|
GCOS Terrestrial ECV Definition: Soil Moisture is the total amount of water, including the water vapor, in an unsaturated soil. (from the AMS Glossary of Meteorology) Introduction: Soil moisture has an important influence on land-atmosphere feedbacks at climate time scales, in particular, because it has a major effect on the partitioning of incoming radiation into latent and sensible heat and on the allocation of precipitation into runoff, subsurface flow and infiltration. Changes in soil moisture have a serious impact on agricultural productivity, forestry, and ecosystem health. Monitoring soil moisture is critical for managing these resources. This variable must be developed within the GCOS framework to ensure proper coordination with other land surface variables. In situ soil moisture activities can build on the soil-moisture data archive at Vienna University. A satellite-based soil moisture product from the Advanced Scatterometer (ASCAT) has recently been made available and could potentially contribute to a longer-term record by building on data from earlier scatterometers. The various ways of representing soil moisture from both satellite and in situ measurements, in combination with climate models, need harmonization and ultimately standardisation. This could be achieved by an expanded network of reference stations to support the validation of satellite measurements with in situ data. With this objective, a Global Terrestrial Network for Soil Moisture (GTN-SM) will be initiated with the aid of TOPC. Satellite Measurements: Soil moisture plays a key role in the hydrological cycle. Evaporation rates, surface runoff, infiltration and percolation are all affected by the level of moisture in the soil. Changes in soil moisture have a serious impact on agricultural productivity, forestry and ecosystem health. Monitoring soil moisture is critical for managing these resources and understanding long-term changes, such as desertification, and should be developed in proper coordination with other land surface variables. There is a pressing need for measurements of soil moisture for applications such as crop yield predictions, identification of potential famine areas, irrigation management, and monitoring of areas subject to erosion and desertification, as well as for the initialisation of NWP models. Direct measurement of soil moisture from space is difficult. Most of the active and passive microwave instruments provide some soil moisture information for regions of limited vegetation cover. However, under many conditions remote sensing data are inadequate and information regarding moisture depth remains elusive. While recent studies have successfully demonstrated the use of infrared, passive microwave, and non-SAR sensors to obtain soil moisture information, the potential of active microwave remote sensing based on SAR instruments remains largely unrealised. The main advantage of radar is that it provides observations at a high spatial resolution of tens of metres compared to tens of kilometres for passive satellite instruments, such as radiometers, or non-SAR active instruments, such as scatterometers (e.g. QuikSCAT and ERS). The main difficulty with SAR imagery is that soil moisture, surface roughness and vegetation cover all have an important and nearly equal effect on radar backscatter. These interactions make retrieval of soil moisture possible only under particular conditions, such as bare soil or surfaces with low vegetation, or through complex modelling to ‘subtract’ the contributions/effects of vegetation. An appropriate instrument for measurements of soil moisture would appear to be the passive microwave radiometer, although some success has been achieved by radar – despite the complications of analysing the signals reflected from the ground. Microwave radiation emitted at the ground can be monitored to infer estimates of soil moisture. Passive microwave sensors can be used to do this, based on detection of surface microwave emissions, although the signal is very small. Reliable data (high signal to noise ratio) need to be taken over a large area – which introduces the problem of understanding how to interpret the satellite signal, since it consists of radiation from many different soil types. (Satellite Missions) (Source: CEOS EO Handbook - Earth Observations Plans by Measurement) References:
Data, Product, Metadata and Information Access [ECV Matrix Main Page] [About the ECV Matrix] [Reference Documents] [Contact] [Updated June 29, 2010]
|
|