The water held in the pore spaces of the unsaturated soil zone between the land surface and the water table. It is a critical variable linking precipitation, runoff, and evapotranspiration.
Soil moisture refers to the water content of the unsaturated soil zone, typically expressed as volumetric water content (volume of water per volume of soil) or gravimetric water content (mass of water per mass of dry soil). It occupies a central position in the hydrologic cycle as the state variable that controls the partitioning of precipitation between infiltration and surface runoff, and the partitioning of available energy between sensible and latent heat flux. Key soil moisture thresholds include saturation (all pores filled), field capacity (water retained against gravity after drainage), and permanent wilting point (water held too tightly for plant extraction). Soil moisture is measured using techniques including time-domain reflectometry (TDR), capacitance sensors, neutron probes, and gravimetric sampling. Remote sensing methods using passive microwave radiometers (such as SMAP and SMOS satellites) provide large-scale soil moisture estimates for the top few centimeters. Soil moisture is critical for agricultural water management, drought monitoring, flood prediction, weather forecasting, and land-atmosphere interaction studies. Antecedent soil moisture conditions strongly influence catchment runoff response and are among the most important initial conditions for hydrological forecasting.