The contribution of space to hydrology

What is space hydrology?

Space hydrology involves the study of the distribution, movement, and management of water resources on Earth using satellite observations and remote sensing techniques. It combines principles of hydrology, remote sensing, and geospatial analysis to monitor various aspects of the water cycle, such as precipitation, evaporation, soil moisture, groundwater storage, snow cover, and surface water bodies, on regional to global scales.

By utilizing data from satellites equipped with sensors capable of measuring different aspects of the Earth’s surface and atmosphere, space hydrology enables researchers to monitor water resources in remote or inaccessible regions, where traditional ground-based observations may be limited. Also, this information is crucial for understanding and managing water availability, predicting floods and droughts, assessing the impact of climate change on water resources, and supporting water resource management and decision-making processes.

What is the contribution of space hydrology to the science of hydrology?

Space hydrology provides several key benefits to the field of hydrology:

Global Coverage: Firstly, Satellite observations offer a global perspective, allowing researchers to monitor water resources in remote and inaccessible regions where ground-based observations are limited or nonexistent. This comprehensive coverage facilitates a better understanding of large-scale hydrological processes and their spatial variability.

Temporal Resolution: In addition, Satellites can provide frequent and consistent measurements over time, allowing for the monitoring of short-term variations and long-term trends in hydrological variables such as precipitation, soil moisture, and snow cover. Furthermore, this temporal resolution is particularly valuable for tracking changes in water resources over different seasons and climatic conditions.

Data Continuity: Furthermore, Space-based observations provide continuous and consistent datasets, enabling researchers to analyze long-term trends and variability in water resources. Moreover, this continuity is crucial for assessing the impacts of climate change, land use change, and human activities on hydrological processes.

Multi-Scale Analysis: Satellite data can be used to study hydrological processes at multiple spatial scales. Furthermore, it ranges from small watersheds to large river basins and entire continents. This multi-scale analysis helps researchers understand how local processes contribute to regional and global hydrological patterns. Also, it helps researches understand changes at one scale may affect others.

Improved Modeling and Forecasting: Satellite data can be integrated into hydrological models to improve their accuracy and reliability for many  predicting water availability, floods, and droughts. By assimilating satellite observations into models, researchers can better constrain model parameters and initial conditions. Thus, it leads to more accurate forecasts and early warning systems for water-related hazards.

Data Integration and Synthesis: In fact, space hydrology facilitates the integration of diverse datasets from multiple sources. This includes satellite observations, ground-based measurements, and model simulations. Thus, this integrated approach allows combining different types of data to gain a more comprehensive understanding of complex hydrological processes and their interactions with other Earth system components.

For those of you willing to delve further into the topic, here’s a selection of ten significant academic articles and papers related to space hydrology that we use as references at BWI:

1. Rodriguez-Iturbe, I., & Entekhabi, D. (1996). Water Resources Variability and Remote Sensing: Hydrology, Water Resources, and Remote Sensing Satellite Observations. Water Resources Research, 32(7), 2147–2156. [DOI: 10.1029/96WR00792]
2. Wood, E. F., & Lettenmaier, D. P. (2006). An Ensemble Approach for Uncertainty Estimation of Soil Moisture and Precipitation-Induced Streamflow Variability. Water Resources Research, 42(8). [DOI: 10.1029/2005WR004745]
3. Crow, W. T., Huffman, G. J., Bolten, J. D., Joyce, R. J., Hsu, K. L., Braithwaite, D., & Adler, R. F. (2012). Integrated Data Fusion for Improved Global Precipitation Estimation. Journal of Hydrometeorology, 13(2), 368–382. [DOI: 10.1175/JHM-D-11-020.1]
4. Western, A. W., Blöschl, G., & Grayson, R. B. (2001). Toward capturing hydrologically significant connectivity in spatial patterns. Water Resources Research, 37(1), 83–97. [DOI: 10.1029/2000WR900249]
5. Alsdorf, D. E., Rodriguez, E., & Lettenmaier, D. P. (2007). Measuring Surface Water from Space. Reviews of Geophysics, 45(2). [DOI: 10.1029/2006RG000197]
6. Jackson, T. J., & Schmugge, T. J. (1991). Passive Microwave Remote Sensing of Soil Moisture. Journal of Hydrology, 122(1–4), 81–103. [DOI: 10.1016/0022-1694(91)90146-V]
7. Wagner, W., Scipal, K., Pathe, C., & de Rosnay, P. (2007). Evaluation of the microwave polarization difference index for soil moisture monitoring. Geophysical Research Letters, 34(17). [DOI: 10.1029/2007GL030450]
8. Huffman, G. J., Adler, R. F., Bolvin, D. T., & Nelkin, E. J. (2010). The TRMM Multi-satellite Precipitation Analysis (TMPA). Chapter 1 in Satellite Rainfall Applications for Surface Hydrology, Springer, 3–22. [DOI: 10.1007/978-90-481-2915-7_1]
9. Dorigo, W. A., Wagner, W., Hohensinn, R., Hahn, S., Paulik, C., Xaver, A., Gruber, A., Drusch, M., Mecklenburg, S., van Oevelen, P., Robock, A., & Jackson, T. (2011). The International Soil Moisture Network: a data hosting facility for global in situ soil moisture measurements. Hydrology and Earth System Sciences, 15(5), 1675–1698. [DOI: 10.5194/hess-15-1675-2011]
10. Brocca, L., Melone, F., Moramarco, T., Wagner, W., Naeimi, V., Bartalis, Z., & Hasenauer, S. (2010). Assimilation of Surface-and Root-Zone ASCAT Soil Moisture Products into Rainfall–Runoff Modelling. Hydrology and Earth System Sciences, 14(5), 2813–2827. [DOI: 10.5194/hess-14-2813-2010]

These papers cover various aspects of space hydrology, including remote sensing of precipitation, soil moisture, and surface water. They also cover the integration of satellite data into hydrological models for improved water resource management and forecasting.

What are some good examples of space missions that contribute to hydrology?

Several satellites and satellite missions provide valuable data for hydrology by monitoring various aspects of the water cycle. Here are some key satellites and their contributions:

1. NASA’s Soil Moisture Active Passive (SMAP): SMAP provides high-resolution global measurements of soil moisture and freeze/thaw state using both active (radar) and passive (radiometer) microwave sensors. These measurements help improve our understanding of soil moisture dynamics. Also, it is crucial for agriculture, water resource management, and climate studies.
2. NASA’s Gravity Recovery and Climate Experiment (GRACE) and its successor, GRACE Follow-On (GRACE-FO): These missions use twin satellites to measure variations in Earth’s gravity field which are primarily caused by changes in the distribution of water mass on or near Earth’s surface. GRACE/GRACE-FO data are used to monitor changes in terrestrial water storage, including groundwater depletion, ice melt, and surface water storage, providing valuable insights into regional and global hydrological processes.
3. NASA’s Global Precipitation Measurement (GPM) Core Observatory: The GPM mission combines data from a constellation of satellites to provide global observations of precipitation, including rain and snowfall rates. To sum up, these measurements are essential for improving our understanding of precipitation patterns, extreme weather events, and their impacts on water resources, agriculture, and natural hazards such as floods and droughts.
4. European Space Agency’s (ESA) Sentinel-1 and Sentinel-2: These satellites are part of the Copernicus program. They provide radar and optical imagery, respectively, for monitoring land and water resources. Sentinel-1’s radar data are particularly useful for mapping floods, monitoring wetlands, and measuring soil moisture. Likewise, Sentinel-2’s optical imagery can be used to assess changes in land cover, vegetation health, and water quality.
5. Japan Aerospace Exploration Agency’s (JAXA) Advanced Land Observing Satellite (ALOS) series: The ALOS satellites, including ALOS-1 (Daichi) and ALOS-2 (Daichi-2), provide high-resolution radar imagery for various applications, including land cover mapping, forest monitoring, and surface water detection. ALOS data are valuable for studying changes in land use, vegetation cover, and hydrological processes.
6. China’s Fengyun series of meteorological satellites: China’s Fengyun satellites provide a range of meteorological observations. This includes cloud cover, atmospheric temperature, humidity, and precipitation. These data are used for weather forecasting, climate monitoring, and hydrological modeling.
7. SWOT (Surface Water and Ocean Topography) is a satellite mission developed by NASA, CNES, and other partners. Launched in December 2022, SWOT provides high-resolution measurements of Earth’s surface water and ocean topography. SWOT carries a radar altimeter and a Ka-band radar interferometer. These work together to precisely measure water surface elevation and characterize its variations. These measurements allow scientists to map the height of water bodies with unprecedented accuracy and detail. They provide insights into their volume, extent, and dynamics. Some of the key contributions of SWOT to hydrology include:
   1. Improved Monitoring of Inland Water Bodies: SWOT provides high-resolution observations of rivers, lakes, and reservoirs. This enables researchers to monitor changes in water levels, flow rates, and storage capacities over time. Also, this information is essential for water resource management, flood forecasting, and ecosystem conservation efforts.
   2. Mapping of River Networks: SWOT’s measurements help create detailed maps of river networks. They include small and remote watercourses that are often poorly monitored. These maps improve our understanding of river morphology, connectivity, and hydrological connectivity. They also support studies on sediment transport, habitat fragmentation, and river basin management.
   3. Quantification of Global Water Storage: Firstly, SWOT measures changes in surface water. As a result, it contributes to the estimation of global water storage variations. Further, this includes changes in terrestrial water storage, groundwater depletion, and surface water fluxes. In addition, this information is valuable for understanding the water cycle. This is because of assessing water resources sustainability, and monitoring the impacts of climate change on hydrological systems.
   4. Coastal and Estuarine Monitoring: SWOT’s observations extend to coastal regions and estuaries, where interactions between freshwater and saltwater environments play a crucial role in ecosystem dynamics, water quality, and coastal hazards. SWOT data helps monitor coastal inundation, erosion, and habitat changes, supporting coastal zone management and adaptation strategies.

   SWOT represents a significant advancement in satellite-based hydrology. Thus, it provides researchers with new tools for studying Earth’s surface water dynamics at an unprecedented spatial resolution. Also, SWOT observations enhance our understanding of hydrological processes. Consequently, it would help improve water resource management practices. Also, it supports efforts to address water-related challenges in both terrestrial and coastal environments.

8. Firstly, EUMETSAT satellites in general, and in particular (but not limited to, see below), Jason-3 is a satellite mission that contributes significantly to the study of oceanography, particularly in measuring sea surface height and monitoring ocean dynamics. While Jason-3 primarily focuses on oceanography, its data also have applications in hydrology, particularly in coastal regions and areas where rivers discharge into the ocean. Here’s how Jason-3 contributes to hydrology:
   1. Monitoring of Coastal Sea Levels: Jason-3 provides precise measurements of sea surface height, including variations along coastlines. Moreover, Changes in coastal sea levels can impact coastal communities, estuarine environments, and coastal aquifers. Thus, by monitoring these changes, Jason-3 data contributes to understanding coastal hydrodynamics and coastal zone management.
   2. River Discharge into the Ocean: Rivers are a major source of freshwater input into the ocean, and their discharge rates can vary seasonally and in response to precipitation events. Also, Jason-3’s measurements of sea surface height can indirectly provide information about changes in river discharge.  This information is valuable for assessing water availability, freshwater inflows to coastal ecosystems, and the impact of riverine inputs on coastal water quality.
   3. Flood Monitoring: While Jason-3 is not specifically designed for flood monitoring, its sea surface height measurements can indirectly provide insights into coastal flooding events. Also, extreme precipitation events can lead to increased river discharge and runoff, which can elevate coastal sea levels. Thus, by monitoring changes in sea surface height, Jason-3 data can help identify areas at risk of coastal flooding and support flood forecasting and emergency response efforts.

   While Jason-3’s primary focus is on oceanography, its measurements of sea surface height have implications for hydrology, particularly in coastal regions where interactions between land and sea influence water dynamics. Integrating Jason-3 data with other hydrological observations can enhance our understanding of coastal hydrodynamics, river-ocean interactions.

   As a gentle reminder, EUMETSAT (the European Organisation for the Exploitation of Meteorological Satellites) is an intergovernmental organization dedicated to the operation of meteorological satellites for the monitoring of weather, climate, and environmental conditions. While EUMETSAT’s primary focus is on meteorology, its satellite data also contribute to various aspects of hydrology and water resource management.

   Here are some other ways in which EUMETSAT’s satellite data benefit hydrology:

   1. Meteorological Observations: EUMETSAT operates a fleet of satellites, including the Meteosat series for geostationary meteorological observations and the Metop series for polar orbiting meteorological observations. These satellites provide essential data on atmospheric conditions, including precipitation, humidity, temperature, and atmospheric circulation patterns, which are crucial inputs for hydrological modeling, weather forecasting, and climate studies.
   2. Monitoring of Precipitation: EUMETSAT’s satellites contribute to the monitoring of precipitation patterns and intensities through instruments such as the Advanced Scatterometer (ASCAT) and the Microwave Humidity Sounder (MHS). These instruments provide data on rainfall rates, snowfall, and atmospheric moisture content, which are important for assessing water availability, drought conditions, and flood risks.

   EUMETSAT’s primary mandate is meteorology. However, its satellite data and products play a valuable role in advancing our understanding of hydrological processes. They support water resource management practices, and enhancing resilience to water-related hazards and climate variability.

All the above-mentioned satellites, along with others not mentioned here, are good examples of how space missions play critical roles in advancing our understanding of hydrological processes. Also they helpp improving water resource management, and enhancing our ability to monitor and respond to water-related hazards and environmental changes on regional and global scales. At Blue Water Intelligence, we’re been working on trying and make happen a complementary space mission named SMASH (Small Altimetry Satellites for Hydrology), that aims at providing daily water level data, globally. Therefore, this mission would be useful to hydrology as a science, but also to water risks professionals as well as disaster monitoring and water resource management authorities.

All in all, the contribution of space hydrology to the field of hydrology as a science lies in its ability to provide comprehensive, consistent, and timely information about water resources at both local and global scales, thereby enhancing our understanding of the water cycle and supporting informed decision-making for water resource management and environmental conservation.