Glossary of Soil Moisture Terms
Below is a glossary of terms related to soil moisture, including units of measurement, soil moisture monitoring processes, and soil moisture's role in the water, energy and carbon cycles. Learn more about soil moisture and its role in drought monitoring.
Carbon storage: The storage of carbon out of the atmosphere, such as into soils. Carbon is either transformed into complex carbons and stored as an organic component of soil or released to the atmosphere during certain microbial processes. Soil moisture is a key factor regulating the balance between release and storage of carbon below ground.
Data assimilation: Data assimilation is a technique to update a continuously running model with incomplete and uncertain information acquired from observations.
Dust generation: The movement of soil sediment via wind, which is mitigated by having wet soil at the soil-atmosphere interface.
Erosion: The movement of soil sediment or the breakdown of rocks via water. Movement of soil sediments is impacted by the amount of soil moisture at the ground surface.
Evaporation: The movement of water from the soil surface into the atmosphere as a water vapor. Evaporation is primarily controlled by net radiation at the soil surface, soil moisture, and atmospheric humidity.
Fractional Available Water (FAW): A measure of how much water is available for plants to extract from the soil. FAW is calculated by dividing the volume of water in the soil that is available to plants by the total available water volume the soil can hold, taking into account soil type and texture. Numbers near zero mean it is difficult for plants to extract water. Higher numbers mean the soil is nearing saturation.
Field Capacity (FC): The amount of water soil can hold within its pores before it begins to drain due to gravity. When field capacity is exceeded, water may drain to deeper depths or pool on the soil surface. The volume of soil moisture associated with field capacity varies by soil type, as different soils (clay, sand, loam) have different water holding capacities.
During heavy precipitation events, soil moisture conditions can exceed field capacity such that the additional moisture will be quickly drained (to deeper depths or horizontally) before plants can access that moisture—and is therefore unavailable to the plant.
Infiltration: The process of water moving from the surface into the soil.
In situ: Measurements collected on location. For soil moisture measurements, in situ data may be collected via sensors physically buried in the ground that measure small volumes of the soil in their direct environments or by cosmic ray neutron sensors, which are located above ground and estimate soil moisture conditions on a scale of acres surrounding the sensor, across multiple depths.
Irrigation: The application of water to the ground surface to supplement rainfall. This can take a variety of forms, including drip, broadcast, center pivot, or flood irrigation.
Model assimilation: When a model uses inputs of in situ data to correct or adjust the estimates the model puts out.
Percolation: The process of water moving through the soil, either into deeper soil layers or laterally.
Recharge: The movement of water into the saturated zone within the soil, called groundwater. Pre-existing moisture conditions impact the amount of recharge that results from precipitation events.
Permanent Wilting Point: The level of soil moisture below which most plants cannot extract sufficient water to avoid irreversible wilting. This is often approximated as the water content associated with -1,500 kilopascals (kPa) matric potential, but actual wilting points may vary by plant type.
Plant Uptake: The action of water being drawn into the root structure of plants. Plant water uptake can be controlled by the magnitude of soil moisture available to roots. Rate and volume of plant uptake may also be influenced by conditions experienced by plants, such as air temperature and relative humidity, carbon dioxide concentration, and plant health.
Soil Matric Potential: The amount of suction required to remove water from the soil: a measure of how difficult it is to extract water from soil. Common units include kilopascals (kPa) or megapascals (MPa). Matric potential accounts for soil texture, so measurements reported in these units can be compared across locations that may have different soils. Because different plant species vary in their ability to extract water from the soil, the matric potential associated with wilting points may vary by plant type.
Transpiration: The exhalation of water vapor from the stomata of a plant into the atmosphere in response to photosynthesis. Transpiration is controlled by absorbed solar radiation, ambient humidity, and amount of water available to plant roots.
Volumetric Water Content (VWC): A common unit of measurement for representing absolute soil moisture conditions. The unit describes volume of water per volume of soil, for example m3/m3. A VWC of 100% (or 1.0 m3/m3) would represent pure water. A VWC of 0% (0.0 m3/m3) represents soils that are baked dry, in which no water remains. Different soil types (e.g., clay, loam, sand) reach saturation at different volumes of water, so 35% VWC (0.35 m3/m3) might indicate the point at which water will no longer infiltrate in one soil type, but not another.