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The Potential of Hydrogeodesy to Address Water‐Related
and Sustainability Challenges
Fernando Jaramillo
1
, Saeid Aminjafari
1
, Pascal Castellazzi
2
, Ayan Fleischmann
3
,
Etienne Fluet‐Chouinard
4
, Hossein Hashemi
5
, Clara Hubinger
1
, Hilary R. Martens
6
,
Fabrice Papa
7,8
, Tilo Schöne
9
, Angelica Tarpanelli
10
, Vili Virkki
11,12
,
Lan Wang‐Erlandsson
13,14,15
, Rodrigo Abarca‐del‐Rio
16
, Adrian Borsa
17
,
Georgia Destouni
1,18,19
, Giuliano Di Baldassarre
20
, Michele‐Lee Moore
13,21
,
José Andrés Posada‐Marín
22
, Shimon Wdowinski
23
, Susanna Werth
24
, George H. Allen
24
,
Donald Argus
25
, Omid Elmi
26
, Luciana Fenoglio
27
, Frédéric Frappart
28
, Xander Huggins
29
,
Zahra Kalantari
18
, Simon Munier
30
, Sebastián Palomino‐Ángel
23,31
, Abigail Robinson
1
,
Kristian Rubiano
32,33
, Gabriela Siles
34
, Marc Simard
35
, Chunqiao Song
36,37
,
Christopher Spence
38
, Mohammad J. Tourian
26
, Yoshihide Wada
39
, Chao Wang
40
,
Jida Wang
41,42
, Fangfang Yao
43
, Wouter R. Berghuijs
44
, Jean‐François Cretaux
7
,
James Famiglietti
45
, Alice Fassoni‐Andrade
8
, Jessica V. Fayne
46
, Félix Girard
47
, Matti Kummu
11
,
Kristine M. Larson
48
, Martin Marañon
1,49,50
, Daniel M. Moreira
47,51
, Karina Nielsen
52
,
Tamlin Pavelsky
40
, Francisco Pena
1
, J. T. Reager
25
, Maria Cristina Rulli
53
, and
Juan F. Salazar
54
1
Department of Physical Geography and Bolin Centre for Climate Research, Stockholm University, Stockholm, Sweden,
2
Commonwealth Scientific and Industrial Research Organisation (CSIRO), Environment, Urrbrae, SA, Australia,
3
Mamirauá Institute for Sustainable Development, Tefé, Brazil,
4
Earth Systems Science Division, Pacific Northwest
National Laboratory, Richland, WA, USA,
5
Department of Water Resources Engineering, Lund University, Lund, Sweden,
6
Department of Geosciences, The University of Montana, Missoula, MT, USA,
7
Université de Toulouse, LEGOS (CNES/
CNRS/IRD/UT3), Toulouse, France,
8
Institute of Geosciences, University of Brasília, Brasilia, Brazil,
9
Department
Geodesy, Helmholtz‐Centre GFZ German Research Centre for Geoscience, Potsdam, Germany,
10
Research Institute for the
Geo‐hydrological Protection, National Research Council, Perugia, Italy,
11
Water and Development Research Group, Aalto
University, Espoo, Finland,
12
Department of Environmental and Biological Sciences, University of Eastern Finland,
Joensuu, Finland,
13
Stockholm Resilience Centre, Stockholm University, Stockholm, Sweden,
14
Potsdam Institute for
Climate Impact Research, Potsdam, Germany,
15
Anthropocene Laboratory, Royal Swedish Academy of Sciences,
Stockholm University, Stockholm, Sweden,
16
Departamento de Geofisica, Universidad de Concepción, Concepción, Chile,
17
Scripps Institution of Oceanography, University of California San Diego, San Diego, CA, USA,
18
Department of
Sustainable Development, Environmental Science and Engineering, KTH Royal Institute of Technology, Stockholm,
Sweden,
19
Stellenbosch Institute for Advanced Study, Stellenbosch, South Africa,
20
Department of Earth Sciences, Uppsala
University, Uppsala, Sweden,
21
Department of Geography and Centre for Global Studies, University of Victoria, Victoria,
BC, Australia,
22
Grupo de Investigación INDEES, IU Digital de Antioquia, Bogota, Colombia,
23
Institute of Environment,
Department of Earth and Environment, Florida International University, Miami, FL, USA,
24
Department of Geosciences,
Virginia Polytechnic Institute and State University, Blacksburg, VA, USA,
25
Jet Propulsion Laboratory, California Institute
of Technology, Pasadena, CA, USA,
26
Institute of Geodesy, University of Stuttgart, Stuttgart, Germany,
27
Institute of
Geodesy and Geoinformation, University of Bonn, Bonn, Germany,
28
ISPA, Institut National de Recherche pour
l'Agriculture, l'Alimentation et l'Environnement (INRAE), Villenave d'Ornon, France,
29
Department of Civil Engineering,
University of Victoria, Victoria, BC, Canada,
30
CNRM, Université de Toulouse, Météo‐France, CNRS, Toulouse, France,
31
Stockholm Environment Institute, Latin America Centre, Bogotá, Colombia,
32
Department of Biology, Faculty of Natural
Sciences, Universidad del Rosario, Bogotá, Colombia,
33
Subdirección Científica, Jardín Botánico de Bogotá ‘José
Celestino Mutis’, Bogotá, Colombia,
34
Département des sciences géomatiques, Université Laval, Québec, QC, Canada,
35
Radar Science and Engineering Section, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA,
USA,
36
Key Laboratory of Lake and Watershed Science for Water Security, Nanjing Institute of Geography and
Limnology, Chinese Academy of Sciences, Nanjing, China,
37
University of Chinese Academy of Science Nanjing
(UCASNJ), Nanjing, China,
38
Environment and Climate Change Canada, Water Science and Technology Directorate,
Saskatoon, SK, Canada,
39
Biological and Environmental Science and Engineering Division, King Abdullah University of
Science and Technology, Thuwal, Saudi Arabia,
40
Department of Earth, Marine and Environmental Sciences, University of
North Carolina at Chapel Hill, Chapel Hill, NC, USA,
41
Department of Geography and Geographic Information Science,
University of Illinois Urbana‐Champaign, Urbana, IL, USA,
42
Department of Geography and Geospatial Sciences, Kansas
State University, Manhattan, KS, USA,
43
Environmental Institute, University of Virginia, Charlottesville, VA, USA,
44
Department of Earth Sciences, Free University Amsterdam, Amsterdam, The Netherlands,
45
School of Sustainability,
Arizona State University, Tempe, AZ, USA,
46
Department of Earth and Environmental Sciences, University of Michigan,
Ann Arbor, MI, USA,
47
Géosciences Environnement Toulouse, Université de Toulouse, Toulouse, France,
48
DETECT,
REVIEW ARTICLE
10.1029/2023WR037020
Special Collection:
Hydrogeodesy: Understanding
changes in water resources using
space geodetic observations
Key Points:
•This is a community view on
hydrogeodesy, the science that
measures the Earth's solid and aquatic
surfaces, gravity field, and their
changes
•Hydrogeodesy encompasses geodetic
technologies such as Altimetry,
Interferometric Synthetic Aperture
Radar, Mass gravimetry, and Global
Navigation Satellite Systems
•We study the evolution of
hydrogeodesy and its role within
current hydrological, sustainability
science, and management frameworks
Correspondence to:
F. Jaramillo,
fernando.jaramillo@natgeo.su.se
Citation:
Jaramillo, F., Aminjafari, S., Castellazzi,
P., Fleischmann, A., Fluet‐Chouinard, E.,
Hashemi, H., et al. (2024). The potential of
hydrogeodesy to address water‐related and
sustainability challenges. Water Resources
Research,60, e2023WR037020. https://
doi.org/10.1029/2023WR037020
Received 9 JAN 2024
Accepted 11 SEP 2024
Author Contributions:
Conceptualization: Fernando Jaramillo,
Fabrice Papa, Shimon Wdowinski
Data curation: Fernando Jaramillo
Formal analysis: Fernando Jaramillo,
Saeid Aminjafari, Ayan Fleischmann,
Etienne Fluet‐Chouinard,
Hossein Hashemi, Clara Hubinger, Hilary
R. Martens, Vili Virkki, Zahra Kalantari,
Sebastián Palomino‐Ángel,
Abigail Robinson, Kristian Rubiano,
Martin Marañon
Funding acquisition: Fernando Jaramillo
Investigation: Fernando Jaramillo
© 2024. The Author(s).
This is an open access article under the
terms of the Creative Commons
Attribution License, which permits use,
distribution and reproduction in any
medium, provided the original work is
properly cited.
JARAMILLO ET AL. 1 of 38
Universität Bonn, Bonn, Germany,
49
Centro Andino para la Gestión y Uso del Agua, Universidad Mayor de San Simón,
Cochabamba, Bolivia,
50
Centro de Planificación y Gestión, Universidad Mayor de San Simón, Cochabamba, Bolivia,
51
SGB, Geological Survey of Brazil, Rio de Janeiro, Brazil,
52
Department of Space Research and Technology, Geodesy and
Earth Observation, Geodesy, Denmark,
53
Department of Civil and Environmental Engineering, Politecnico di Milano,
Milan, Italy,
54
GIGA, Escuela Ambiental, Facultad de Ingeniería, Universidad de Antioquia, Medellín, Colombia
Abstract Increasing climatic and human pressures are changing the world's water resources and
hydrological processes at unprecedented rates. Understanding these changes requires comprehensive
monitoring of water resources. Hydrogeodesy, the science that measures the Earth's solid and aquatic surfaces,
gravity field, and their changes over time, delivers a range of novel monitoring tools that are complementary to
traditional hydrological methods. It encompasses geodetic technologies such as Altimetry, Interferometric
Synthetic Aperture Radar (InSAR), Gravimetry, and Global Navigation Satellite Systems (GNSS). Beyond
quantifying these changes, there is a need to understand how hydrogeodesy can contribute to more ambitious
goals dealing with water‐related and sustainability sciences. Addressing this need, we combine a meta‐analysis
of over 3,000 articles to chart the range, trends, and applications of satellite‐based hydrogeodesy with an expert
elicitation that systematically assesses the potential of hydrogeodesy. We find a growing body of literature
relating to the advancements in hydrogeodetic methods, their accuracy and precision, and their inclusion in
hydrological modeling, with a considerably smaller portion related to understanding hydrological processes,
water management, and sustainability sciences. The meta‐analysis also shows that while lakes, groundwater and
glaciers are commonly monitored by these technologies, wetlands or permafrost could benefit from a wider
range of applications. In turn, the expert elicitation envisages the potential of hydrogeodesy to help solve the 23
Unsolved Questions of the International Association of Hydrological Sciences and advance knowledge as
guidance toward a safe operating space for humanity. It also highlights how this potential can be maximized by
combining hydrogeodetic technologies simultaneously, exploiting artificial intelligence, and accurately
integrating other Earth science disciplines. Finally, we call for a coordinated way forward to include
hydrogeodesy in tertiary education and broaden its application to water‐related and sustainability sciences in
order to exploit its full potential.
Plain Language Summary Increasing climatic and human pressures are changing the world's water
resources and hydrological processes at unprecedented rates. Understanding these changes requires
comprehensive monitoring of water resources. Hydrogeodesy, the science that measures the Earth's solid and
aquatic surfaces, gravity field, and their changes over time, delivers a range of novel monitoring tools
complementary to traditional hydrological methods. It encompasses technologies such as Altimetry,
Interferometric Synthetic Aperture Radar (InSAR), Gravimetry, and Global Navigation Satellite Systems
(GNSS). Beyond quantifying these changes, we need to understand the potential of hydrogeodesy to contribute
to more ambitious goals of water‐related and sustainability sciences. Addressing this need, we combine a meta‐
analysis of over 3,000 articles to chart the range, trends, and applications of hydrogeodesy with an expert
elicitation that systematically assesses this potential. We find a growing body of literature relating to
advancements in hydrogeodetic methods, their accuracy and precision, and their inclusion in hydrological
modeling. The expert elicitation envisages the large potential to solve hydrological problems and sustainability
challenges. It also highlights how this potential can be maximized by combining several hydrogeodetic
technologies, exploiting artificial intelligence, and accurately integrating other Earth science disciplines.
1. Introduction
The water cycle and associated surface and subsurface flows and storages are changing at unprecedented rates via
complex and interconnected processes that increasingly challenge humanity (Bierkens & Wada, 2019; Konikow
& Kendy, 2005; Porkka et al., 2024; Yao et al., 2023). For example, global terrestrial water storage (TWS) has
decreased considerably in some regions due to freshwater consumption for energy and agriculture (Rodell
et al., 2018). Around 4 billion people have inadequate access to water during one or more months per year
(Mekonnen & Hoekstra, 2016), and ∼2.2 billion people live in regions facing both water stress and storage
depletion (Huggins et al., 2022). In turn, water consumption and flow regulations have impacted freshwater
ecosystems, with 59% of the world's largest river systems estimated to be either moderately or strongly affected
Methodology: Fernando Jaramillo,
Pascal Castellazzi
Validation: Fernando Jaramillo
Visualization: Fernando Jaramillo,
Saeid Aminjafari, Shimon Wdowinski
Writing – original draft:
Fernando Jaramillo, Pascal Castellazzi,
Ayan Fleischmann, Clara Hubinger, Hilary
R. Martens, Tilo Schöne,
Angelica Tarpanelli, Vili Virkki,
Lan Wang‐Erlandsson, Rodrigo Abarca‐
del‐Rio, Adrian Borsa, Georgia Destouni,
Giuliano Di Baldassarre, Michele‐
Lee Moore, José Andrés Posada‐Marín
Writing – review & editing:
Fernando Jaramillo, Saeid Aminjafari,
Ayan Fleischmann, Etienne Fluet‐
Chouinard, Hossein Hashemi,
Fabrice Papa, Shimon Wdowinski,
Susanna Werth, George H. Allen,
Donald Argus, Omid Elmi,
Luciana Fenoglio, Frédéric Frappart,
Xander Huggins, Simon Munier,
Gabriela Siles, Marc Simard,
Chunqiao Song, Christopher Spence,
Mohammad J. Tourian, Yoshihide Wada,
Chao Wang, Jida Wang, Fangfang Yao,
Wouter R. Berghuijs, Jean‐
François Cretaux, James Famiglietti,
Alice Fassoni‐Andrade, Jessica V. Fayne,
Félix Girard, Matti Kummu, Kristine
M. Larson, Daniel M. Moreira,
Karina Nielsen, Tamlin Pavelsky,
J. T. Reager, Maria Cristina Rulli, Juan
F. Salazar
JARAMILLO ET AL. 2 of 38
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by fragmentation (Grill et al., 2019) and 65% of riverine freshwater habitats already under threat (Vörösmarty
et al., 2010). Overall, water scarcity is driven by water use, land use, and changing hydroclimatic conditions
(Rijsberman, 2006; Schewe et al., 2014; Schmidt, 2019; Seckler et al., 1999; Singh & Kumar, 2019) and can
further be exacerbated by climate change (X. Li et al., 2022). Global water use and climate change already impair
essential functions of the water cycle and, at the global scale, may have already transgressed specific water‐related
thresholds describing a safe operating space for humanity (Destouni et al., 2013; Jaramillo & Destouni, 2015;
Porkka et al., 2024; Richardson et al., 2023).
Only a limited fraction of global freshwater is considered an accessible resource (0.76%; Shiklomanov &
Rodda, 2003), and freshwater resources are fragmented and fractioned across the landscape. For instance, mil-
lions of inland water bodies exist, many dispersed across remote or inaccessible regions (Hegerl et al., 2015; Pekel
et al., 2016; Verpoorter et al., 2014). The distribution of these water bodies is unequal across Earth's land area,
implying an even smaller percentage of freshwater per unit area in many regions. Consistent monitoring is
required for understanding and managing the functioning and evolution of such a large number of fragmented
freshwater bodies, especially under a rapidly changing global water cycle, and considering the need to prepare for
extreme water‐related events such as floods and droughts (Yang et al., 2021). Spaceborne remote sensing ap-
proaches can provide such comprehensive surveillance of surface water bodies and groundwater across our
planet's land area.
Satellite‐based geodetic observations can help track freshwater availability by measuring the temporal variation
of geometry and gravity over Earth's landscapes in 3D (e.g., Adams et al., 2022; Jin & Feng, 2013; Singh
et al., 2013; Tourian et al., 2022). Geodetic techniques have not only complemented the use of optical sensors to
understand changes in surface water extent but have additionally and considerably increased our understanding of
other characteristics of water availability, such as changes in water level, storage volume, and connectivity of
water bodies. The use of satellite‐based geodetic observations to understand changes in water availability, dis-
tribution, and movement can be termed “Hydrogeodesy” (Wdowinski & Eriksson, 2009). The term is the
combination of Geodesy─which studies Earth's size, shape, orientation, gravitational field, and the variations of
these quantities over time─and Hydrology, which studies the occurrence, distribution, fluxes, movement, and
properties of water on Earth. Although the term Hydrogeodesy has been used to highlight the potential of specific
geodetic technologies to study water resources (e.g., White et al., 2022), it scopes a wider range of technologies.
The main technologies related to Hydrogeodesy include (a) Nadir‐looking Altimetry (hereafter “Altimetry”), (b)
InSAR, (c) Mass Gravimetry (hereafter “Gravimetry”), and (d) GNSS (Figure 1). Combining the principles
behind these techniques has led to technical advances in the study of water from space, as embodied in the
recently launched Surface Water and Ocean Topography (SWOT) satellite (L.‐L. Fu et al., 2024), which com-
bines both nadir‐looking Altimetry and a wide‐swath Ka‐band InSAR.
Comprehensive scientific reviews of these technologies already exist in the literature, highlighting each tech-
nology's limitations, requirements, and applications with respect to tracking water resources (e.g., Adams
et al., 2022; Chawla et al., 2020; Fassoni‐Andrade et al., 2021; J. Lee, 2017; H. Lee et al., 2020; Papa & Frap-
part, 2021; White et al., 2022). However, (a) how the use of these technologies has recently evolved, and (b) what
their role within current water‐related science and water management is, are questions that, to our knowledge,
remain overlooked and, thus, are the focus of this review.
This study addresses two main research questions: (a) How has the field of Hydrogeodesy developed throughout
the last three decades? and (b) How can Hydrogeodesy contribute to addressing the goals of key hydrological and
sustainability science frameworks and water management? To answer the first question, we introduce the four
main technologies of Hydrogeodesy and study the coevolution of their application for water resources with a
comprehensive meta‐analysis covering more than 3000 articles. The meta‐analysis evaluates the use of these
hydrogeodetic technologies and identifies their trends of use, combinations, main applications, and the water
resources of interest for their usage. For the second, we conduct an expert knowledge elicitation on the potential of
Hydrogeodesy to address key water‐related and sustainability science questions, including the 23 Unsolved
Problems of the Hydrological Community (Blöschl et al., 2019) and the Planetary Boundaries Framework for
guidance towards a safe operating space for humanity (Rockström et al., 2009). Finally, we discuss how the
potential can be maximized by combining different technologies and even by addressing the challenges of
teaching and learning hydrogeodesy.
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2. Hydrogeodesy in a Nutshell
By using geodetic methods to measure or infer hydrological quantities and their changes over time, Hydrogeodesy
supports hydrological monitoring, management, and research via measurements that standard hydrological ob-
servations cannot obtain. While hydrological observations are usually direct measurements of hydrological
variables collected on‐site or from space, hydrogeodetic observations are obtained indirectly from geodetic data.
These data are then translated to hydrological quantities, such as TWS, snow depth, and surface water level
(Figure 1) via translating concepts or geophysical models. The main advantages of spaceborne hydrogeodetic
observations are their wide spatial coverage, low costs to the end user, and relatively high spatial and temporal
resolution, which enable comparison between multiple freshwater bodies across regions. For example, InSAR
observations can achieve a spatial resolution of 1–100 m, depending on acquisition parameters; GNSS‐IR
(interferometric reflectometry) integrates observations of soil moisture and snow depth over an area of
roughly 100 m
2
; and GNSS‐ and GRACE‐inferred estimates of TWS do so over tens to hundreds of kilometers
(e.g., the spatial resolution of GRACE is more than 150,000 km
2
). The dimensions of an observed area vary
depending on the measurement techniques, from 10 to 100 m, in the case of GNSS‐IR, to thousands of kilometers
in GRACE measurements (Figure 2). The main disadvantages include fairly short series of data as hydrogeodetic
missions are relatively new (i.e., earliest 1978, Table 1), and the corresponding need to standardize data across the
time span of different missions with the same technology, which can be challenging.
While satellite altimetry was initially designed for oceanography in the 1970s, it is now used also to monitor
inland water and ice sheet surface elevation by measuring the range (distance inferred from a signal's travel time)
between the satellite and continental water surfaces (Abdalla et al., 2021). Satellite altimeters measure surface
Figure 1. Illustration of the various technologies of Hydrogeodesy and their applications. The table includes the hydrological parameters most commonly measured in
the context of Hydrogeodesy, the principle behind such measurement, and the usual temporal and spatial resolution. Icons are under the Creative Commons License and
used from https://uxwing.com.
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heights by considering the two‐way travel time of radar or laser pulses be-
tween the satellite and Earth and applying specific corrections (Cretaux
et al., 2017; Cretaux et al., 2023). Altimetry has been used to track water
levels in rivers and large lakes (Crétaux et al., 2011,2016; Yao et al., 2023),
reservoirs (Birkett et al., 2011; Y. Li et al., 2023; Schöne et al., 2018), wet-
lands (Enguehard et al., 2023; J.‐W. Kim et al., 2009; Kitambo et al., 2022),
and increasingly used over smaller lakes (Brasseur et al., 2022; Cooley
et al., 2021; Luo et al., 2022). Such measurements are used to calibrate (Sun
et al., 2012; Zhong et al., 2020), validate (Finsen et al., 2014; Velpuri
et al., 2012) or parametrize hydrological (Durand et al., 2008; Emery
et al., 2020; Michailovsky et al., 2013; Paiva et al., 2013) and hydraulic
models (Coppo Frias et al., 2023), to estimate stage‐discharge relationships in
rivers (Papa et al., 2012; Paris et al., 2016; Tourian et al., 2013) or to reference
water level stations (Calmant et al., 2013). They can also be used to estimate
the snow height, changes in ice thickness on the sea or the ground surface
(Liang et al., 2021; Moholdt et al., 2010; Siles et al., 2022) or to estimate the
bathymetry (Armon et al., 2020; Fassoni‐Andrade et al., 2020) and local
geoid variations of lakes (Jiang et al., 2019).
InSAR can form spatially continuous maps of surface elevation (i.e., digital
elevation models; DEMs) and surface elevation changes with time (e.g.,
subsidence). To create a DEM, InSAR employs two or more radar acquisi-
tions collected from slightly different viewing geometries—either from an-
tennas separated by a boom (e.g., Farr et al., 2007) or by shifting the
platform's orbit (e.g., Krieger et al., 2007). Changes in surface elevation,
commonly called ground deformation in the geomorphology literature, can be
retrieved with up to millimetric accuracy (Wdowinski et al., 2004). These
changes have been related to changes in soil moisture (Mira et al., 2022;
Ranjbar et al., 2021), groundwater changes (Levy et al., 2020; Wu
et al., 2022), fluvial sediment (Higgins et al., 2014), water mass changes in
lakes or reservoirs (Cavalié et al., 2007; Darvishi et al., 2021; Doin
et al., 2015), snow topography (Guneriussen et al., 2001; Molan et al., 2018;
Oveisgharan & Zebker, 2007), permafrost thaw (Chen et al., 2020; Short
et al., 2014; C. Wang et al., 2018), and ice flow (Fatland & Lingle, 2002;
Forster et al., 2003; Palmer et al., 2009). InSAR applications to surface hy-
drology have been mostly used to measure water level changes in wetlands
due to the double bounce of the SAR sensor on vegetation and the surface of
the water, which yields a coherent signal that can be translated to water level
changes (e.g., Hong & Wdowinski, 2014; S.‐W. Kim et al., 2013; Liao
et al., 2020; Siles et al., 2020; C. Xie et al., 2013) but are now increasingly used to assess changes in water extent
and hydrologic connectivity (Jaramillo et al., 2018; D. Liu et al., 2020; Oliver‐Cabrera & Wdowinski, 2016;
Palomino‐Angel et al., 2019). Furthermore, although with limitations, its potential for inferring lake water levels
is also gaining some attention (Palomino‐Ángel et al., 2022); the SAR signal can also bounce on vegetation on the
shores of the lakes or emerging vegetation on the water surface, also guaranteeing a coherent signal (Aminjafari,
Brown, Mayamey, & Jaramillo, 2024).
Time‐variable mass gravimetry, especially from the Gravity Recovery and Climate Experiment (GRACE) and
GRACE Follow‐On (GRACE‐FO), measures temporal variations of Earth's gravity field to estimate changes in
water mass (Tapley et al., 2004). GRACE and GRACE‐FO are identical satellites orbiting together and separated
by ∼220 km along their track (Landerer et al., 2020; Tapley et al., 2004). The missions measure and track the
changes in the distance between the satellites, which correlate to changes in the gravity field and, thus, mass
anomalies (Swenson et al., 2003). Once the effects of the atmosphere and oceans are accounted for, the remaining
signal is generally associated with monthly to interannual changes in TWS (Landerer & Swenson, 2012). Most of
these changes are related to large‐scale surface and subsurface water resource variations and can elucidate
regional hydrological changes (Rodell & Reager, 2023). These changes may be climate‐driven, such as the
Figure 2. Temporal and spatial scales of hydrological processes (diagonal
and horizontal ellipses with gray text) and the capacity of hydrogeodetic
technologies (colored shaded rectangles and text) to observe such processes.
The combination of missions and technologies in time could also lead to
longer temporal scales—water components taken from Blöschl and
Sivapalan (1995). The boxes for each hydrogeodetic technology encompass,
in the lower limit, the spatial and temporal resolution provided by the
technologies, and in the upper limit, the available record length obtained by
combining several missions (Table 1). For example, regarding the temporal
scale, altimetry generally provides information on water levels in a range of
every ∼10–35 days (depending on the mission), but combining various
missions can give lake water level data every three days in some cases (An
et al., 2022). Integrating data on various altimetry missions can result in a
data series of ∼30 years (e.g., Aminjafari, Brown, Mayamey, &
Jaramillo, 2024). Regarding the spatial resolution, altimetry synthesizes
spatial information from 50 to 200 m for a water level data point. When
several altimetric missions are combined, maps of water level for an entire
basin, such as the Amazon (Birkett et al., 2002; Fassoni‐Andrade
et al., 2021) or the Congo (Kitambo et al., 2022) can be obtained. Large‐
swath altimetry (Surface Water and Ocean Topography) now offers a quasi‐
global coverage of rivers, wetlands, and lakes with a resolution of ∼100 m
(L.‐L. Fu et al., 2024).
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Table 1
Space Geodetic Technologies Relevant to Hydrology and the Study of Water Resources, as Modified and Updated After Wdowinski and Eriksson (2009)
Technology Acronym/type Agency Time Applications
Global Navigation Satellite Systems (GNSS)
GPS Global Positioning System DoD 1980‐present Solid Earth, Hydrology, Glaciology, atmosphere,
Ionosphere, Natural hazards
GLONASS Global Positioning System USSR/Russia 1982‐present
Galileo Global Positioning System ESA 2005‐present
Beidou‐1/2/3 Global Navigation Sat. System CNSA 2000‐present
IRNSS Regional Navigation Sat. System ISRO 2013‐present
QZSS Quasi‐Zenith Satellite System JAXA 2018‐present
Altimetry
SeaSAT Radar Altimetry DoD 1978 Oceanography
GeoSAT Radar Altimetry DoD 1985–1989 Oceanography, Hydrology, Glaciology,
Geoid determination
GeoSAT‐Follow Radar Altimetry NASA 1998–2008
ERS‐1 Radar Altimetry ESA 1991–1996
TOPEX/Poseidon Radar Altimetry NASA/CNES 1992–2005
Jason‐1/2/3 Radar Altimetry NASA/CNES 2002‐present
ERS‐2 (RA‐1) Radar Altimetry ESA 1995–2011
ENVISAT (RA‐2) Radar Altimetry ESA 2002–2012
ICESat Laser Altimetry NASA 2003–2009
ICESat‐2 Laser Altimetry NASA 2018‐present
CryoSAT‐2 SAR/Interfer. Radar Altimeter ESA 2010‐present
Sentinel‐3 SAR Altimetry ESA 2016‐present
SWOT Radar interferometer/Altimeter NASA/CNES 2022‐present
Sentinel‐6MF Radar Altimetry ESA/NASA/CNES 2020‐present
SARAL/AltiKa Radar Altimetry CNES/ISRO 2013‐present
GEDI Laser Altimetry NASA 2018‐present
(InSAR) Interferometric Synthetic Aperture Radar
SeaSAT L‐band, HH polarization (pol) DoD 1978 Oceanography
ERS‐1 C‐band, VV pol ESA 1991–1996 Solid Earth, Hydrology, Glaciology, Oceanography,
Geotechnical, Natural hazards
ERS‐2 (SAR) C‐band, VV pol ESA 1996–2012
JERS‐1 L‐band, HH pol JAXA 1992–1998
RADARSAT‐1 C‐band, HH pol CSA 1995–2013
ENVISAT (ASAR) C‐band, VV +VH, HH +HV pol ESA 2002–2012
ALOS (PALSAR) L‐band, quad‐pol JAXA 2006–2011
RADARSAT‐2 C‐band, quad‐pol CSA 2007‐present
TerraSAR‐X X‐band, quad‐pol DLR 2007‐present
TanDEM‐X X‐band, quad‐pol DLR 2010‐present
COSMO‐SkyMed X‐band, quad‐pol ASI 2007‐present
Risat‐1 C‐band, quad‐pol ISRO 2012–2017
KOMPSAT‐5 X‐band, quad‐pol KARI 2013‐present
ALOS‐2 L‐band, quad‐pol JAXA 2014‐present
Sentinel‐1 A C‐band, dual‐pol ESA 2014‐present
Sentinel 1‐B C‐band, dual‐pol ESA 2016‐present
PAZ X‐band, quad‐pol PNOTS 2018‐present
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melting of the ice caps (Velicogna et al., 2020) or long‐term droughts (Tapley et al., 2019), or man‐made, such as
groundwater withdrawal (Adams et al., 2022; Richey et al., 2015; Rodell et al., 2018; W. Wang et al., 2021).
Although satellite gravity's low spatiotemporal resolution makes it difficult for usage in smaller aquifers (Melati
et al., 2019), and there are still some discrepancies among gravimetric products (Jing et al., 2019), gravity‐
determined water mass changes are commonly combined with hydrological modeling to obtain water mass
variations at the higher resolution needed for water management applications (B. Li et al., 2019; Zaitchik
et al., 2008).
Lastly, precise ground‐based GNSS monitoring systems reside on the Earth's surface and are primarily used for
measuring 3‐D positions (e.g., East, North, and Up) and their changes. The positions are based on advanced
modeling (orbits, clocks, and the atmosphere) and simultaneous observations from multiple satellites (Ble-
witt, 2007). Networks of GNSS stations can track changes at larger spatial scales, and different GNSS appli-
cations can even be used to track changes in soil and plant moisture content, snow, and ice (White et al., 2022).
Hence, changes in the position of the ground or water surfaces obtained by GNSS can elucidate the effect of
changes in sea level, glaciers, and ice caps (e.g., Hugonnet et al., 2021; Khan et al., 2022), snow depth,
groundwater storage, or storage in rivers, lakes, and soils. Specifically, for hydrology, techniques have been
created to measure crustal loading and deformation across a Global Positioning System (GPS) station network to
infer a hydrologic load at the Earth's surface. Pioneering papers (Argus et al., 2014,2017; Borsa, Agnew, &
Cayan, 2014; Borsa, Moholdt, et al., 2014; Y. Fu et al., 2015) introduced the viability of this approach to detect
long‐period signals in total water storage for hydrologic science.
Reflected GNSS signals can be observed in space and potentially be considered to fall in the category of
hydrogeodetic techniques after applying the loosest constraints on the definition and accepting that other
reflectometric techniques, such as radar and radiometry, can be included by that standard. The technology
known as GNSS‐R uses scattered signals reflected from the surface captured by receivers on low Earth‐
orbiting satellites (Cardellach & Rius, 2008). The receiver processes information about the time delay,
phase shift, amplitude, and polarization of the reflected signals to infer the properties and elevation of the
reflected surface. For instance, spaceborne acquisitions from CYGNSS have been used to retrieve soil
moisture variations (Chew & Small, 2018; Eroglu et al., 2019) and for flood mapping and monitoring
inundation extent (Chew et al., 2023; Zeiger et al., 2023). Another hydrogeodetic technique based on GNSS
signals is called GNSS interferometric reflectometry (GNSS‐IR). It has been used to measure soil moisture
(Larson et al., 2008), permafrost melt (L. Liu & Larson, 2018), tides (Larson et al., 2013; Löfgren
et al., 2014), lake and river levels (Holden & Larson, 2021; Zeiger et al., 2021), freeboard ice (S. Xie, 2022),
and snow/ice surfaces (Larson & Nievinski, 2013; Siegfried et al., 2017). These environmental products use
Table 1
Continued
Technology Acronym/type Agency Time Applications
SAOCOM‐1 L‐band, quad‐pol CONAE 2018‐present
Radarsat Constell. C‐band, dual‐pol CSA 2019‐present
NISAR L‐band NASA Expected 2024
Gravimetry
LAGEOS‐1/2 Laser Geodynamics Satellites NASA 1976‐present Geoid determination, Oceanography,
Hydrology, Glaciology
Ajisai Experimental Geodetic Satellite JAXA 1986‐present
CHAMP Challenging Minisat. Payload DLR 2000–2010
GRACE The Gravity Recovery and NASA/DLR 2002–2017
GRACE‐FO Climate Experiment NASA/DLR 2018‐present
GOCE Gravity field and steady‐state Ocean
Circulation Explorer
ESA 2009–2013
Note. The names of the missions were used to perform the meta‐analysis of articles in Hydrogeodesy (see Methods). Time starts with the launch of the mission.
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the interference between the direct and reflected GNSS signals to calculate the height difference between the
GNSS antenna and the reflecting surface.
3. Materials and Methods
We searched for English‐language articles on hydrogeodesy published on the Clarivate Web of Science through
November 2023 (https://www.webofscience.com/wos/woscc/basic‐search). The query to retrieve these scientific
articles focused on the names of the hydrogeodetic technologies introduced in Figure 1, that is, Gravimetry,
Altimetry, GNSS, and InSAR. Optical and SAR satellite imagery have also been utilized in different
Hydrogeodesy‐related applications (Elmi et al., 2016). However, they generally encompass a broad spectrum of
Earth system monitoring, and for water resources, their utility is focused on the spatial depiction of surface waters.
Given that most applications of these two geodetic techniques fall outside the scope of Hydrogeodesy, we have
excluded them from the query. Likewise, hydrogeodetic technologies can be used in either space‐borne or air‐
borne missions. Air‐borne hydrogeodetic missions, such as AirSWOT (Altenau et al., 2019; Atimetric and
InSAR) or UAVSAR (Jones et al., 2016; enabling InSAR), are important for experimental purposes and focused
assessments but have shorter periods of operation and smaller spatial extents of application, resulting in limited
data availability. Space‐borne missions usually have a longer life span and a larger spatial extent of application
than air‐borne missions, leading to more studies and corresponding publications. Hence, we decided to limit the
meta‐analysis to space‐borne missions.
As these technologies are often referred to in the context of specific sensors, missions, or constellations (e.g.,
ICESat for Altimetry, GRACE for Gravimetry, or GPS for GNSS), in our preliminary search, we also included the
names of the most common hydrogeodetic sensors (See Table 1). We used the root of the words rather than the
complete word to avoid omitting relevant manuscripts. For instance, instead of searching for the word “Altim-
etry,” we searched for “Altimet,” which is the root of the words “Altimeter,” “Altimetric” and “Altimetry.” We
initially searched for any of these words in the abstract or keywords, as sometimes these technologies are not
mentioned in the article's title. However, due to the large number of false positive articles (over 10,000), and since
many of the studies using these technologies are not necessarily focused on water resources, we decided to also
look for specific words pertaining to water resources. Hence, the initial general query looked for the simultaneous
occurrence of (at least) one word associated with Hydrogeodesy in the abstract and (at least) one word associated
with water resources in the title or the keywords. This combination removed most unrelated articles, for instance,
those using GNSS for positioning rather than to assess properties or changes in water resources or those using of
hydrogeodetic techniques for seismological, volcanic, and geological studies. We decided to include studies
related to glaciology to compare the use of hydrogeodesy on ice surfaces with that of water in liquid form. We
looked in total for 24 words related to water resources and grouped them as follows: “lake,” “lagoon,” and
“reservoir” we tagged as Lake; “wetland,” “floodplain,” “estuary” as Wetland; “watershed,” “catchment,”
“hydrological basin” as Watershed; “river,” “discharge,” “stream” as River; “groundwater,” “ground water,”
“aquifer” as Groundwater; “ice,” “glacier,” “Antarctic,” “Arctic” as Ice; “total water storage,” “terrestrial water
storage” as Total Water; “snow” as Snow; “soil moisture,” “soil humidity” as Soil Moisture; and “permafrost,”
“active layer” as Permafrost. The initial query yielded 3,279 articles.
We sorted the articles into five technology categories based on the hydrogeodetic words found (i.e., Altimetry,
InSAR, Gravity, GNSS, Combined). The last category “Combined” was used when two or more of the four
hydrogeodetic technologies were mentioned in the abstract. In addition, we manually classified missions carrying
both an altimeter and a SAR sensor (e.g., Envisat, ERS‐2) and discarded studies using just SAR backscatter data
rather than interferometry (i.e., InSAR). It is worth noting that studies using SAR or optical data not involving
altimetry and interferometry (e.g., SAR studies to classify wetlands or optical studies determining soil moisture,
among others) were deliberately not accounted for in the list of articles. This deliberate scope refinement allows
for a more targeted examination of articles that specifically integrate altimetry and interferometry. Articles that
appeared in the search dealing specifically with landslides, earthquakes, and volcanoes or using primarily
techniques such as drones, airborne missions (e.g., Uninhabited Aerial Vehicle SAR (UAVSAR)), unmanned
aerial systems or Ground Penetrating Radar were also manually removed from the list.
Since we considered it essential to know the scientific audiences targeted by the articles' authors, and especially
the split between water resources and remote sensing, we determined how articles were distributed between
journals related to Water Resources, Glaciology, Remote Sensing, and Multidisciplinary studies. We used
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categories from the Science Citation Index Expanded (SCIE; https://mjl.clarivate.com/collection‐list‐downloads)
of the Web of Science Core Collection based on the journal title. This categorization is based on the type of
journal publishing the article. If the journal is classified under “Water Resources” among other categories, we
tagged the journal as “Water Resources.” We did likewise for the category “Remote Sensing,” also including all
journals focusing on Geodesy. The “Multidisciplinary” category includes all journal categories with the words
“multidisciplinary,” “geoscience” or “engineering.” To make a broad differentiation between glaciology and
studies of water resources, that is, hydrology, we generated an additional category called “Glaciology.” When
several of these four categories were listed for a journal, we chose the category highest on the prioritized order:
Water Resources, Glaciology, Remote Sensing, and Multidisciplinary. These categories also helped remove
articles from the initial list that were not targeting any of these categories based on the journals where they are
published.
We also used meta‐analysis of these studies to understand the authors' scientific motivations and their use cases
for hydrogeodetic technologies. Each study's objective was categorized manually, as automatization proved
difficult. We randomly selected approximately 120 articles for each of the five technology categories (∼20% of
the number of articles in the initial query) and manually inspected the wording in the abstracts. We searched for
the main objective in the article's abstract, specifically in the sentences describing the study's main goal, aim,
objective, research question, or hypothesis. If this was not explicitly mentioned in the abstract, we performed an
overall assessment based on the metadata available. The main categories selected to categorize the objective of the
study (and the way we refer to them in parenthesis) were the following:
•Technical advances (Technical)—Studies seeking to advance coding, algorithms, procedures (such as
generating digital elevation models), schemes, and theory of geodetic tools with applications focused on water
resources. We also include studies comparing results from different missions or technologies, and studies
reporting on the development of public access data sets.
•Determination of key hydrological variables (Hydrovariable)—Studies aiming to determine a hydrological
parameter or variable, such as water level, water table, water storage, soil moisture, ice elevation, and their
temporal change, without pursuing a case application (to separate this category from the effects of water
management, for example) or attempting to understand the hydrological or geomorphological system. In
addition, it includes studies determining the accuracy, precision, performance, and potential of a specific
mission or technology to track changes in the hydrological parameter or variable.
•Model development (Modeling)—Studies using hydrogeodetic technologies to assimilate into, parametrize,
calibrate, or validate a hydrological, hydraulic, or hydrogeological model.
•Effects of water management (Management)—Studies focusing on the geomorphological effects of irrigation,
water impoundment for regulation, river diversion, groundwater abstraction, or water consumption. Also,
studies focus on impacts on channels, dams, pipelines, aqueducts, and urban infrastructure regarding ground
subsidence and uplift.
•Geomorphological and surface water processes (Processes)—Studies of processes not related to direct human
activities but to natural variability, aiming to understand a hydro or geomorphological process beyond the sole
calculation of the typical hydrological variables estimated by the technologies. Such processes include glacier
growth and mass balance, melting and movement, permafrost thaw, iceberg movement, ground seepage, and
infiltration. Regarding water in liquid form, studies focusing on sheet flow, hydroperiod, hydrological con-
nectivity, seasonality of water availability, estimating discharge, or quantifying components by water mass
balance. Studies focusing on landslides with no relationship to water resources were excluded from the
selection.
It is worth noting that although an article may address several of these aspects, we selected the most prominent
and relevant objective based on its importance, as stated in the abstract. For instance, studies implementing a
novel algorithm to quantify a hydrologic variable would fit both the “technical” and “hydro variable” categories.
It was then our task to assign the objective to select the one weighing more in the overall outcome of the article
and, if needed, refer to the general manuscript beyond the abstract. These special cases also, once flagged, if
needed, would receive a second opinion from another researcher to make a final decision.
To assess how Hydrogeodesy can contribute to significant advancements in hydrological science, we focused on
the 23 Unsolved Problems in Hydrology (UPH) proposed by Blöschl et al. (2019). We asked all coauthors of this
study to rate how each of the four technologies could benefit the research towards each UPH, taking advantage of
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the various areas of expertise among coauthors concerning the four technologies and a wide range of related space
missions. The rating ranged from scores 1 (low) to 5 (high) for the potential of Hydrogeodesy to contribute to
resolving each UPH. Depending on their areas of expertise, the coauthors also elucidated the potential of
hydrogeodetic technologies to help advance the science to help solve the 23 UPH and the limitations that would
need to be overcome to do so. Finally, we discussed the potential of Hydrogeodesy in relation to global sus-
tainability frameworks, with special emphasis on the Planetary Boundaries framework that aims to delimit a
biophysically safe operating space for humanity (Rockström et al., 2009).
4. Results and Discussion
4.1. An Increasing Trend of Publications Involving Hydrogeodetic Technologies
The number of publications using hydrogeodetic technologies to understand changes in water resources has been
increasing at an accelerating pace, with 3278 articles published from January 1990 to November 2023 in peer‐
reviewed journals indexed in the Web of Science (Figure 3a). There are more than 800 articles involving
Gravimetry (806), followed by studies using GNSS (739), InSAR (626), Altimetry (547), and their combination
(561). This acceleration coincides with the increasing availability of hydrogeodetic missions and sensors in orbit
and the extended period available for observations when several missions are combined. The four technologies
analyzed here show a substantial increase in published articles following the launch of specific space missions for
each technology (Table 1). For instance, the launch of the Sentinel‐1 satellite constellation in 2014 by the Eu-
ropean Space Agency (ESA) substantially increased the annual publications of InSAR studies using C‐band SAR
data to perform ground deformation analysis related to groundwater changes (Figure 3a). Its 6‐day revisit time
(for the A and B satellites together) and its global coverage now allow a high temporal resolution of the ground
surface changes worldwide. Likewise, the launch of the Sentinel‐3A radar altimeter in 2016 and the ICESat‐2
laser altimeter in 2018 by the National Aeronautics and Space Administration (NASA) can also explain the
gain in publications using Altimetry to determine water levels in lakes and ice changes in glaciers. Additionally,
(the launches of GRACE‐FO Gravimetry) and Galileo (GNSS) have been followed by an increasing number of
publications per year.
Moreover, the number of studies combining two or more technologies has steadily increased. Around 16%
(n=561) of all publications in Hydrogeodesy used two or more technologies, with the use of Altimetry in
combination with Gravimetry (n=117) and GNSS with InSAR (n=111) the most frequent combinations among
publications (Figure 3b). A smaller number of studies combine up to three technologies (n=44), such as T. Yuan
Figure 3. Development of Hydrogeodesy (a) Annual number of scientific peer‐reviewed publications in the Web of Science in the field of Hydrogeodesy (n=3278),
differentiated by technology: Gravimetry (806), Global Navigation Satellite Systems (739), Interferometric Synthetic Aperture Radar (626), Altimetry (547), and their
combination (561). Publications from December 2023 are excluded due to the time of writing. (b) Venn diagram showing the total number of publications up to
November 2023 for each technology and for publications that combine more than one technology.
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et al. (2017), who estimated absolute water storage in the Congo River floodplains by computing water depths and
storage volumes by integrating InSAR and Altimetry. They later compared it with large‐scale estimates of total
water storage as obtained from gravimetric measurements of the GRACE satellite.
4.2. Hydrogeodesy Is Published Mostly in Remote Sensing and Multidisciplinary Journals
Regarding the journals hosting these publications, 33%–59% of articles are published in Multidisciplinary
journals, depending on the technology, and thus are not necessarily solely directed to the Hydrological or Remote
Sensing audiences (Figure 4). This is evident across all four technologies. It is worth noting that Gravimetry is the
technology that has permeated the hydrological community the most, with 43% of all publications in water
resource‐related journals, while Altimetry, InSAR and GNSS studies are more prevalent in journals of Remote
Sensing and Multidisciplinary categories (only 18%–21% in water‐related journals). Finally, water resources
journals have a larger share of publications than Glaciology journals across all four technologies. The wider
spread of Gravimetric studies in water‐resource journals may stem from the fact that gravimetric missions
(GRACE/GRACE‐FO) have hydrology as their primary application, whereas Altimetry, GNSS and InSAR have a
broader range of applications across other disciplines. It may also be related to the long‐standing public avail-
ability and accessibility of GRACE and GRACE‐FO data, which reduces the skill and knowledge barriers
required to process and generate the data. For instance, NASA has open and processed gravimetric data for the
GRACE/GRACE‐FO satellites, provided on 0.5‐degree global grids and updated monthly (e.g., https://grace.jpl.
nasa.gov/data/get‐data/). The user's processing requirements are low. Monthly gridded data sets for land water
storage are already available, reducing data processing costs for the user and making them ready for hydrologic
analysis.
Yet, the small share of altimetric studies published in water‐related journals (21%) is not explained by limited data
availability and accessibility, as several altimetric data sets have global coverage and are available at no cost to
end users. The first such data set was River and Lake launched by ESA https://altimetry.esa.int/riverlake/shared/
main.html. Now, there are several altimetric databases that track many lakes worldwide, such as the Global
Reservoirs and Lakes Monitor (G‐REALM; https://blueice.gsfc.nasa.gov/gwm/lake/Index) via the NASA and
USDA/FAS Water Measurements web portal; the Hydroweb next (https://hydroweb.next.theia‐land.fr/) of CNES
and LEGOS; DAHITI (https://dahiti.dgfi.tum.de/en/) from the German Geodetic Research Institute at the
Technical University of Munich (DGFI‐TUM) delivering rivers and lake level data for 10,676 targets; and
HydroSat (http://hydrosat.gis.uni‐stuttgart.de) by the Institute of Geodesy, University of Stuttgart, featuring time
series of water levels in the rivers and lakes worldwide through almost 25,000 virtual stations.
GNSS and InSAR studies also have relatively low penetration in water‐related journals (18%), in comparison to
journals more related to Geodesy (under the Remote Sensing category; Figure 4). Regarding InSAR (20%),
although data sets of ground deformation and water level changes are becoming more open and accessible at the
regional level, the hydrogeodetic community still needs a centralized global data set of InSAR products to study
changes in water levels in lakes, reservoirs, and wetlands (Wdowinski & Hong, 2015). This is difficult due to the
intense processing and the specialized (and sometimes costly) software required for interferometry. Still, the
Figure 4. Percentage of publications featuring each of the four main hydrogeodetic technologies or their combination
grouped by “Multidisciplinary,” “Remote Sensing,” “Water Resources,” and “Glaciology” categories, according to the
Science Citation Index Expanded (SCIE) of the Web of Science Core Collection.
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availability of InSAR processing tools for hydrologists is increasing, with the
spread of open‐source software and services such as Hyp3 (Hogenson
et al., 2016) and OpenSARLab at the Alaska Satellite Facility enabling cloud‐
based interferometric processing (Hogenson et al., 2021), reducing both local
processing costs and time. Additionally, the interferometric ground defor-
mation analysis results for the entire European continent since 2015 are
openly accessible through online services such as the Copernicus European
Ground Motion Service (Crosetto et al., 2021), and NASA plans to publish an
interferogram of all imaged areas by the NISAR mission as a standard and
freely available data product.
4.3. Ice, Lakes, and Groundwater Are Widely Investigated With
Hydrogeodesy
The four hydrogeodetic technologies target different surface or below‐
ground water resources. Glaciers and lakes are the most studied water re-
sources (Figure 5; Ice, 32%), especially by GNSS and Altimetric studies. Ice
sheets/caps and glaciers have been monitored mainly by Altimetry since the
1980s, with the first studies focusing on the Antarctic and Greenland ice
sheets and sea ice in the Baltic Sea (Scott et al., 1994). Many altimetric
missions track ice topography, determine ice surface elevations, map the
boundaries of ice shelves, and identify icebergs and ice‐surface features
(Mcintyre, 1991).
In ice‐free regions, the water‐related words most targeted by Hydrogeodesy are lakes (15%) and watersheds
(15%). Regarding the first, although radar altimeters were designed to measure the global sea level, altimetric
sensors now track water level changes in numerous lakes, reservoirs, and ponds worldwide due to their improved
spatial coverage and along‐track resolution. While most applications focused on large water bodies only (Birkett
et al., 2011; Crétaux et al., 2011,2016; Y. Li et al., 2023; Schöne et al., 2018; Yao et al., 2023), advances in
retracking algorithms minimize the impact of non‐water reflections and allow now to track smaller ones of only a
few hectares, under the condition that the satellite track covers the water bodies (Boy et al., 2022; Egido &
Smith, 2017). Laser altimeters can be applied to sporadically monitor water levels of small water bodies (Cooley
et al., 2021; Sulistioadi et al., 2015). Additionally, they can be combined with optical or radar imagery to increase
temporal resolution and reconstruct water levels over a longer period (Yao et al., 2024), which can also produce
ground height products for measuring the banks of the lakes (Arsen et al., 2014).
Regarding watersheds, the focus of hydrogeodetic studies at these larger spatial scales is mostly related to
gravimetric studies aiming to study TWS changes. This is because changes in groundwater storage are determined
by time‐variable gravity data from GRACE (10% of all studies) after isolating the groundwater storage contri-
butions within the TWS observations. The groundwater storage change is typically considered the residual after
all non‐groundwater contributions are subtracted from the GRACE TWS in a process referred to as decompo-
sition. This requires model output or observations of soil moisture, snow water equivalent, surface water storage,
and canopy water. It also requires a good conceptualization of the dominant water stores and all potential non‐
water mass changes across a study area (i.e., glacial isostatic adjustment, large earthquakes, mining exports).
For typical large‐scale applications in which diffuse water storage contributors (soil moisture) dominate the signal
or where signal leakages in or out of the study area can be important, the challenges of separating contributors of
TWS and the inherent low resolution of the GRACE observations (∼300 km, Luthcke et al., 2013) forces hy-
drologists to synthesize data on water storage changes at the watershed scale.
For regional scale applications, Gravimetry can also be used to distinguish temporal variations of TWS arising
from focused, spatially discrete masses such as lakes (e.g., Urmia Lake; Saemian et al., 2020), glaciers (Cas-
tellazzi et al., 2019) and large impounded reservoirs such as the Grand Ethiopian Renaissance Dam (Kansara
et al., 2021) or the Three Gorges Dam in China (Huang et al., 2015). Such approaches are feasible if the spatial
distribution of the expected mass change can be inferred via auxiliary data (Longuevergne et al., 2013).
Furthermore, downscaling GRACE data can relate mass changes to human groundwater use and consumption
(Argus et al., 2022; Castle et al., 2014; Famiglietti et al., 2011; Rodell et al., 2007; Rodell & Famiglietti, 2002;
Figure 5. Number of publications grouped by type of water resource
investigated (see Methods for grouping and assigning of water resource
names).
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Scanlon et al., 2012), which represents one of many interdisciplinary applications of Gravimetry at the interface of
water science and sustainability.
Earth surface deformation sensors like InSAR and GNSS follow Gravimetry as the technology most used to track
groundwater changes. Deformation data can be used to track aquifer mechanical responses due to changes in
groundwater pressure, causing poroelastic compaction or deflation of sub‐surface layers, which is experienced as
changes in ground elevation at the surface. If the deformation is elastic, once the aquifers are recharged, water
content, pressure, and the ground will again rise (Adams et al., 2022). While GNSS and InSAR sensors are
sensitive to poroelastic deformation signals, especially high‐resolution deformation maps of InSAR are useful to
resolve aquifer‐related deformation accurately and have served as a proxy of groundwater storage change (4%;
e.g., Motagh et al., 2008). Such studies can relate ground and infrastructure subsidence to human groundwater
withdrawal. For example, poroelastic deformation in aquifers has been used to determine water use/withdrawal
rates in the San Joaquin Valley in California and Mexico City using InSAR data (Khorrami et al., 2023; Levy
et al., 2020; Manuel Pardo et al., 2013; Ojha et al., 2018; Smith et al., 2017), and to a lesser extent using GNSS
observations. However, in most regions worldwide, GNSS data do not provide sufficient spatial coverage for the
deformation signal to resolve water use/withdrawal rates accurately. High‐resolution deformation data can be
combined with groundwater level data to calculate storage coefficients and groundwater heads in compacting
aquifer layers (e.g., Chen et al., 2016). The major limitation of using deformation data for groundwater studies is
the intricate relationship between surface deformation and water volume change, particularly immediately after
drought when residual or delayed compaction coincides with groundwater pressure increase (Lees et al., 2022;
Murray & Lohman, 2018; Shirzaei et al., 2019). In addition, most groundwater studies applying InSAR defor-
mation data focus on urban or agricultural areas with features such as infrastructure that enable a coherent signal.
This also challenges large‐scale applications of high‐resolution InSAR data, which inherently cover a wide range
of land covers and spatially variable InSAR noise levels (Castellazzi et al., 2021; Du et al., 2023). However,
recent improvements in InSAR processing (Ao et al., 2024; Castellazzi & Schmid, 2021; Ohenhen et al., 2024;
Ojha et al., 2018; Zebker & Chen, 2023) and machine learning have been proven useful to address this challenge
(Naghibi et al., 2022). Another frequent challenge in groundwater studies applying deformation data is the
separation of the groundwater‐related signal from other, spatially coinciding sources, such as stacked aquifers and
clayey soils occurring in large alluvial fans (Castellazzi et al., 2021), tectonic deformation, sediment compaction,
and elastic loading (Kang & Knight, 2023; Larochelle et al., 2022).
Regional TWS changes, including groundwater storage, can be inferred from the elastic deformation response of
the Earth's crust to surface and near‐surface water loads (Figure 5; 3% for groundwater and TWS). Green's
functions for crustal load displacements (e.g., Farrell, 1972) are applied to invert for gridded mass changes that
best explain the observed deformation. So far, most loading studies have relied on GNSS observations because the
amplitude of the loading signal is relatively small compared to other deformation processes, that is, a few mm up
to 1 cm for annual deformation (Argus et al., 2014), and the signal includes large spatial wavelengths that were
difficult to resolve with InSAR until recently. Hence, this approach is mostly applied in locations with contin-
uously and densely operating GNSS networks, as the temporal and spatial resolution of resulting TWS change
maps depends on the sampling rate and spacing of the GNSS stations. Up to ∼50 km spatial resolution has been
achieved in a few densely monitored regions, like the US west coast (Borsa, Agnew, & Cayan, 2014; Borsa,
Moholdt, et al., 2014; Carlson et al., 2022). The temporal resolution of this approach is limited by short‐period
non‐loading signals and other noise in the deformation data, but it is at least 7 days for daily GNSS positions (e.g.,
Adusumilli et al., 2019). Terrestrial water storage changes estimated from GNSS include water cycling through
the Himalayas (Y. Fu & Freymueller, 2012), seasonal water changes in the western USA (Argus et al., 2014; Y.
Fu et al., 2015), multiannual cycles of drought and recovery (Adusumilli et al., 2019; Argus et al., 2017; Borsa,
Agnew, & Cayan, 2014; Borsa, Moholdt, et al., 2014), and the impact of individual storms (Milliner et al., 2018).
Recent progress in InSAR processing for yielding large‐scale deformation maps in global reference frames that
are combined and validated with GNSS observations can provide high‐resolution maps of the loading response as
recently achieved in Mexico City (Khorrami et al., 2023) and are promising for further applications of this
approach in regions worldwide where GNSS networks are sparse or absent.
GNSS observations are also used to measure water levels of rivers and lakes and inundation dynamics (Holden &
Larson, 2021; Zeiger et al., 2021), especially in the tropics. To retrieve soil moisture variations (6%) from ground‐
based receivers, GNSS‐R, which consists of analyzing the GNSS signals reflected by the Earth's surface, is used
increasingly. The launch of the first spaceborne GNSS‐R missions, and the Cyclone Global Navigation Satellite
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System (CYGNSS) (Ruf et al., 2016), have offered new opportunities to
monitor surface soil moisture variations (Chew & Small, 2018).
4.4. Wetlands Are Understudied With Hydrogeodetic Technologies
Few hydrogeodetic studies focus on wetlands as compared to other water
resources (Figure 5;∼4%), despite the growing importance of these ecosys-
tems for climate mitigation, biodiversity conservation, and sustainable
development (Jaramillo et al., 2019; Thorslund et al., 2017). A possible
explanation may be the challenge of measuring water levels in these eco-
systems; the vegetation covering wetlands and peatlands limits certain tech-
nologies, and their inaccessibility limits the ground‐truthing of the
hydrogeodetic measurements. InSAR and GNSS are probably the best tech-
nologies for solving this issue (Zeiger et al., 2022). It is generally agreed that
SAR and GNSS‐R with a longer wavelength and lower frequency, such as L‐
band sensors, better penetrate vegetation canopy than the higher frequencies
of C‐band sensors (Freeman & Durden, 1998; Hess et al., 1990). These
characteristics help distinguish water below vegetation and, thus, determine
corresponding water level changes in wetlands. Drawbacks of InSAR for
wetlands study include difficulties in obtaining a coherent radar signal and the
fact that it can only resolve relative water levels in time and space (H. Lee
et al., 2020). This change in time is “relative” to other points on the water
surface, requiring Altimetry or in‐situ observations to determine absolute
water level changes. Nevertheless, the maps of water level change are useful
due to non‐uniform changes across wetlands from sheet flow, contributions
from different river inflows and groundwater, and hydrological barriers of
flow between water bodies (Gondwe et al., 2010; Jaramillo et al., 2018; D. Liu
et al., 2020; Lu & Kwoun, 2008). In recent years, InSAR has been effectively
used to study peatland evolution and carbon emissions. Hoyt et al. (2020) and
Khodaei et al. (2023) have used InSAR to map peatland ground deformation in tropical and temperate regions,
respectively, to calculate the contribution of peatlands to carbon emission/sink at both regional and local scales.
4.5. A Large Portion of Hydrogeodetic Studies Have a Technical Focus
In agreement with recent studies (Fassoni‐Andrade et al., 2021; Topp et al., 2020), we find that a large fraction of
Hydrogeodesy articles can be considered of a technical nature (71%), either aiming to improve methods/tech-
nologies (Figure 6, Technical category, 36%), to estimate a specific hydrological variable and its variation in time
and space (Figure 6; Hydrovariable category, 28%), or as an aid for hydrological, geomorphological or hydraulic
modeling (Figure 6; Modeling category, 7%). The first is the most recurrent type of objective of the studies,
involving new algorithms, remote sensing methodologies, and statistical methods and refinements to improve the
quality, precision, and accuracy of the data, decrease uncertainty, and remove the noise of the various signals of
the sensors (e.g., Canisius et al., 2019; Seo et al., 2020; T. Wang et al., 2022; Y. Wang & Morton, 2022).
Gravimetry studies have the smallest proportion of studies falling under this category.
Regarding the second category (Figure 6; Hydrovariable), many of the articles also aim to quantify the direct
hydrological or geological variables that the technology can track (see Figure 1). The quantification of these
variables is crucial for regions or water resources where it is important to understand regional and temporal
patterns of change and to assess and track water availability. These regions include Greenland, Patagonia, West
Antarctica and the Antarctic Peninsula, where ice‐cap and glacier loss are accelerating; and California (United
States), Northern India, the Middle East, Caspian and Aral Sea regions, and Eastern China, where drought and
groundwater depletion are reducing water availability for human use (Rodell et al., 2018); or for instance,
northwestern South America, where water availability is decreasing due to an increasing frequency of El Niño
Southern Oscillation events (Bolanos et al., 2021).
Hydrogeodetic studies addressing hydrological problems beyond technical developments, such as those aiming to
understand hydrological and geomorphological processes or attribute changes to human or climatic impacts, are
Figure 6. Sankey diagram of the main objective behind hydrogeodetic
studies. The article objectives were tagged based on random samples of
articles for each technology or their combination (n=120). See Methods for
a description of the categories of primary objectives. Gray numbers on the
right represent the number of articles of each technology addressing a
specific category of article objective. Light‐gray boxes are the objectives of
technical nature while the dark‐gray boxes are those going beyond technical
(into hydrological processes, water management and sustainability).
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less numerous (Figure 6; 29%, which is the sum of Processes (23%) and Management (6%)). Processes analyzed
by hydrogeodetic techniques often relate more to the cryosphere than liquid water, involving ice‐cap dynamics,
changes in ice thickness, iceberg movement, or dynamics of river and lake ice. Indeed, satellite techniques such as
InSAR and altimetry, along with auxiliary derived information from sources like climate models, have revealed
notable changes in river/lake ice patterns (e.g., Kouraev et al., 2007; Siles & Leconte, 2023) and attributed to
climate change. These impacts are also discernible in liquid water resources. Rodell et al. (2018) quantified
freshwater mass trends observed by GRACE satellites and attributed them to natural interannual variability,
unsustainable groundwater consumption, climate change, or combinations thereof. More recently, Yao
et al. (2023) combined altimetry missions with satellite images, hydroclimate models, and field surveys to
quantify and attribute global lake water storage trends. They found that more than water levels in half of the
world's largest lakes have declined over the past three decades, with human water consumption, warming climate,
and sedimentation largely responsible for these water losses. Additionally, hydrogeodetic techniques have been
used on a much smaller scale to understand surface water processes, such as dynamics in estuaries and coastal
wetlands, and determine river water surface slopes.
Anthropogenic effects of management of water resources focus on groundwater depletion for agricultural or
urban consumptive use, at large spatial scales with Gravimetry and small scales by InSAR and GNSS. The effects
of fragmentation and regulation on water seasonality and connectivity are, to some degree, studied with
Altimetry. InSAR and GNSS are also used to determine the geomorphological changes occurring by water storage
changes in managed reservoirs. For instance, ground deformation around Lake Mead and vertical displacements
of the Hoover Dam (United States) have been found to relate to water storage changes in the reservoir (Cavalié
et al., 2007; Darvishi et al., 2021). InSAR and GNSS technologies are usually combined with geomorphological
modeling to understand the dynamics of elastic deformation necessary to guarantee the stability of water‐related
infrastructure such as dams and ancillary structures (e.g., Neelmeijer et al., 2018).
4.6. The Potential of Hydrogeodesy to Help Solving Key Hydrological Problems
The International Association of Hydrological Studies (IAHS) has outlined the main topics of focus for the
Hydrological Community during the last three decades. The first decade (2003–2012) was termed the Decade on
Predictions in Ungauged Basins, aiming to develop and improve methods and techniques for estimating hy-
drological and hydraulic parameters in ungauged basins where little or no hydrological data is available. The
decade's goals aligned with the relevance of using hydrogeodetic applications to make accurate predictions and
assessments of water resources and flood risk, where traditional hydrological data collection was limited or
absent.
The second decade (2013–2023) was termed Panta Rhei (“everything flows”) and highlighted the challenges
imposed by global changes on traditional assumptions, such as hydrological stationarity, setting the pathway for
socio‐hydrology. During this decade, the challenges of global environmental change, including issues related to
water resources management, extreme events, and climate change, were prioritized to quantify changes in the
global hydrological system and their impact on society. The hydrogeodetic community has supported this
initiative by spaceborne gauging and observing thousands of rivers, lakes, reservoirs, and glaciers to synthesize
their changes and societal implications (Figures 3–5). As part of this initiative, the IAHS has also proposed the 23
Unsolved Problems in Hydrology (UPH; Blöschl et al., 2019). These UPHs represent major challenges faced by
the hydrology field that, if solved, could potentially transform the management of water resources worldwide and
considerably increase the understanding of hydrological processes.
Our insights on the potential of Hydrogeodesy to target these UPHs─through a survey among co‐authors ranking
the applicability of the four technologies to solve each of them─highlight the convenience of using hydrogeodetic
techniques for such an endeavor (Table 2). Addressing the UPHs related to Measurements and data (UPHs 16–18)
and Modeling Methods (UPHs 19–20) could also largely benefit from Hydrogeodesy. Hydrogeodetic technol-
ogies have revolutionized water resource monitoring by increasing the temporal and spatial resolution and record
lengths of hydrological observations worldwide, mostly regarding unmonitored water resources. Furthermore,
there is a growing interest of hydrogeodesists to support hydrological modeling, either for its parametrization,
calibration, validation, or assimilation (Figure 6; Modeling). It is worth noting the case of Gravimetry, where due
to the many water components included in TWS observations, TWS changes are usually validated with hydro-
logical models and reanalysis products to obtain specific fluxes and stocks on the surface or below (Niu &
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Table 2
How Hydrogeodetic Technologies Could Help Answer the Unsolved Problems in Hydrology of the International Association of Hydrological Studies
Unsolved problem in hydrology (UPH) Altimetry InSAR Gravimetry GNSS Total
Time variability and change
1. Is the hydrological cycle regionally accelerating/decelerating, and are there tipping points
(irreversible changes)?
3.6 3.5 4.1 3.1 3.6
2. How will cold region runoff and groundwater change in warmer climates ? 2.9 4.1 4.1 2.9 3.5
3. How does climate change and water use alter ephemeral rivers and groundwater in (semi‐) arid
regions?
2.7 3.2 3.2 3.3 3.1
4. How do land cover change and soil disturbances impact water and energy fluxes on land and
groundwater recharge?
2.1 3.2 3.4 2.6 2.8
Space variability and scaling
5. What causes spatial heterogeneity/homogeneity/sensitivity to controls in hydrological and
material fluxes?
2.4 3.4 2.7 3.4 3.0
6. What are the hydrologic laws at the catchment scale, and how do they change with scale? 2.7 3.1 2.9 3.3 3.0
7. Why is most flow preferential across multiple scales, and how does such behaviour co‐evolve
with the critical zone?
1.9 2.1 2.1 2.3 2.1
8. Why do streams respond so quickly to precipitation inputs when stormflow is so old, and
what is the transit time distribution of water in the terrestrial water cycle?
2.5 2.9 2.7 3.6 2.9
Variability of extremes
9. How do flood‐rich and drought‐rich periods arise, are they changing? 3.5 3.3 4.0 3.8 3.7
10. Why are runoff extremes in some catchments more sensitive to land use/cover and geomorphic
change?
2.7 3.4 2.2 3.0 2.8
11. Why, how and when do rain‐on‐snow events produce exceptional runoff? 2.0 3.1 2.1 4.1 2.8
Interfaces in hydrology
12. What processes control hillslope–riparian–stream–groundwater interactions, and how do they
connect?
2.2 3.1 2.7 2.9 2.7
13. What processes control groundwater fluxes across boundaries ? 1.7 3.0 3.3 2.3 2.6
14. What factors contribute to the long‐term persistence of sources responsible for water quality
degradation?
1.8 2.2 1.7 1.1 1.7
15. What are the extent, fate and impacts of contaminants? How are subsurface microbial
pathogens removed/inactivated?
1.3 1.2 1.5 1.1 1.3
Measurements and data
16. How can we use innovative technologies to measure surface and subsurface properties,
states and fluxes?
4.2 4.6 4.6 3.5 4.2
17. What is the value of traditional hydrological observations vs. qualitative observations from lay
persons, data mining? Under what conditions can we substitute space for time?
3.7 3.8 3.5 4.1 3.8
18. How can we extract information from available human and water systems data to inform the
building process of socio‐hydrological models and conceptualizations?
3.4 4.1 3.5 3.7 3.7
Modeling methods
19. How can hydrological models be adapted to extrapolate to changing conditions? 3.5 4.1 3.6 4.1 3.8
20. How can we disentangle and reduce model structural/parameter/input uncertainty in
hydrological prediction?
3.8 4.0 4.1 3.4 3.8
Interfaces with Society
21. How can the (un)certainty in hydrological predictions be communicated to decision makers/
general public?
3.1 3.2 3.1 2.9 3.1
22. What are the synergies and tradeoffs between societal goals related to water management ? 2.7 3.4 3.2 2.7 3.0
23. What is the role of water in migration, urbanization and the dynamics of human civilizations,
and what are the implications for contemporary water management?
3.6 3.8 3.1 2.9 3.4
Note. The survey results consist of answers from each co‐author to the question: What is the potential of this hydrogeodetic technology to help solve this unresolved
question (UPH)? The answers ranged from 1 (low potential for Hydrogeodesy to contribute, red) to 5 (high potential for Hydrogeodesy to contribute, blue). The numbers
below each technology show the average score of all co‐authors answering the survey for that specific technology. The column “Total” is the average of all scores from
all answers.
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Yang, 2006; Ramillien et al., 2021). To give some examples, GRACE has been used to evaluate TWS with the
World Climate Research Program's Coupled Model Intercomparison Project Phase 5 (CMIP5) (Freedman
et al., 2014) or regional‐scale hydrologic modeling with the Soil and Water Assessment Tool (SWAT) in Sub‐
Saharan Africa (H. Xie et al., 2012). InSAR ground displacement and water level change outputs can calibrate
and parameterize modeling of groundwater such as 1D compaction models (e.g., Lees et al., 2022) or 3D finite
element groundwater flow and geomechanical model (Boni et al., 2020).
The problems regarding Hydrology Interfaces with Society (UPHs 21–23) also rank high among hydrogeodesists,
especially the UPH related to the role of socio‐hydrology and focusing on the role of water in migration (Wolde
et al., 2023), urbanization and human dynamics (Sardo et al., 2023), and the implications for water management
(UPH 23). Sociohydrology studies the interplay between water, infrastructure, and society (Di Baldassarre
et al., 2013,2015; Hall, 2019; Sivapalan et al., 2012). This interplay includes the water‐energy‐food nexus across
all spatial scales of analysis (Cudennec et al., 2018; D’Odorico et al., 2018; Lant et al., 2019; J. Liu et al., 2017).
We argue that Hydrogeodesy is essential to complement models and to go beyond the specific case studies
constrained by data availability on changes to water resources. By exploring large data sets of change in water
resources from multiple places around the world, Hydrogeodesy can help (a) unravel generic patterns and trends
in the way that societies extract and transport energy sources, produce and convert energy, irrigate crops for
biofuel production (Rulli et al., 2016), and produce water‐intensive renewable energy, (b) advance our under-
standing of the relationship between economic growth and water flows, (c) develop a complete picture of changes
in global water resources by reducing the bias in in‐situ monitoring toward the Global North, and (d) uncover the
interconnected nature of food‐energy‐water systems and potentially enhance our ability to address all Sustainable
Development Goals (Di Baldassarre et al., 2019).
Regarding water levels, several altimeters (e.g., Jason‐3 and Sentinel‐3A/B) sensors have been used to calibrate
hydraulic models (Malou et al., 2021; Schroeder et al., 2019). Special attention is paid to Cryosat‐2, which can
accurately monitor river profiles and slopes due to its short inter‐track distances, benefiting hydraulic applications
and river discharge estimation (Schneider et al., 2018). Given the role of surface water bodies and rivers in
providing water storage during drought periods and conveying and storing water during flood events, their
observation is critical, and the role of water elevation in this regard allows mitigation of the impacts of these
hydrological extremes. Concerning water resource management, monitoring reservoirs by measuring water
height with altimeters (and changing spatial extent from optical sensors) allows their volume to be quantified
when combined with water extension changes obtained by optical and radar imagery (Tourian et al., 2022). For
instance, in Brazil, the water and risk management agencies (ANA, SGB) have started incorporating hydro-
geodetic technologies to monitor reservoirs and provide services to many communities and economic sectors
across the country.
Addressing the agenda set out by IAHS and the UPHs requires consideration of both the changes that can be
measured by these technologies and related research problems whose solutions are relevant to water management.
While hydrogeodetic technologies may be able to better quantify reductions in water availability, there is still
limited experience in applying this information in practice, such as in formal or informal water allocation
decision‐making (Curran et al., 2023). Hydrogeodetic observations could be particularly beneficial in areas with
insufficient data coverage or for management practices that require, for example, estimating groundwater use
when monitoring well data is restricted or otherwise unavailable (Molle & Closas, 2020). Moreover, these
technologies could also be operationalized to inform a broader range of water management decision‐making
processes (Sheffield et al., 2018). For instance, clarifying the distinctions between changes that result from
over‐allocation (i.e., policy‐only decisions that can be altered locally) versus biophysical changes resulting from
broader climate change can be helpful for water managers (e.g., Grafton et al., 2013; Ricciardi et al., 2022). As
another example, lags in decision‐making often mean that responses to sudden or slower changes in water
availability are too delayed to be effective (Barnett et al., 2015; Punzo & Arbabi, 2023). While some lag time may
always remain, the ability of hydrogeodetic technologies to detect signals and changes in near real‐time could
shorten these lags, leading to more responsive and effective management.
4.7. Maximizing the Potential of Hydrogeodesy and Potential Limitations to Solve the Problems
The four hydrogeodesy technologies discussed here can be used to solve specific UPHs (Table 3; second column),
although there are potential limitations the researcher/user will face in the process (Table 3; third column). The
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Table 3
Selected Extracts From the Expert Elicitation on How Hydrogeodesy Can Help Address the UPHs and the Limitations That Need to Be Overcome
Unsolved problem in hydrology (UPH)
Examples on the potential of hydrogeodesy to contribute
to the UPH Limitations that need to be overcome
Time variability and change
1. Is the hydrological cycle regionally
accelerating/decelerating under climate
and environmental change, and are there
tipping points (irreversible changes)?
The occurrence of tipping points and determination of
accelerating change in hydrological fluxes requires
tracking in time. This can be done with many
hydrogeodetic sensors. To name a few, the Ka‐band
Radar Interferometer (KaRIn) onboard SWOT can
observe surface water fluxes in millions of lakes,
reservoirs, and rivers at an unprecedented
spatiotemporal scale (<100 m, sub‐monthly). The
ICESat‐2 altimeter provides high‐accuracy
measurements of mass changes in global glaciers and
ice sheets. The forthcoming NISAR satellite mission
will collect radar images in both L‐ and S‐bands and
measure changes in ice masses and soil moisture at a
spatial scale of farm fields every 6–12 days.
Combining these data with hydrological models, we
can determine if the changes are (irreversible or
transient.
Each geodesy sensor has its tradeoffs between temporal
and spatial coverages or resolutions. Many of the
existing sensors are relatively young. For example,
altimetry satellites have only been available since the
early 1990s, implying that observed trends are sensitive
to the short‐term variability of the natural climate
system, and additional mechanistic analyses and data
are often needed to disentangle short‐term signals from
longer‐term trends.
2. How will cold region runoff and
groundwater change in warmer climates
(e.g., glacier melt and permafrost thaw)?
Groundwater change in warmer climates can be tracked
with InSAR applications to determine rates of
ground deformation once the relationship between
groundwater change and ground deformation is
established. We can combine data on several SAR
missions to determine ground deformation and
estimate groundwater volume change. Traditional
monitoring methods, that is, observation wells, are
insufficient for obtaining detailed spatial and
temporal groundwater data to understand the
dynamics of large‐scale aquifer systems. InSAR,
combined with the power of AI, can upscale the point
measurements in the monitoring wells to the entire
aquifer both in time and space. Hence, the integration
of InSAR measurement and AI is emerging as a
promising solution in the realm of groundwater
monitoring and management.
Data availability limitations of SAR sensors in important
regions where groundwater resources need to be
monitored. Data on groundwater levels needed to
establish the relationships between deformation and
groundwater level may be scarce. Coherence of the
signal in some locations may be low, limiting the
application of InSAR. Lastly, although InSAR is
becoming a common approach to detecting
deformation in groundwater‐induced subsidence in
agricultural regions, vegetation decorrelation results in
the partial spatial coverage of land deformation.
3. What are the mechanisms by which climate
change and water use alter ephemeral
rivers and groundwater in (semi‐) arid
regions?
As with InSAR, Gravimetry can