Vadose

The vadose zone is the part of the subsurface that lies between the land surface and the groundwater table. In geology and hydrology it is commonly referred to as the unsaturated zone, because its pore spaces are not fully filled with water; they contain both air and liquid water. The depth and extent of the vadose zone vary with climate, geology, and topography, and it can range from shallow horizons to several meters or more in deeper soils or porous rock.

Physically, the vadose zone is characterized by moisture that is variable in space and time. Water content, matric potential, and hydraulic conductivity change with soil type, texture, and structure. In this zone, water movement is governed by gravity, capillary forces, and plant water uptake. Infiltration from the surface can percolate downward toward the groundwater, while evapotranspiration at the surface tends to draw moisture upward or remove it near the surface. Capillary rise can bring groundwater-derived moisture into shallower layers, especially in fine-grained soils.

The vadose zone plays a crucial role in groundwater recharge, nutrient cycling, and contaminant transport. It is a key domain for modeling groundwater-surface water interaction. Hydrologists describe flow in this zone with the Richards equation, which combines Darcy’s law with capillarity. In practice, measurements often rely on soil moisture sensors, tensiometers, or neutron probes, while modeling supports management decisions in agriculture, engineering, and environmental protection.