Earth Observations to Enable Management in Support of Water Security
Water is essential for of all life on Earth. It is the only known substance that can exist naturally as a gas, liquid and solid within the relatively small range of air temperatures and pressures found on the surface of Earth. Furthermore, the chemical properties of water make it the best natural solvent and a widely used medium for waste disposal and waste dilution. Water connects almost everything in nature and society, representing a nexus of water–energy–food security interests. The extreme rate of aquatic biodiversity loss and the deep links between water and land use are among the most important water-related challenges to be addressed in our quest for sustainability.
Soil Moisture Ocean Salinity (SMOS) satellite.
Water cycles through the Earth System with the vast majority being retained in stores such as oceans and aquifers. In all, Earth’s water content is about 1.39 billion km3 and the vast bulk of it, about 97.5%, is in the global oceans. The other 2.5% is stored as freshwater in the polar icecaps, glaciers, permanent snow, groundwater, lakes, rivers, streams and soil. The amount of water that falls on land in the form of precipitation each year (often referred to as the renewable amount) is 113000 km3 which represents approximately 0.003% of the total water stored on the globe’s surface. This freshwater either returns to the atmosphere through evapotranspiration, infiltrates the soil to add to soil moisture and groundwater or runs off in streams and rivers. While soil water is important for agriculture, it is the runoff that can be most readily captured and managed for the betterment of society. This amounts to 40000 km3 which flows to the oceans in streams and rivers. Furthermore, a certain percentage of this water is rendered unusable due to water pollution.
The present global water withdrawal is approximately 4000 km3 annually (or about 10% of the annual renewable water flows through streams and ground water to the oceans) of which an estimated 2600 km3 is consumed annually. Seventy percent of this water use occurs in agriculture. As freshwater flows are reduced, freshwater ecosystems are among the most heavily affected by biodiversity loss. In many parts of the world, water withdrawal for agricultural use alone is already high compared to the locally available renewable water resources, and this will become much greater as Earth’s population increases.
SMOS global soil moisture and ocean salinity map
Demand for freshwater is expected to increase significantly in the coming decades. Furthermore, as the world’s population increases, the additional food required to feed future generations will put added pressure on fresh water resources. Future management of freshwater resources will be complicated by the uncertainties in rainfall patterns introduced by climate change, with observations and models suggesting increased frequency and intensity in both extreme precipitation and drought events, depending on the region.
Freshwater availability and use, as well as the conservation of aquatic resources, are key to human well-being. The quantity and quality of surface and groundwater resources, and life-supporting ecosystem services are being jeopardised by the impacts of population growth, rural to urban migration, and increasing wealth and resource consumption, as well as by climate change. If present trends continue, it is estimated that 1.8 billion people will be living in countries or regions with water scarcity by 2025, and two thirds of the world population could be subject to water stress. Freshwater is a finite and vulnerable resource, essential to sustain life, development and the environment. Management of this resource is expected to emerge as one of the greatest challenges in achieving the goal of sustainable development during the 21st century.
The Water Cycle
The combination of increased scarcity of global water resources and increased uncertainties in Earth’s water cycle has added urgency to the need to improve predictions of rainfall and water resources by developing an integrated water cycle-observing system. This requires extending our understanding of the water cycle and its variability due to climate change and human activities such as land-use change that affect the flow of water over the terrain.
This movement of water, in continuous circulation from the ocean to the atmosphere to the land and back again to the ocean, is termed the global water cycle and is at the heart of the Earth System, affecting every physical, chemical and ecological component. Changes to this system are among the highest priority issues confronting the Earth science and environmental policymaking communities. Often, water-related policies are implemented without a means of tracking the resulting changes in water-use patterns. This is compounded by uncertainty in models of future precipitation distribution.
TRMM Image of Hurricane Katrina before it hit New Orleans. The
satellite’s 3D look inside the storm provided unique information on
the rainfall structure as it approached land.
The GPM constellation.
Better predictions of water cycle behaviour are needed for:
− Monitoring climate variability and change;
− Effective water management;
− Sustainable development of the world’s water resources;
− Understanding trends in water use and long-term projections;
− Improved weather forecasts and monthly-to-seasonal climate predictions, including forecasts for mitigation of drought and flood impacts.
Owing to the complexity of the global water cycle, long-term observational datasets are needed to characterise its behaviour. A number of important parameters need to be measured, including precipitation, evapotranspiration, soil moisture, surface water and groundwater:
− Global precipitation is needed as the basic driver for Numerical Weather Prediction systems, climate models and hydrologic models;
− Evapotranspiration measurements are important for understanding the influence of the plant canopy on the water vapour content of the atmosphere and for estimating the rate of plant growth;
− Soil moisture affects the partitioning of rainfall into infiltration and runoff, as well as the partitioning of energy between sensible and latent heat that is transported into the atmosphere. When water is retained in some soils, it creates a reserve for plant growth, thereby promoting plant productivity during the growing season;
− Surface water is crucial for domestic use, irrigation, energy production and ecosystems. Runoff and river discharge are critical measurements for flood prediction and evaluating the hydrologic impacts of drought;
− Groundwater as surface water supplies diminish or become more contaminated, people in many parts of the world increasingly rely on groundwater for their water supply. Groundwater data are essential for assessing changes in groundwater resources and evaluating the vulnerability and sustainability of strategic aquifers;
− Water storage in snow, glaciers and ice cover, lakes and wetlands;
− Sea-surface temperature as a significant factor influencing rainfall patterns, often markedly as in the El Niño phenomena; coupled with wind and air temperatures it also provides a measure of air–sea fluxes;
− Land-surface temperature is an important variable in estimating the factors that contribute to evapotranspiration, snow and glacier melt, crop growth and other properties;
− Atmospheric temperature and water vapour.
In recent years and in large parts of the world, the collection and dissemination of in-situ water-related information has been in decline. In order to strengthen cooperation on information gathering amongst countries, the WMO – in association with the World Bank – established the World Hydrological Cycle Observing System (WHYCOS) in 1993. WHYCOS is based on a global network of reference stations, which transmit hydrological and meteorological data in near-real time, via satellites, to national and regional centres. A number of international scientific research programmes have been developed to address the key challenges relating to the global water cycle, most notably under the auspices of the World Climate Research Programme and its Global Energy and Water Cycle Experiment (GEWEX).
The main forum for coordination of supporting observation programmes – including those of the satellite and in-situ measurement communities – is the GEO Integrated Global Water Cycle Observations Theme (IGWCO) Community of Practice (COP). COP provides a framework for guiding international decisions regarding priorities and strategies for the maintenance and enhancement of water cycle observations in support applications development and science objectives, including the provision of systematic observations of trends in key hydrologic variables.
COP also has close links with the Global Water System Project (GWSP), GEWEX (WCRP), UNESCO and other partners that have a strong interest in the management of water for sustainable development. For this reason, IGWCO is putting great emphasis on the development of integrated data products that cross data collection types, and the Water Cycle Integrator, which will cross many scales and functions. It is also exploring ways in which Earth observations can be used to support Integrated Water Resources Management (IWRM). This will be elaborated in the GEO Water Cycle Strategy, currently under development.
Collaboration between research and operational Earth observation activities is vital. GCOS had played a very important role in supporting the application of satellite data and in-situ observations to the climate change issue. It is hoped that the success of GCOS can be repeated in the water domain through collaboration between IGWCO COP, GEO, WMO and other partners. To complement the satellite data, existing ground-based measurement networks and systems must continue to acquire data that can be compared meaningfully with past records.
The Role of Earth Observation Satellites
Earth observation satellites play a major role in the provision of information for the study and monitoring of the water cycle and represent an important element of the observation strategy. Currently, new integrated, multi-year datasets are being generated, taking advantage of the opportunity presented by the simultaneous operation of key satellites by Europe, Japan and the USA. Satellite data provide many opportunities to increase the information available for water management. Their global nature also helps to address the problems of data continuity in trans-boundary basins where complete, consolidated, and consistent information may be difficult to obtain.
Atmospheric temperature, water vapour and cloud data have been provided operationally by polar-orbiting meteorological satellites for decades, provided by the USA (NOAA series) and more recently Europe (EUMETSAT’s MetOp series), as well as China and Russia. The use of high-resolution infrared soundings (IASI), radio occultation techniques (which look at the interaction of radio signals with the atmosphere to derive characteristics of the atmosphere), and the Global Positioning Satellite (GPS) signal (e.g. by the COSMIC satellite constellations and GRAS on MetOp) have further augmented the contribution from space.
Water vapour observations from a geostationary satellite.
Sea-surface temperature measurements are also provided by the operational meteorological satellites, Envisat (AATSR) and the Terra and Aqua missions (MODIS). More recently, NOAA’s VIIRS instrument, launched on Soumi NPP in 2011, has added to sea-surface temperature measurements and is expected to continue through the JPSS series, and eventually be joined by the OLCI instrument on the Sentinel-3 series.
Precipitation is clearly a key parameter in the water cycle. Traditionally, visible/infrared images from geostationary meteorological satellites like GOES, GMS and Meteosat provided the best source of satellite information, with indirect but frequent estimates of rainfall derived from measurements of cloud top temperature. This data is used in the WCRP’s GEWEX Global Precipitation Climatology Project (GPCP), which provides monthly mean precipitation data from 1979 up to the present, as well as its International Satellite Cloud Climatology Project (ISCCP).
Cloud and precipitation systems tend to be somewhat random in character and also evolve very rapidly, especially during the summer in convection regimes. These factors make clouds and precipitation difficult to quantify. Reliable ground-based precipitation measurements are difficult to obtain over regional and global scales because more than 70% of Earth’s surface is covered by water, and many countries are not equipped with precision rain measuring sensors (rain gauges and/or radars). The only practical way to obtain useful regional and global-scale precipitation measurements is from the vantage point of a space-based remote sensing instrument.
The advent of the Tropical Rainfall Mapping Mission (TRMM, NASA/JAXA) in 1997 provided a breakthrough in the provision of 3D information on rainfall structure and characteristics. TRMM was the first satellite dedicated to rainfall measurement and is still the only satellite that carries a weather radar. Now in its 15th year, the TRMM mission has provided a wealth of knowledge on severe tropical storms such as hurricanes and short-duration climate shifts such as El Niño. Such active sensors have proved themselves to be an essential tool for the measurement of precipitation. Continuity will be provided via the Global Precipitation Measurement (GPM) Core mission, the core of a GPM constellation.
The Water Cycle.
In 2011, Megha-Tropiques was launched, devoted to the study of the water cycle and energy exchanges in the atmosphere. Its low-inclination orbit focuses observations on the tropics, using microwave radiometers to monitor precipitation, clouds and water vapour on a frequent basis. Megha-Tropiques increases the sampling rate of the GPM constellation in this crucial region of the globe. This will be complimented by JAXA’s Global Change Observation Water Mission series (GCOM-W). All these missions apply microwave-based techniques (using either passive remote sensors, or weather radars) to provide the next generation of rainfall measurements.
The Soil Moisture and Ocean Salinity (SMOS) mission is ESA’s second Earth Explorer Opportunity mission, developed jointly with CNES and CDTI, and launched in November 2009. SMOS provides soil moisture and ocean salinity measurements. It was followed by SAC-D/Aquarius (CONAE/NASA), which was launched in June 2011, which also focused on ocean salinity measurements. Soil moisture and ocean salinity are important parameters as they help understand the energy balance between Earth’s surface and the atmosphere. Their global distribution is of interest for climatic and weather research, in particular in improving model and forecasts. Both SMOS and Aquarius rely on measurements in the L-band, and significant issues with interference from unauthorised terrestrial sources have had to be overcome to ensure their sustainable operation.
Evapotranspiration is generally estimated from satellite data using a range of models, with model inputs from the visual and thermal bands from the GEO, MODIS and Landsat satellites.
An emerging application area is the use of GRACE, and its gravimetric measurements, which are being used to quantify groundwater changes. Plans are currently being formulated for GRACE Follow On and GRACE-II missions, and it is expected this area of research will continue to expand. Other areas of emerging interest include the use of optical wavelengths to assess plankton and other waterborne materials, and the exploration of radar altimetry to measure water levels in lakes and rivers.
In addition to producing data products, CEOS and its collaborators work with applications groups to ensure the information are used. Following the 2002 Johannesburg World Summit on Sustainable Development, space agencies, including ESA with its TIGER initiative, responded to the need to expand capacity-building efforts, where data and capabilities from developed countries would be applied in the developing world. Through the efforts of GEO and the IGWCO COP, a number of other regional capacity-building efforts have been launched, including AWCI (Asian Water Cycle Initiative), CIEHLYC (Water Cycle Capacity Building activity in Latin America) and AfWCCI (African Water Cycle Coordination Initiative), focusing on the use of space technology for water resource management and providing concrete actions to match the Resolutions.
New technologies for measuring, modelling and organising data on Earth’s water cycle offer the promise of deeper understanding of water cycle processes and of how management decisions may affect them. Currently, Earth observation satellites provide high-resolution measurement coverage that is unprecedented in the geophysical sciences. The challenges to be faced in utilisation of these new capabilities include:
− Development of new analytical methodologies to exploit existing long time series of satellite measurements;
− Investigating novel approaches to convert satellite measurements into useful parameters that can be applied in scientific models, and that can be inter-compared and inter-calibrated among the different satellite missions;
− Development of assimilation methodologies to integrate satellite and in-situ observations;
− Capacity building, particularly in developing countries so that those countries in most dire need of water information have the means of access, analysis and understanding required to derive maximum benefit from the data;
− Continuing to collect consistent and accurate data over many years in order to detect the trends necessary for climate change studies;
− Succeeding in the technology developments aimed at accurately measuring key parameters from space, including precipitation and soil moisture.
To complement the satellite data, existing ground-based measurement networks and systems must continue operating to obtain current data that can be compared meaningfully with past records. The development of new and innovative satellite applications needs to continue, and continuity of data records for current satellite data streams needs to be assured.
CEOS Virtual Constellation for Precipitation
Recognising the vital importance of timely and accurate precipitation measurements in support of a broad range of societal needs, including climate studies, weather forecasting, flood predictions for extreme events, water resource management and agriculture, CEOS created a Virtual Constellation for Precipitation. The Constellation seeks to improve international coordination of Earth observation satellite planning in support of common needs, and its goals include:
- Providing a framework to advocate and facilitate the timely implementation of the GPM mission and encourage more nations to contribute to the GPM constellation. Although GPM offers impressive new measurement capabilities, the mission period is only three years;
- Sustaining and enhancing an accurate and timely global precipitation data record, including a Fundamental Climate Data Record, essential for understanding the integrated weather/climate/ecological system, managing freshwater resources and monitoring and predicting high-impact natural hazard events. This data record should be fit for the purpose specified by GCOS for the monitoring of precipitation as an essential climate variable.
NASA and JAXA are co-leading the development of the GPM mission that is the cornerstone of the Constellation. The GPM mission is expected to be augmented significantly by a collaboration agreement with Megha-Tropiques (ISRO, CNES). Brazil (INPE), Europe (ESA & EUMETSAT) and Japan (JAXA) are also supporting the creation of enhanced, merged multi-satellite global precipitation products. It is also hoped to investigate the capabilities and potential of adding meteorological missions operated by China and Russia to the Constellation.
The challenges typify the nature of such international projects and include the difficulties of formalising the sensitive inter-agency agreements among many different countries for the sake of the common good, with participants being guaranteed open access to the resultant datasets.