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| Soil moisture
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| Definition |
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The water content of a defined soil sample.
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| Rationale |
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Of all measured climate variables soil moisture is the most important determinant of NPP and of vegetation structure, composition and density. The variable rate at which soil moisture is released back into the atmosphere by the vegetation results in a shift in time scales from that of precipitation, which occurs over a scale of minutes, to weeks. The partitioning of precipitation into soil moisture and run-off is highly dependent upon the holding capacity of the soil, the soil water content, the rain intensity, and the distribution in space and time of the rainfall. This results in another variable time scale, ranging from minutes to hours for a given rainfall pattern, to months and seasons for soil moisture.
Changes in soil moisture and evapotranspiration are likely to have large impacts on terrestrial vegetation, since the distribution and abundance of plant communities are controlled to a large extent by the quantity and seasonality of moisture. If changes in the water balance are significant (e.g. through climate change) major shifts in vegetation patterns and conditions are a likely to occur.
Soil moisture is the integrator of surface and sub-surface water fluxes and is thus the key diagnostic variable for surface water budgets. Many BGC processes are fundamentally related to soil moisture conditions. Hydrological models use soil moisture to determine runoff and evaporation rates.
Soil moisture has been shown to be a critical component of temperature and precipitation forecasts. These forecasts require soil moisture measurements down to depths of 1 to 2 metres.
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| Users |
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Ecologists using ET to estimate or model NPP, vegetation composition, and vegetation structure. Water and agricultural resource managers, hydrological modellers, and atmospheric modellers estimating land-atmosphere water fluxes.
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| Assessment method |
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Tiers 1-3: in-situ recording sensors and site-specific water budget models;
Tier 5: microwave emission and microwave backscattter (experimental).
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| Units of Measure |
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Atmospheres of pressure (Pascals) or cm/100 cm; both as available soil moisture capacity between wilting point and field capacity.
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| Frequency of measurement |
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For data fields and point sets, weekly to annual; for stations, centres and surface experiments, daily maximum and minimum or diurnal (4 x daily) measurements.
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| Spatial resolution |
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Tiers 1, 2 and 3
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| Accuracy/precision required |
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Area nearest 25 x 25 km grid;
± 5 mm up to 50 mm;
± 10 mm up to 600 mm.
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| Associated measurements |
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Soil moisture is a function of precipitation, soil texture, porosity and the rate of moisture withdrawal from the soil (water uptake by plants and evapotranspiration from them, which in turn is controlled by stomatal opening, relative humidity and wind).
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| Present status |
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There is no method to measure soil moisture directly on the needed time and space scales. Therefore, soil moisture is inferred from 4-dimensional data assimilation using models, which have sufficiently sophisticated land-surface parameterisations. Runoff, evapotranspiration and precipitation are the most important soil moisture related variables in these models. Runoff and evapotranspiration are usually derived from models, but measured precipitation must be obtained. Despite the success of research in rainfall estimation through satellite techniques, precipitation estimates still use a combination of in situ and satellite data. Therefore the networks of rain gauges managed by weather services around the world are critical. It is evident that the actual density of these networks, despite the efforts of international agency programmes, is insufficient to provide global fields of precipitation at the precision needed for soil moisture estimates.
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| R and D needed |
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- Radar imagery appears capable of measuring soil moisture, after several years of airborne and ground-based experiments, but it needs to be developed and instrumentation provided to satellite vehicles.
- Airborne and ground-based experiments and instrument development should proceed for eventual radar or SAR measurements of soil moisture.
- Some current 4-dimensional data assimilation models are predicting soil moisture fields. Mechanisms are needed to make these data routinely and economically available.
- Considerable improvements in modelling soil moisture should result from a one-off effort to improve the specificity of global soil maps to include detailed physical properties and greater spatial detail.
- Efforts should be made to support existing rainfall and runoff measurement networks, and to enhance their efficiency by using modern data collection, transmission, and dissemination systems.
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