land Elevation Satellite 2 [ICESat‐2] lidar altimetry mission with 6 simultaneous high‐resolution laser beams provides the opportunity to define river cross‐section geometries as well as observe water surface elevation [WSE] and water surface slope spatially resolved along the river chainage. This paper describes a method to utilize terrain altimetry and water surface slope estimates to define complete river geometries from ICESat‐2 data products, using the diffusive wave approximation to calculate depth in the submerged section not penetrated by the lidar. Exemplifying the method, cross‐sections are defined for a stretch of the Mekong River. Hydrodynamic model results of the stretch are compared with ICESat‐2 WSE estimates and in situ gauging station time series. Insights in river characteristics from satellite imagery and the ICESat‐2 slope estimates allow for fine‐tuning of the cross‐sections using spatially varying Manning numbers. The final model achieves a root mean square error against the ICESat‐2 WSE of 0.676 m and average Kling‐Gupta Efficiency against gauging station time series of 0.880. The method is limited by the diffusive wave approximation resulting in inaccurate cross‐section estimates in sections with supercritical flow or significant acceleration. Errors can be identified from ICESat‐2 WSE estimates and reduced with additional cross‐sections. Combined with hydrological models, the method will allow for cross‐section definition without in situ data. The depth and width of the river channel are important factors when seeking to predict floods. To predict water level in a river, computer models for flood forecasting must be informed with river channel geometry at cross‐sections along the river. Such cross‐sections are traditionally measured in the field which is expensive and time consuming. This paper describes a method to estimate river cross‐sectional shape from land and water height measurements from the satellite ICESat‐2. When the satellite path crosses the river, the precise laser instrument onboard outlines the surface to indicate the river channel shape, but it does not penetrate the water. We found that it was possible to estimate the shape of the submerged part of the cross‐section using the water surface slope obtained from the satellite's unique instrument. Evaluating the method on a stretch of the Mekong River, we were able to model water level along the river with accuracy sufficient for flood forecasting. In some sections, where the flow speed changes quickly, a hydraulic model with the defined cross‐sections does not reproduce observed water levels. Using satellite crossings from days with steadier flow reduces the error. The method allows for building hydraulic river models without cross‐section surveys. Land elevation and water surface slope estimates from the ICESat‐2 lidar altimetry mission are combined to define river cross‐sectionsRMSE of 0.676 m and KGE of 0.88 is achieved for a hydrodynamic model of a stretch of the Mekong River using the derived cross‐sectionsSections with significant flow acceleration lead to inaccurate cross‐sections, but errors are reduced with additional ICESat‐2 crossings Land elevation and water surface slope estimates from the ICESat‐2 lidar altimetry mission are combined to define river cross‐sections RMSE of 0.676 m and KGE of 0.88 is achieved for a hydrodynamic model of a stretch of the Mekong River using the derived cross‐sections Sections with significant flow acceleration lead to inaccurate cross‐sections, but errors are reduced with additional ICESat‐2 crossings

This study applies a hydroeconomic optimization method for water resources management in the highly water stressed Haihe River basin. A multi-objective, multi-temporal deterministic hydroeconomic optimization model has been built to quantify the economic trade-offs and reveal “minimum cost strategies” when reducing groundwater abstraction to sustainable levels. A complex basin representation, with ∼140,000 decision variables is formulated where each decision variable represents a flow-path from a water source to a sink. Available water sources are runoff generated by the sub-basins upstream the nine major surface water reservoirs, the inter-basin transfers from Yellow River and South to North Water Transfer Project (SNWTP) and the natural groundwater recharge to the three main groundwater aquifers. Water demands, i.e. sinks, are aggregated for each model sub-basin in categories of the major agricultural users, domestic, industrial and ecological water demands. Each demand is associated with a curtailment cost and groundwater abstraction with a pumping cost. Groundwater overdraft is constrained in each model scenario, ranging from unlimited overdraft in the plain area groundwater aquifer to sustainable abstractions over an 8-year period. Inflow upstream Yuqiao reservoir, and the inter-basin transfers from SNWTP and Yellow River are identified as the water resources with the highest increase in average shadow prices when limiting groundwater overdraft. An increase in inflow shadow prices of 37.5% indicates that these water sources will be most valuable if sustainable groundwater abstraction should be achieved. The shadow prices of water sources reveal when and where in the Haihe River basin users are curtailed if water resources are managed in the most optimal way. Average shadow prices of 1.6 yuan/m3for all surface water sources in the sustainable abstraction scenarios shows that overdraft can be avoided by curtailment of users with a willingness-to-pay ≤1.6 yuan/m3. The shadow prices of the existing surface water reservoirs represented in the model shows that no costs can be saved from expanding their capacities. Finally, the cost of achieving sustainable groundwater abstraction with present water resource availability is found to be minimum 8.2 billion yuan/year.

With increasing coverage, density, and accuracy of the global inland water altimetry record, remote sensing observations of water surface elevation (WSE) and water surface slope (WSS) are becoming available for the world's rivers. In steady, uniform flows, WSS is invariable, while there is a unique one‐to‐one relationship between WSE and discharge, the rating curve. While the assumptions of steady uniform flow are appropriate for many rivers, they are violated upstream of river confluences. We present a simple analytical hydraulic model of river confluences using the theory of steady, gradually varied flow. We apply the model to four river confluences in the Mississippi‐Missouri river system. We determine the spatial extent of the backwater‐affected zones and map WSE‐discharge and WSS‐discharge relationships. We show that coincident measurements of WSE and WSS from new satellite altimetry missions effectively constrain discharge estimates from space in the backwater‐affected zones upstream of river confluences. New satellite altimetry missions monitor water levels in global rivers at unprecedented spatio‐temporal resolution and accuracy. One principal objective of this monitoring effort is to map river flow in space and time, understand the impacts of climatic change on river flows, and provide river flow estimates for practical water resources applications. In many locations and situations, there is a one‐to‐one relationship between river flow and river water level. This so‐called rating curve can be used to directly translate water level observations into river flow estimates. However, the concept of the rating curve breaks down whenever the flow in the river is significantly different from steady uniform flow, which is the flow occurring in a long and uniform river reach with constant boundary inflow. Upstream of river confluences, conditions are often non‐uniform, because high flow in one tributary can coincide with low flow in the other, leading to significant backwater effects in both tributaries. This study models water level and water surface slope dynamics upstream of river junctions and highlights the value that data sets from new satellite missions provide for understanding the hydraulics around river confluences and for estimating flow upstream of river confluences from space. The classic stage‐discharge rating curve concept fails upstream of river confluences because of backwater effectsHydraulic modeling confirms that unique stage‐slope‐discharge relationships exist upstream of river confluencesNew satellite altimetry observations of river stage and slope reveal stage‐slope‐discharge relationships for selected river confluences The classic stage‐discharge rating curve concept fails upstream of river confluences because of backwater effects Hydraulic modeling confirms that unique stage‐slope‐discharge relationships exist upstream of river confluences New satellite altimetry observations of river stage and slope reveal stage‐slope‐discharge relationships for selected river confluences

The geological and morphological evolution of the Kalahari Basin of Southern Africa has given rise to a complex hydrogeological regime that is affected by water quality issues. Among these concerns is the occurrence of saline groundwater. Airborne and ground‐based electromagnetic surveying is an efficient tool for mapping groundwater quality variations and has been used extensively to explore the Kalahari sediments, e.g., in Botswana and Namibia. Recently, airborne and ground‐based mapping of groundwater salinity was conducted in the Machile–Zambezi Basin, southwestern Zambia, using the versatile time‐domain electromagnetic system and WalkTEM system, respectively, incorporating earlier ground‐based ProTEM 47D measurements. The data were inverted using the laterally constrained inversion technique followed by a separate spatially constrained inversion scheme. WalkTEM data were inverted as ordinary single‐site one‐dimensional inversions. The regional electrical resistivity signature of the Machile–Zambezi Basin was found to be characterized by high elevation (1000 m–1050 m above mean sea level), high electrical resistivity (above 100 Ωm) areas that form the western and eastern boundaries of a low‐resistivity (below 13 Ωm) valley that extends southwestwards into the Makgadikgadi salt pans. The electrical resistivity distribution is indicative of a full graben related to the Okavango–Linyati Fault system as a result of propagation of the East African Rift Valley System into Southern Africa. The saline lacustrine sediments infilling the Machile Graben are responsible for the low formation resistivity (below 13 Ωm) and high salinity (above 7000 μS/cm) observed in the groundwater and are probably related to the complex evolutionary history of Palaeo‐Lake Makgadikgadi.

Karst aquifers represent important but very vulnerable sources for water supply to a significant part of the Earth’s population. For sustainable use of these resources, development of integrated management tools based on numerical groundwater models is required. In principle karst aquifers are characterized by the presence of two distinct flow domains: the limestone matrix fractures and the conduits. A flow model of karst aquifers requires detailed, spatially distributed information on the hydrologic characteristics of the aquifer and flow paths. Geophysical methods determining the distribution of the electrical resistivities within the subsurface could provide such information. An international scientific research project was initiated to explore the potential of airborne electromagnetic mapping for providing such innovative information for improving groundwater modelling of karst aquifers. The project study area is located in the Sian Ka’an Biosphere Reserve located in Yucatán, Mexico, a coastal wetland of international importance. As a first step ground geoelectric and ground electromagnetic measurements were performed in March 2006 to determine the electrical properties of the Sian Ka’an Biosphere Reserve subsurface environment. These results were used for 3D forward modelling to calculate the expected airborne electromagnetic response. Based on these promising results, an airborne pilot survey was performed in 2007 to evaluate the applicability of airborne electromagnetic methodology. This survey covers an area of 40 square kilometres above the well‐mapped Ox Bel Ha cave system. The results showed that the signature of the cave system could be clearly detected. The pilot survey offered as well the chance to define the limits of current state‐of‐the‐art airborne data acquisition and inversion. The study helped to define the needs for further developments and improvements to establish the frequency domain electromagnetic method as a practical karst exploration method.

Geodetic altimeters provide unique observations of the river surface longitudinal profile due to their long repeat periods and densely spaced ground tracks. This information is valuable for calibrating hydraulic model parameters, and thus, for producing reliable simulations of water level for flood forecasting and river management, particularly in poorly instrumented catchments. In this study, we present an efficient calibration approach for hydraulic models based on a steady‐state hydraulic solver and CryoSat‐2 observations. In order to ensure that only coherent forcing/observation pairs are considered in the calibration, we first propose an outlier filtering approach for CryoSat‐2 observations in data‐scarce regions using a simulated runoff produced by a hydrologic model. In the hydraulic calibration, a steady‐state solver computes the water surface elevation (WSE) profile along the river for selected discharges corresponding to the days of CryoSat‐2 overpass. In synthetic calibration experiments, the global search algorithm generally recovers the true parameter values in portions of the river where observations are available, illustrating the benefit of dense spatial sampling from geodetic altimetry. The most sensitive parameters are the bed elevations. In calibration experiments with real CryoSat‐2 data, validation performance against both Sentinel‐3 WSE and in situ records is similar to previous studies, with Root Mean Square Deviation ranging from 0.43 to 1.14 m against Sentinel‐3 and from 0.60 to 0.73 against in situ WSE observations. Performance remains similar when transferring parameters to a one‐dimensional hydrodynamic model. Because the approach is computationally efficient, model parameters can be inverted at high spatial resolution to fully exploit the information contained in geodetic CryoSat‐2 altimetry. We use satellite altimetry observations from CryoSat‐2 and a steady‐state solver to calibrate hydraulic model parametersWe develop an outlier filtering method for CryoSat‐2 observations in ungauged catchments based on rainfall‐runoff model simulationsWe integrate the altimetry observations in an efficient global calibration approach at low cost compared to a one‐dimensional hydrodynamic model We use satellite altimetry observations from CryoSat‐2 and a steady‐state solver to calibrate hydraulic model parameters We develop an outlier filtering method for CryoSat‐2 observations in ungauged catchments based on rainfall‐runoff model simulations We integrate the altimetry observations in an efficient global calibration approach at low cost compared to a one‐dimensional hydrodynamic model

Secular polar drift underwent a directional change in the 1990s, but the underlying mechanism remains unclear. In this study, polar motion observations are compared with geophysical excitations from the atmosphere, oceans, solid Earth, and terrestrial water storage (TWS) during the period of 1981–2020 to determine major drivers. When contributions from the atmosphere, oceans, and solid Earth are removed, the residual dominates the change in the 1990s. The contribution of TWS to the residual is quantified by comparing the hydrological excitations from modeled TWS changes in two different scenarios. One scenario assumes that the TWS change is stationary over the entire study period, and another scenario corrects the stationary result with actual glacier mass change. The accelerated ice melting over major glacial areas drives the polar drift toward 26°E for 3.28 mas/yr after the 1990s. The findings offer a clue for studying past climate‐driven polar motion. The Earth's pole, the point where the Earth's rotational axis intersects its crust in the Northern Hemisphere, drifted in a new eastward direction in the 1990s, as observed by space geodetic observations. Generally, polar motion is caused by changes in the hydrosphere, atmosphere, oceans, or solid Earth. However, short‐term observational records of key information in the hydrosphere (i.e., changes in terrestrial water storage) limit a better understanding of new polar drift in the 1990s. This study introduces a novel approach to quantify the contribution from changes in terrestrial water storage by comparing its drift path under two different scenarios. One scenario assumes that the terrestrial water storage change throughout the entire study period (1981–2020) is similar to that observed recently (2002–2020). The second scenario assumes that it changed from observed glacier ice melting. Only the latter scenario, along with the atmosphere, oceans, and solid Earth, agrees with the polar motion during the period of 1981–2020. The accelerated terrestrial water storage decline resulting from glacial ice melting is thus the main driver of the rapid polar drift toward the east after the 1990s. This new finding indicates that a close relationship existed between polar motion and climate change in the past. Past climate‐driven polar motion was quantified by modeling terrestrial water storage under two different scenariosOne scenario was based on GRACE and reanalysis data; another scenario was based on extra glacier change observationsRapid terrestrial water storage decline caused by ice melting over glacial areas drove the polar drift toward the east in the 1990s Past climate‐driven polar motion was quantified by modeling terrestrial water storage under two different scenarios One scenario was based on GRACE and reanalysis data; another scenario was based on extra glacier change observations Rapid terrestrial water storage decline caused by ice melting over glacial areas drove the polar drift toward the east in the 1990s

Image cross‐correlation techniques, such as particle image velocimetry (PIV), can estimate water surface velocity (vsurf) of streams. However, discharge estimation requires water depth and the depth‐averaged vertical velocity (Um). The variability of the ratio Um/vsurfintroduces large errors in discharge estimates. We demonstrate a method to estimate vsurffrom Unmanned Aerial Systems (UASs) with PIV technique. This method does not require any ground control point (GCP): the conversion of velocities from pixels per frame into length per time is performed by informing a camera pinhole model; the range from the pinhole to the water surface is measured by the drone‐borne radar. For approximately uniform flow, Umis a function of the Gauckler‐Manning‐Strickler coefficient (Ks) and vsurf. We implement an approach that can be used to jointly estimate Ksand discharge by informing a system of two unknowns (Ksand discharge) and two nonlinear equations: i) Manning's equation and ii) mean‐section method for computing discharge from Um. This approach relies on bathymetry, acquired in situ a priori, and on UAS‐borne vsurfand water surface slope measurements. Our joint (discharge and Ks) estimation approach is an alternative to the widely used approach that relies on estimating Umas 0.85·vsurf. It was extensively investigated in 27 case studies, in different streams with different hydraulic conditions. Discharge estimated with the joint estimation approach showed a mean absolute error of 19.1% compared to in situ discharge measurements. Ksestimates showed a mean absolute error of 3 m1/3/s compared to in situ measurements. Drone‐borne sensors can measure stream water surface velocity and water surface slopeWe developed a new method to estimate stream roughness and discharge from drone‐borne water surface velocity and slope measurementsDrone‐borne discharge measurements compared well with in situ measurements in 27 different field sites Drone‐borne sensors can measure stream water surface velocity and water surface slope We developed a new method to estimate stream roughness and discharge from drone‐borne water surface velocity and slope measurements Drone‐borne discharge measurements compared well with in situ measurements in 27 different field sites

Tibetan lakes are an effective indicator of climate change as they are highly sensitive to and directly affected by climate change. The past decade has seen the seven warmest years on record globally. Such observations have prompted questions about lake changes over the Tibetan Plateau. The dense coverage of the CryoSat‐2 altimeter reveals large‐scale patterns in this climate change signal. We investigate lake level variations of more than 200 lakes using altimetry observations from CryoSat‐2 during the period 2010 to 2019. Combined with GRACE/GRACE‐FO, we evaluate the water storage change of lakes and terrestrial water storage (TWS). We find that most studied lakes generally went through three phases of change, that is, rising‐hiatus/decline‐rising, albeit lakes in the north Tibetan Plateau, show higher rising rates. Results also show that lake levels are widely affected by the 2015/16 El Niño event across the entire Inner Plateau via reduced precipitation. Above normal precipitation during 2016–2018, resulted in a sharp rise of ~1.22 m on average, accounting for 56% of the decadal lake level rise (mean/median: 2.19/1.85 m). TWS in the Inner Tibetan Plateau accumulated a net gain of 70.5 km3, which is dominated by the net gain of lake water storage (ca. 63.3 km3). The interannual TWS variation is found to be associated mainly with precipitation. In particular, extreme conditions such as the 2015/16 El Niño, had a profound negative impact on the TWS. The findings in this study shed new light on the response of Tibetan lakes to recent decadal environmental changes. More than 200 lakes have been monitored by CryoSat‐2 over the last decade (2010–2019)CryoSat‐2 reveals three phases (rising‐hiatus/decline‐rising) of lake level change; hiatus/decline is widely observed during 2015/16 El NiñoMean change of lake level over the last decade is 2.19 m, of which 56% occurs in the three most recent years

Satellite radar altimetry has become an important data source supplementing in situ water level. For practical reasons, a radar altimeter can only record reflected echoes within its range window. Therefore, water surface elevation must fall within the range window. To this end, the Sentinel‐3 radar altimetry mission is configured with a new tracking function (i.e., open loop) to position its range window based on an onboard a priori elevation of ground targets. We found that Sentinel‐3 is unable to observe recently built reservoirs due to the incorrect onboard elevations. To overcome this issue, four approaches are proposed to improve the a priori elevation for inland water bodies, particularly reservoirs. These approaches can significantly increase data availability of Sentinel‐3 over reservoirs and can be used to prepare the onboard elevations of reservoirs and lakes for future missions such as Sentinel‐6, Sentinel‐3C/‐3D, and the Surface Water and Ocean Topography (SWOT) mission. Satellite radar altimetry is increasingly being used for hydrological studies. However, it is still challenging to deliver high‐quality data over inland water bodies, that is, lakes, rivers, and reservoirs. One of the reasons is that the altimeter cannot easily identify the water surface elevation due to highly variable terrain elevation along the satellite flight path. In other words, the data delivered may not contain valid information for water bodies. To address this issue, Sentinel‐3 is configured with a new function, which informs the altimeter about the expected elevation of the water surface. However, this function demands a priori knowledge of the altitude of the ground targets. We present four approaches to improve the a priori information to increase data availability over inland water bodies, specifically reservoirs. The Sentinel‐3A altimeter cannot deliver useful data over many reservoirs (constructed after 2005) in open‐loop tracking mode with OLTC V5Defining correct a priori elevation information is the key to increasing data availability over reservoirsFour approaches are proposed to define the most accurate onboard a priori elevation