Soil moisture is a key parameter for the understanding of the Earth.s climatology and the global water cycle. As a measurement, a growing body of scientific literature shows that observations of soil moisture would lead to significant improvements in weather and flood forecasting. Other applications of remote soil moisture observations include irrigation, pesticide, and herbicide scheduling, early detection and monitoring of crop disease and drought, fire monitoring and forecasting, soil trafficability, and prediction of aerosol dust production. An emerging priority is measurements of important ecological and physiological functions such as evapotranspiration (ET) to be used to improve process characterizations in predictive models and develop more refined land cover analyses to assess the role of surface energy fluxes and water storage as regulators of biogeochemical cycles such as carbon, methane and nutrients.
Hydrometeorological applications are those involving analysis of the surface water and energy fluxes, including surface runoff, infiltration, sub-surface vertical and horizontal flows, evapotranspiration and the associated latent heat flux, as well as sensible heat and radiative fluxes. Soil moisture is paramount in controlling these surface energy processes. Estimates of surface profile soil moisture are critical for improving the performance of NWP models, in which it has been shown that accurate initialization of soil moisture, perhaps more than any other initialized field, results in substantial gains in forecast skill. At the local scale, agricultural applications typically focus on the field scale (< 1 km) and are thus more suited for sub-orbital observational platforms. However, in regions of large-scale agriculture, including row crops or pasturelands, space-based soil moisture observations at a spatial scale of 10 km may still be of some value with 1 km resolution desirable.
NASA's Earth Science program is dedicated to advancing Earth remote sensing and pioneering the scientific use of global satellite measurements to improve human understanding of our home planet. Earth Science objectives and outcomes are organized across six inter-disciplinary Focus Areas, one of which is Water and Energy Cycle. The Water and Energy Cycle Focus Area encompasses studies of the distribution, transport and transformation of water and energy within the Earth System. Since solar energy drives the water cycle and energy exchanges are modulated by the interaction of water with radiation, the energy cycle and the water cycle are intimately entwined. The long-term goal of this focus area is to enable improved predictions of water and energy cycle as consequences of global change. This key goal requires not only documenting and predicting trends in the rate of the Earth's water and energy cycling as the climate changes, but also predicting changes in the frequency and intensity of related meteorological and hydrologic events. Over the next decade, this Focus Area strives toward balancing the water budget at global and regional spatial scales and providing global observation capability of precipitation over the day's cycle and important land surface quantities such as soil moisture and snow quantity and processes such as evapotranspiration. These quantities and processes are also intimately coupled to the Weather and Carbon Cycle and Ecosystems Focus Areas. One of the emerging priorities of the latter Focus Area is measurements of the important ecological and physiological functions such as evapotranspiration to be used to improve process characterizations in predictive models and develop more refined land cover analyses.
NASA has defined Earth science questions that can be best addressed using the Agency's unique capabilities: How are global precipitation, evaporation, and water cycle dynamics changing? and How are surface hydrologic processes and variations in local water resources related to global climate variation? Answering these science questions involves measurements of soil moisture and evapotranspiration and requires comprehensive research programs that include scientific research and analysis, suborbital and space missions, field campaigns, data management, computational modeling, and advanced technology development. Global soil moisture is identified in the Science Mission Directorate Science Plan as a future representative mission element for an Earth System Science Pathfinder mission.
The National Research Council was recently tasked by NASA Office of Earth Science, NOAA national Environmental Satellite Data and Information Service, and the USGS Geography Division to conduct a decadal survey to generate consensus recommendations from the Earth and environmental science and applications communities regarding a systems approach to the space-based an ancillary observing programs of these three agencies. A soil moisture mission was recommended in the first tier of missions to be implemented in the 2010-2013 timeframe.
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