Soil Moisture and Ocean Salinity (SMOS)

(MIRAS on RAMSES)

The baseline SMOS payload is an L-band (1.4 GHz), Y shaped, 2D interferometric radiometer. It is proposed to launch SMOS on a sun synchronous orbit.

The main objective of the SMOS mission is to deliver crucial variables of the land surfaces: soil moisture, and of ocean surfaces: sea surface salinity fields. The mission should also deliver information on root zone soil moisture, vegetation and biomass, and lead to significant research in the field of the cryosphere.

Jean-Christophe Calvet is co-ordonator of the land climatology and meteorology sub-group.


SCIENTIFIC OBJECTIVES OVER LAND

   From all the lower boundary conditions which drive the atmosphere, land-surfaces are of particular interest to mankind as their direct and local impact is of great importance to human activities. The challenges posed by land-surfaces for all meteorological and climatological applications lie in the fact that they are very variable over a broad range of temporal and spatial scales. In contrast to oceans for instance, diurnal variations of temperature and fluxes are an order of magnitude larger. Another major difference is that moisture for evaporation is available in limited supplies but constitutes at the same time a memory for the system. The surface hydrology is one of the keys to our understanding of the interaction between the continental surfaces and the atmosphere as it determines the portioning of energy between the different fluxes. The proposed SMOS project is a research mission intended to apply a new instrumental technique to tackle a number of scientific objectives, mainly in the field of meteorology and climatology: weather forecast, climate change sensitivity studies, and data analysis. The science issues considered in this proposal are related to the parameterization of land surface processes and to the development of methods to retrieve surface variables from satellite data, in order to improve the representation of surface fluxes, soil moisture content, soil hydraulic characteristics, and plant stress in mesoscale and global models. The initialisation of soil moisture in atmospheric models, including numerical weather forecast models, is of great concern and a subject of active research. The current methods to estimate soil moisture are all very indirect and other ways of inferring soil moisture, with global coverage, are needed.

   For watershed hydrologic model applications, there is an urgent need to have access to distributed soil water fluxes at regular temporal resolutions over large areas. The yearly integrated land surface and base flow water budgets are generally well predicted by the new generation of hydrologic models. However, the estimation of the ratio between base flow and surface runoff, as well as the ratio between deep drainage and soil moisture content are still very imprecise. The soil stratum and in particular the unsaturated zone between the soil surface and the groundwater table (vadose zone) plays a crucial role: the estimation of soil moisture in the vadose zone is an important issue for short and medium term meteorological modelling, hydrological modelling, and the monitoring of plant CO2 assimilation and plant growth. The vadose zone hydrology being inaccurately described, attempts to monitor water quality and flooding risks often fail. New ways to parameterise effective soil characteristics are needed.

   In most cases, the vadose zone hydrology and the surface fluxes are controlled by vegetation. Modelling the rate of soil water extraction by the plant roots and the stomatal feedback is important for either atmospheric, hydrologic, and environmental studies. The current models manage to describe first order responses but do not encompass the complete behaviour of the plant, especially at the mesoscale, where several landscape units may contribute to the surface fluxes. In most parts of the world, plant water supply is the dominant factor that affects plant growth and crop yields. Monitoring soil moisture is an interesting way to detect water stress period (excess or deficit) for yield forecasting or biomass monitoring, especially in areas where the density of climatic stations is low. Time series of soil moisture at the mesoscale would be a very interesting input to the representation of vegetation in land surface schemes, also.

   Concerning the characterization of the atmosphere, there is a need to estimate the surface emissivity at the wavelengths of the atmospheric sounders in order to improve the retrievals. The all-weather surface characterisation capability of L-band radiometry could help estimate these values.

   These objectives may be achieved by estimating near-surface soil moisture content (wS). In most cases, the root-zone or the vadose-zone soil moisture (wVZ) is required, also, together with effective hydraulic soil characteristics. In the next section, it is explained how the latter variables can be inferred from wS, provided this variable is monitored with a good (3-4 days) temporal resolution.

   The use of the near surface soil moisture wS to help characterise the surface fluxes, bulk soil moisture and plant stress, must be considered through assimilation and aggregation or disaggregation techniques.

   Water storage in the soil, either in the top surface layer (e.g. 5 cm) or in deeper layers, affects not only direct evapotranspiration but the heat storage ability of the soil, its thermal conductivity, and the partitioning of energy between latent and sensible heat flux. It is therefore a key variable of landsurface-atmosphere interaction.

   The value of the top surface layer wS conditions direct evaporation from bare soil or soil partially covered by vegetation, and determines the possibility of surface runoff after rainfalls. This parameter is currently not extensively measured.

   The vadose zone (wVZ) is the hydrological connection between the surface water component of the hydrological cycle and the groundwater component. Evaporation, infiltration and recharge of the groundwater usually occurs through the unsaturated zone. Because of root water extraction, the vadose zone is the interface between the vegetation and the hydrological systems: the value of wVZ conditions plant transpiration and CO2 uptake through stomatal aperture and possible damage to the photosynthesis apparatus. Furthermore, wVZ is directly linked to the ability of the soil to produce drainage after a rainfall. The soil- vegetation-atmosphere transfer (SVAT) schemes now employed in meteorology and hydrology are designed to describe the basic evaporation processes at the surface together with the water partitionning between the vegetation transpiration, the drainage, the surface runoff and the soil moisture increase or decrease. The current trend in SVAT modelling is the integration of biological processes such as photosynthesis and plant growth, and hydrological transfers, in the same surface model. The 'classical' part of the SVAT performs the atmosphere interface calculations, while new modules provided by the research in physiology and hydrology simulate interactive vegetation and river flow. This is why improving SVAT modelling would benefit to meteorology, climatology, hydrology, and agronomy. In operational simulations, a realistic initial value of wVZ must be provided to the SVAT model. One of the main difficulties in the use of such parameterisations is the initialisation of wVZ: soil wetness is one of the least understood and poorly simulated components of the climate system and is also one of the most sparsely measured.

   Water movement in the unsaturated zone is affected by intrinsic parameters such as hydraulic characteristics depending upon structural properties and, to a lesser extent, upon texture. The textural properties are characterised by a smaller underlying variability than the structural properties, which is mainly due to the fact that biological and anthropogenic factors have a much larger impact on structural properties than on textural ones. Most SVAT models rely on the use of pedotransfer functions which estimate soil characteristics from readily available data, such as texture (i.e. particle size distribution) which are the most common measured soil data across the world. However, pedotransfer functions are doomed to fail when used for the estimation of structural parameters.

   Since microwave techniques provide information about the moisture wS of a shallow surface layer (about 5 cm at L-band), only, it was investigated to what extent vadose zone soil moisture wVZ and soil hydraulic characteristics can be inferred from near surface soil moisture.

   Time series of surface soil moisture content (wS) assessed by L-band microwaves allow for the determination of wVZ and for that of the surface fluxes (evaporatranspiration). When dealing with bare (or sparecely covered) soils, evaporation rate and runoff can be calculated from wS time series. When dealing with soil surfaces covered with vegetation, information on the vadose zone soil moisture (wVZ) is generally needed to describe water and energy fluxes in the soil-plant-atmosphere continuum. Furthermore, the inclusion of CO2 assimilation algorithms in SVAT schemes made it potentially possible to simulate vegetation growth and, hence, to explore biosphere feedback mechanisms in response to changes in rainfall patterns, temperature, and soil water store. Initially, the problem was tackled through assimilation studies using meteorological satellites observations. However it was shown that only under dry conditions the diurnal change in surface skin temperature could be used to retrieve the soil moisture store. Hence, the applicability of satellite infra-red data is limited. Attempts were made to retrieve the root- zone soil moisture from screen level variables (air temperature and humidity) by inverting simple surface schemes: in some conditions, it is possible to adjust the value of the root-zone soil moisture in order to minimize the forecast error on the low-level atmospheric parameters. A difficulty of this method is that the link between screen-level parameters and the root-zone soil moisture is rather indirect. The consequence is that the method does not work when the energy available at the surface is too low (e.g. short diurnal cycles in winter) or when the wind speed is too high. In a recent study Calvet et al. (1998) tested the possibility of using an operational SVAT scheme to retrieve root-zone soil moisture and evapotranspiration fluxes from in situ measurements of the near-surface soil moisture content (the 5 cm top layer) applying atmospheric and precipitation forcing. The SVAT scheme chosen for the inversion procedure was the ISBA code developed by Noilhan and Planton (1989). The experimental data collected during the MUREX (South of France) field campaign (3 years) were used for ground validation. The authors derived assimilation rules for either surface soil moisture or surface temperature and showed that 4 or 5 wS values measured once every four days are sufficient to retrieve wVZ as well as the evapotranspiration flux.



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