Geoscience Reference
In-Depth Information
Table 1
GCOS essential climate variables, variables directly related to the hydrological cycle are marked
in italics
Atmosphere
Surface
Air temperature; precipitation
Air pressure;
Surface radiation budget;
Wind speed and direction; water vapour
Upper air
Earth radiation budget;
Upper air temperature;
Water vapour;
Cloud properties; Wind speed and direction
Composition
Carbon dioxide, methane and other greenhouse gases (GHGs);
Ozone; Aerosol properties
Ocean
Surface
Sea-surface temperature, sea-surface salinity,sea level, sea state, sea ice, ocean colour;
Carbon dioxide partial pressure
Sub-surface
Temperature, salinity; current; nutrients; carbon; ocean tracer; phytoplankton
Terrestrial
Lake level; snow cover; glacier and ice caps; albedo; land-cover fraction of absorbed
photo-synthetically active radiation; Leaf Area Index (LAI) biomass, fire disturbance; soil
moisture
Water use, ground water, river discharge
Permafrost and seasonally frozen ground
The usefulness of remote sensing for assessing products beneficial for hydrological
monitoring dates back to the 1970s, when the potential of the infrared geostationary data
for rainfall and vegetation monitoring was first demonstrated as an important technology
for complementing and enhancing information from in situ observational systems. Over the
last three decades, the amount of relevant hydrological products derived from satellites has
increased, and they have been implemented in derivation of the majority of the Global
Climate Observing System's (GCOS) and the Global Terrestrial Observing System's
(GTOS) terrestrial and atmospheric Essential Climate Variables (ECVs) (see Table
1
).
These include, in accordance with GCOS recommendations, precipitation, water vapour,
cloud properties, soil moisture, leaf area index, water level and estimates of ground water,
evapotranspiration, river discharge, ocean salinity, snow cover, albedo, glaciers and ice
caps, ice sheets, permafrost extent and seasonally frozen ground (GCOS
2003
).
Each of these variables shown in italics can currently be estimated with the use of at
least one Earth observation system although possibly not in all cases with the requisite
spatial and temporal resolution for advancing the understanding of local to global feed-
backs in the hydrological cycle. However, adequate validation of the satellite data is
challenging and often limited implying retrievals lacking satisfactory uncertainty esti-
mates. Commonly, several systems have capabilities to derive identical parameters, and
implementation techniques for their merging and blending were suggested. As a result,
new integrated, multi-year data sets are being generated, taking advantage of the oppor-
tunity presented by the simultaneous operation of key satellites by Europe, Japan and the
USA. For instance, the ESA Climate Change Initiative (CCI) aims to demonstrate the full
potential of the long-term global Earth Observation archives as a significant and timely
contribution to the GCOS Essential Climate Variables (ECVs) databases required by
UNFCCC (United Nations Framework for Combating Climate Change). In its first phase,
the initiative is targeting the following hydrology-related variables: sea level, sea ice,
glaciers and ice caps, ice sheets and clouds. The ongoing discussion on possible operations
