An essential resource

Of all the water present on our planet, only 2.5% is fresh, and only 0.007% is readily available to people via rivers, lakes, and reservoirs. Freshwater is a finite and vulnerable resource, essential to sustain life, development and the environment, and management of this resource is expected to emerge as one of the greatest challenges facing mankind during the 21st century.

Despite significant improvements in recent decades, over one billion people still lack access to safe water, and nearly two billion lack safe sanitation. An estimated 10,000 people die every day from water and sanitation-related diseases, and thousands more suffer from a range of debilitating illnesses. The impact of inadequate water and sanitation services falls primarily on the world’s poor.

Humans currently appropriate more than half of accessible freshwater run-off, and this amount is expected to increase significantly in the coming decades. A substantial amount, 70%, of the water currently withdrawn from all freshwater resources is used for agriculture. With the world’s population set to increase significantly by 2050, the additional food required to feed future generations will put further pressure on fresh water resources. According to recent global water assessments, around 70% of the future world population will face water shortages and 16% will have insufficient water to grow their basic food requirement by 2050. 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.

The combination of increased scarcity of global water resources, and increased uncertainties in the 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, and by extending our understanding of the physical basis of the climate system driven by the water cycle.


Observing and understanding the water cycle

In all, the Earth’s water content is about 1.39 billion cubic kilometres and the vast bulk of it, about 96.5%, is in the global oceans. Approximately 1.7% is stored in the polar icecaps, glaciers, and permanent snow, and another 1.7% is stored in groundwater, lakes, rivers, streams, and soil. Finally, a thousandth of 1% exists as water vapour in the Earth’s atmosphere.

The Earth’s water cycle

Irrigation for agriculture dominates human use of available water. New satellite techniques for measurement of soil moisture will help to account for this element of the water cycle.

Because water continually evaporates, condenses, and precipitates, with evaporation on a global basis approximately equalling global precipitation, the total amount of water vapour in the atmosphere remains approximately the same over time. This movement of water, in a 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’s climate system, affecting every physical, chemical, and ecological component. Amongst the highest priorities in Earth science and environmental policy issues confronting society are the potential changes in global water cycle due to climate change. Climate changes may profoundly affect atmospheric water vapour concentrations, clouds, and precipitation patterns. Many uncertainties remain, however, as illustrated by the inconsistent results given by current climate models regarding the future distribution of precipitation.

Better predictions of water cycle behaviour are needed for:

• monitoring climate variability and change;
• effective water management through better provision of information inputs to decision support tools;
• sustainable development of the world’s water resources requiring knowledge of trends and long-term projections of the intensity of the global water cycle;
• improved weather forecasts and monthly to seasonal climate predictions – including for mitigation against drought and flood.
  Such capabilities will require improved understanding of a range of complex processes, such as:
• evaporation processes from the global ocean (which account for as much as 87% of the water present in the atmosphere);
• land surface hydrologic processes which govern evapotranspiration and the partitioning of rainfall between re-evaporation, storage in the soil, and run-off to rivers;
• relationships between global climate and regional weather systems which govern clouds and rainfall;
• the science of clouds, and how they lead to precipitation.
  Given the complex and global nature of the water cycle, this understanding can only be achieved if scientists are equipped with long-term data to characterise the behaviour of the Earth System with regards to a range of key parameters, including:
• global precipitation: precipitation is the most significant aspect of climate change from the perspective of human interests and the health of ecosystems;
• atmospheric temperature and water vapour: since water vapour is the Earth’s primary greenhouse gas and contributes significantly to uncertainties in projections of future global warming, it is critical to understand how it varies in the Earth System;
• sea surface temperature and ocean salinity: as a significant measure of air-sea fluxes;
• soil moisture and snow accumulation: to assess the freshwater budget of land and ocean.

In large parts of the world, the collection and dissemination of water-related information has been in decline in recent years. In order to strengthen cooperation amongst countries in gathering the necessary information, 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.



Water stress and scarcity will increase globally due to population increases and climate changes


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, including:

• GEWEX: The Global Energy and Water Cycle Experiment whose scientific focus includes studies of atmospheric and thermodynamic processes that determine the global hydrological cycle and water budget and their adjustment to global changes such as the increase in greenhouse gases and land use change;
• CLIVAR: “Climate Variability and Predictability” is the main focus in WCRP for studies of climate variability.

The main forum for co-ordination of the supporting observation programmes – including those of the satellite and in-situ measurement communities, is the Integrated Global Water Cycle Observations Theme (IGWCO) of the IGOS Partnership. IGWCO provides a framework for guiding international decisions regarding priorities and strategies for the maintenance and enhancement of Water Cycle observations so they will support the most important applications and science goals, including the provision of systematic observations of trends in key hydrologic variables.

On the morning of 15th September 2004, the TRMM satellite captured a 3-D look inside Hurricane Ivan – providing unique information on the structure of rainfall inside the storm as Ivan approached landfall


The role of Earth observation satellites

Earth observation satellites play a major role in the provision of information for study and monitoring of the water cycle and represent an important element of the observation strategy defined within IGWCO – the first element of which is the CEOP project (Co-ordinated Enhanced Observing Period), which is taking the opportunity of the simultaneous operation of key satellites of Europe, Japan, and USA, and the GEWEX Continental-scale Experiments, during a multi-year period to generate new data sets of the water cycle.

HYDROS will deliver the first global views of the Earth’s soil moisture content and freeze/thaw state

Novel measurements of soil moisture and ocean salinity will be provided by SMOS

Atmospheric temperature and water vapour data are provided operationally by polar orbiting meteorological satellites. Sea surface temperature measurements are also provided by these satellites, by Envisat (AATSR), and by the Terra and Aqua missions (MODIS). Ocean wind measurements are also provided by these missions – and by NASA’s QuikSCAT which acquires all-weather, high-resolution measurements of near-surface winds over 90% of the global oceans on a daily basis.

Precipitation is clearly a key parameter, but given the high temporal and spatial variability of precipitation it is a fundamentally difficult parameter to measure. Until recently, visible/infrared images from geostationary meteorological satellites provided the best source of information from satellite – with indirect but frequent estimates of rainfall derived from measurements of cloud top temperature. The advent of the Tropical Rainfall Mapping Mission (TRMM of NASA/JAXA) in 1997 provided a breakthrough in the provision of 3-D information on rainfall structure and characteristics.

NASA, JAXA, and ESA will collaborate to follow-up the success of TRMM and will launch the Global Precipitation Mission (GPM) from 2009. GPM aims to provide precipitation measurements on a global basis with sufficient quality, Earth coverage, and sampling to improve prediction of the weather, the Earth’s climate, and specific components of the global water cycle. GPM design involves a large ‘core’ platform equipped with both passive and active microwave instruments and a number of smaller satellites large enough to ensure a repeat observation cycle of approximately 3 hours.

Recognising the central role of the water cycle to our understanding of the Earth System and climate change, the world’s space agencies are operating or developing a number of new missions aimed at addressing key water cycle issues. These include the Aqua mission (NASA), Cloudsat (NASA), EarthCare (ESA/JAXA), Cryosat (ESA), and Megha-Tropiques (CNES/ISRO) (which will study water cycle and energy exchanges in the tropical belt). Revolutionary new measurement capabilities – such as the provision of information on soil moisture and ocean salinity – will be provided in future by missions such as SMOS (ESA), HYDROS (NASA), and Aquarius (CONAE/NASA).

Following the 2002 Johannesburg World Summit on Sustainable Development, ESA launched the TIGER Initiative - focusing on the use of space technology for water resource management in Africa and providing concrete actions to match the Resolutions.



Water vapour observation from geostationary satellite


Future challenges

New technologies for measuring, modelling, and organising data on the Earth’s water cycle offer the promise of deeper understanding of water-cycle processes and of how management decisions may affect them. Earth observation satellites will provide synoptic, high-resolution measurement coverage that is unprecedented in the geophysical sciences. The challenges to be faced in utilisation of these new capabilities include:

• converting 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 for maximum benefit;
• providing 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 for the first time – including soil moisture and ocean salinity.

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.

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