|Soil Moisture Experiments|
Washita, SGP, SMEX, and CLASIC
Soil Moisture Experiments
The significance of a hydrologic state variable is expressed well in the recent description of NASA’s Global Water and Energy Cycle research program. Water is at the heart of both the causes and the effects of climate change. Ascertaining the rate of cycling of water in the Earth system, and detecting possible changes, is a first-order problem with regard to the renewal of water resources and hydrologic hazards. A more complete understanding of water fluxes, storage, and transformations in the land, atmosphere, and oceans will be the central challenge to the hydrological sciences in the 21st century. Improved knowledge and prediction of the water cycle can yield large benefits for resource management and regional economies if variability and uncertainties can be understood, quantified and communicated effectively to decision-makers and to the public. The overarching objective is to improve the understanding of the global water cycle to the point where useful predictions of regional hydrologic regimes can be made. This predictive capability is essential for practical applications to water resource management and for validating scientific advances through the test of real-life prediction.
Soil moisture is the key state variable in hydrology: it is the switch that controls the proportion of rainfall that percolates, runs off, or evaporates from the land. It is the life-giving substance for vegetation. Soil moisture integrates precipitation and evaporation over periods of days to weeks and introduces a significant element of memory in the atmosphere/land system. There is strong climatological and modeling evidence that the fast recycling of water through evapotranspiration and precipitation is the primary factor in the persistence of dry or wet anomalies over large continental regions during summer. As a result, soil moisture is the most significant boundary condition that controls summer precipitation over the central U.S. and other large mid-latitude continental regions, and essential initial information for seasonal predictions.
A common goal of a wide range of agencies and scientists is the development of a global soil moisture observing system. Providing a global soil moisture product for research and application remains a significant challenge. Precise insitu measurements of soil moisture are sparse and each value is only representative of a small area. Remote sensing, if achievable with sufficient accuracy and reliability, would provide truly meaningful wide-area soil wetness or soil moisture data for hydrological studies over large continental regions.
Development and implementation of the remote sensing component of a global soil moisture observing system will require advancements in science and technology. Many aspects of the research require validation and demonstration, which can only be accomplished through controlled large-scale field experimentation. Large-scale field experimentation requires significant resources to be successful that are usually contributed from several programs.