status and availability of soil moisture are expected to be of increasing geopolitical significance in such regions as the Middle East and sub-Saharan Africa.
Despite its unquestioned significance and recognition by science working groups that it is among the most important environmental parameters that NASA's suite of sensing satellites should monitor on a global scale, no means are available today to map soil moisture on any scale, nor are there any specific plans to do so in the near future. Soil moisture has not been widely applied as a variable in any land process models for two primary reasons. It is a difficult variable to measure on a consistent and spatially comprehensive basis. It also exhibits great spatial and temporal variability; hence point measurements have little meaning. Consequently, soil moisture has not been used as a measurable variable in current hydrologic, climatic, agricultural, or biogeochemical models due to a lack of appropriate data. Although soil moisture is listed by NOAA as a critical climate environmental data record (EDR) (NOAA, 1997), it is the only EDR for which no threshold and objective values are provided. The committee 's assessment of the current status and future NPOESS plans for observing and measuring soil moisture from space is discussed in Box 6.1.
Because of its ubiquity, there are numerous potential science applications for frequent and spatially comprehensive measurements of soil moisture. Most of these fit under the following four science issues:
Understanding the role of surface soil in the partitioning of incoming radiant energy into latent and sensible heat fluxes at a variety of scales, from the mesoscale to general circulation model (GCM) scale;
Understanding the relationship between the moisture in the top 5 cm of soil that is observable by microwave techniques and the total profile (1 m or more) of soil moisture that is accessible to plants and is available for transpiration to the atmosphere;
Understanding how spatial and temporal patterns of soil moisture are related to the physical and hydrological properties of soils; and
Understanding how the spatial and temporal patterns of soil moisture can be used to improve our ability to model runoff at a variety of scales and adapt hydrologic models to areas of differing climate, biomes and soils, and geology.
With a potential for measuring soil moisture demonstrated, how might society use such soil moisture measurements? As in the science issues, there are four general areas in which routine measurements of soil moisture could have major impacts on day-to-day life:
Improving medium-range weather forecasting by incorporating measured soil moisture on a 30-km grid daily;
Improving on-farm irrigation scheduling and efficiency, and improving crop yield modeling for domestic and foreign areas, among other agricultural applications, at scales of 10 m to 100 m and 1 to 3 days.
Better quantifying water use, storage, and runoff to monitor existing resources and to assist decision makers in allocating limited resources or coordinating relief efforts in times of flooding, at scales of 100 m to 1 km and daily or on demand.
Improving climate models, particularly for annual and interannual variability, so that they represent the land surface hydrologic processes accurately. Measured soil moisture can be used as a state variable and as a validity measure for GCMs. The scales of interest are 1 to 10 km, possibly averaged to coarser resolution, at time scales of 1 to 7 days.
The status of soil-moisture sensing may change somewhat after the European Space Agency (ESA) launches ENVISAT in June 2001 and the National Space Development Agency (NASDA) of Japan launches PALSAR in