Soil moisture is an important variable in land-atmosphere feedbacks at weather and climate time scales because of its major effect on the partitioning of incoming radiation (available energy) into latent and sensible heat and on the allocation of precipitation into runoff, subsurface flow, and infiltration. Soil moisture is intimately involved in the feedback between climate and vegetation, since local climate and vegetation both influence soil moisture through evapotranspiration, while soil moisture and climate determine the type of vegetation in a region. Changes in soil moisture therefore have a serious impact on agricultural productivity, forestry and ecosystem health. Soil moisture estimates can also assist gas flux estimates in permafrost regions.
Information on soil-moisture changes and their statistics will also help reduce process uncertainties and improve climate models. On seasonal timescales, improved initial conditions for soil moisture in models should increase the model-prediction accuracy.
Soil moisture is a very heterogeneous variable and varies on small scales with soil properties and drainage patterns. Satellite measurements integrate over relative large-scale areas, with the presence of vegetation adding complexity to the interpretation. The in situ measurements do not relate easily to the
large-scale measurements. Calibration and validation activities need to be carefully chosen and well- instrumented sites used.
In situ soil moisture activities can build on the International Soil Moisture Network
(http://www.ipf.tuwien.ac.at/insitu/) currently hosted by the Vienna University of Technology (Austria). Satellite-based soil moisture product from scatterometers (e.g. ERS-1/2, ASCAT) and passive microwave (e.g. SMMR, [JAXA/NASA] Tropical Rainfall Measuring Mission (TRMM), AMSR-E and SMOS) have recently been made available and could potentially contribute to a longer-term record by building on data from these different satellites. Space agencies may need to maintain active long-wavelength microwave observation systems with a high temporal and reasonable spatial resolution, possibly with a polarimetric capability, to measure soil moisture and soil-moisture change.
The various ways of representing soil moisture from both satellite and in situ measurements, in
combination with climate models, need harmonization and, ultimately, standardization. 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 GCOS/GTOS TOPC.
The following is required for this ECV:
Product T.11 Global near-surface soil moisture maps (up to 5cm soil depth) Benefits
• Improved accuracy of general circulation models (GCMs) and soil-vegetation-atmosphere transfer schemes;
• Improved understanding of the feedback between climate and vegetation;
• Assistance in gas flux estimates in permafrost regions. Target Requirements
Variable/ Parameter Horizontal Resolution
Vertical Resolution
Temporal
Resolution Accuracy Stability
Volumetric soil moisture 50km NA Daily 0.04m3/m3 0.01m3/m3 /year
Rationale: The targets are set as an accuracy of about 10 per cent of saturated content and stability of about 2 per cent of saturated content. These values are judged adequate for regional impact and adaptation studies and verification and development of climate models. It is considered premature to consider global-scale changes.
Stating a general accuracy requirement is difficult for this type of observation, as this depends not only on soil type but also on soil moisture content itself. The stated numbers thus should be viewed with some caution.
Requirements for satellite instruments and satellite datasets
FCDR of passive microwave and scatterometer radiances, initially, for example, through:
• The ESA SMOS radiometer, which is expected to provide surface-soil moisture with 30-50km spatial resolution and one- to three-day temporal resolution;
• ASCAT on Metop (2006- present), which is continuing the data record of the scatterometers on ERS-1 and ERS-2, and should provide a continuous and homogenous data record for at least the remainder of the current decade;
• SMMR/AMSR-E-class instruments;
• SAR data from e.g. JAXA ALOS as an additional potential source of data. Calibration, validation and data archiving needs
• Highly accurate absolute and relative radiometric calibration will be needed;
• Validation studies over diverse types of land cover with representative soil-moisture measurements is critical to the development of retrievals;
• Due to scaling issues and different layer depths, the comparison of in situ measurements from
individual sites with satellite retrievals is not straightforward; nevertheless, significant advances in the understanding of soil-moisture-scaling issues have been made in the last few years, and there now exist different strategies to deal with scaling (multiple in situ instruments, models and advanced
techniques such as data assimilation and triple co-location).
Adequacy/inadequacy of current holdings
• There are now many microwave satellites that can provide useful soil-moisture information; however, the higher-level processing and reprocessing capabilities are far from adequate; therefore, much of the capability of the existing space segment remains underexploited;
• There is a long record of passive microwave remote sensing, starting with low-frequency observations from NASA’s Nimbus-7 SMMR in the 1970s, TRMM TMI since 1997, and AMSR-E, Windsat, and SMOS in the last decade. By using all these different satellite datasets, and including the scatterometer data (ERS and ASCAT), one could build a long-term soil moisture record;
• ERS scatterometer data have been used for global soil-moisture estimation at a scale of 50km and are now being continued with ASCAT on Metop; data from the latter are available from the Vienna University of Technology and EUMETSAT;
• The passive microwave instruments AMSR-E and Windsat provide C-band and X-band
measurements of brightness temperature; several soil moisture datasets derived from these instruments have been released in the last few years; C-band capability is limited, due to radio- frequency interference in many populated regions of the world (e.g. the USA, Europe, Japan);
• AMSR-E-derived soil-moisture results are currently available from NASA for the Aqua time period (2002-2011);
• Over Europe, Asia and some other regions of the world, Radio Frequency Interference (RFI) is strong for the ESA SMOS mission, which results in partly unusable data.
Immediate action, partnerships and international coordination
• International cooperation is needed to improve understanding of the relative performance of different satellite instruments (frequency, active versus passive, sampling and radiometric accuracy) and
retrieval approaches (empirical approaches, change detection, semi-empirical models and theoretical models);
• Even though the first soil moisture time series based on merging active and passive soil moisture datasets exist, research to blend surface soil moisture observations with satellite observations remains a key challenge.
The SAR C-band radar system on ENVISAT and Sentinel-1 could possibly contribute to the derivation of a global soil-moisture map, although accurate retrieval of soil moisture from these measurements is still a research topic; a fully polarimetric SAR system may be needed to separate the effects of soil moisture and vegetation.
Link to GCOS Implementation Plan
• [IP-10 Action T13] Develop a record of validated globally-gridded near-surface soil moisture from
satellites;
Other applications
• NWP and nowcasting;
• Hydrological modelling, groundwater management, agricultural management and hazard forecasting, including flood and drought prediction;
• Epidemiology, through prediction of water-borne diseases.