The water cycle is illustrated here. Water is constantly moving on Earth. The water cycle consists of all the processes involved in the transfer and storage of water in Earth’s atmosphere, on its surface, underground, and by organisms living on our planet.

SMAP will provide a capability for global mapping of soil moisture and freeze/thaw state with unprecedented accuracy, resolution, and coverage. SMAP science objectives are to acquire space-based hydrosphere state measurements over a three-year period to:

Understand processes that link the terrestrial water, energy and carbon cycles

Estimate global water and energy fluxes at the land surface

Quantify net carbon flux in boreal landscapes

Enhance weather and climate forecast skill

Develop improved flood prediction and drought monitoring capabilities

Water & Energy Cycles, Weather and Climate

Changes to water availability is a critical practical impact of global warming on society. How will global change affect water supply and food production?

Soil moisture is a key control on evaporation and transpiration at the land-atmosphere boundary. Since large amounts of energy are required to vaporize water, soil moisture control also has a significant impact on the surface energy flux. By exerting a control on the rate of surface evaporation, soil moisture links the water and energy cycles. Thus, soil moisture variations affect the evolution of weather and climate particularly over continental regions.

Evaporation occurring through the plant transpiration path involves exchange of water vapor and Carbon Dioxide gas during photosynthesis. When soil moisture is low and the plant cannot uptake enough water through its roots, transpiration is reduced. By exerting a control on the rate of transpiration, soil moisture links the water and carbon cycles.

Taken all together, the link between the water, energy and carbon cycles over land is through soil moisture. These three cycles are the main cycles that maintain our global climate and determine regional water availability and temperatures. Mis-specification of how the cycles are linked leads to error in predicting weather and climate in the future. SMAP’s measurement of this important link is a step towards reducing mis-specification of the link in models. In this way it contributes to the reduction of uncertainty in weather and climate predictions.

Recent model simulations of the effects of greenhouse gases on climate show that current models agree quite well in predicting temperature change but disagree significantly in predicting surface moisture change and water resource availability. Accurate soil moisture information such as will be available from SMAP will improve the performance of numerical weather prediction models and seasonal climate models and enhance their predictive skill.

Carbon Cycle and Ecosystems

Soil moisture and its freeze/thaw state are also key determinants of the global carbon cycle. Carbon uptake and release in boreal landscapes is one of the major sources of uncertainty in assessing the carbon budget of the Earth system (the so-called missing carbon sink). The SMAP mission will quantify the nature, extent, timing and duration of landscape seasonal freeze/thaw state transitions that are key to the estimation of terrestrial carbon sources and sinks. SMAP freeze/thaw state measurements will also contribute to understanding how ecosystems respond to and affect global environmental change, improving regional mapping and prediction of boreal-arctic ecosystem processes.

The global carbon cycle is the complex set of interactions involving several different carbon-based gases (among them the so-called “greenhouse gases”) that take place between the atmosphere, land, and oceans of the Earth.

The three colored curves demonstrate the production or reduction of carbon in the atmosphere at three northern hemisphere field experiment sites. Each curve corresponds to a different year. Depending on the thaw date (inset table) the same spot in the forest can be generating or removing carbon from the atmosphere. SMAP freeze/thaw measurements will help determine the role of northern forests in the global carbon budget.

The SMAP Project will implement a spaceborne earth observation mission designed to collect measurements of surface soil moisture and freeze/thaw state, together termed the hydrosphere state.

Soil moisture is defined in terms of volume of water per unit volume of soil.

Freeze/thaw state is defined as the phase of the water contained within the landscape including soil and vegetation.

SMAP hydrosphere state measurements will yield a critical data set that will enable science and applications users to:

Understand processes that link the terrestrial water, energy and carbon cycles

Estimate global water and energy fluxes at the land surface

Quantify net carbon flux in boreal landscapes

Enhance weather and climate forecast capability

Develop improved flood prediction and drought monitoring capability

SMAP soil moisture and freeze/thaw measurements have a wide range of applications in Earth science and natural hazards mitigation. From the NRC Earth Science Decadal Survey Report, 2007.

To meet the goals of science and applications users, SMAP has …

To resolve hydrometeorological water and energy flux processes and extend weather and flood forecast skill, spatial resolution of 10 km and temporal resolution of 3 days are required.

To resolve hydroclimatological water and energy flux processes and extend climate and drought forecast capability, spatial resolution of 40 km and temporal resolution of 3 days are required.

To quantify net carbon flux in boreal landscapes spatial resolution of 3 km and temporal resolution of 2 days are required.

In addition, the SMAP mission will validate a space-based measurement approach that could be used for future systematic hydrosphere state monitoring missions.

The control of evaporation on land and the overall productivity of life on land is influenced by soil moisture, freeze/thaw state, and solar radiation. The map illustrates which of these is the primary controller over Earth’s land surfaces (except Antarctica).

Baseline Science Requirements

The SMAP science objectives lead to the following requirements for the baseline science mission: