Hydrologic Processes in the unsaturated soil layer

Water delivered to the land surface (from precipitation) may run off directly, as overland flow or infiltrate into the soil. To which extent the infiltration in the soil occurs, depends on the physical characteristics of the upper soil layer as well as on the actual soil moisture.

Quantification of the process of water distribution in the soil implies calculation of the actual soil moisture SW (t) using continuity equation for the unsaturated soil layer.

Here different processes have been shown. They takes occur either from the ground surface (infiltration, evaporation) or from the bottom layers of the aeration zone (vadose zone) such as groundwater recharge or capillary uprise. This development of soil water potential gradients in the vertical direction creates a predominant vertical movement of soil water in the aeration zone.

Vertical Profile "Soil-Vegetation-Atmosphere" with water balance components (source DVWK)
Distribution of the soil layers

Soil Layer Distribution

Vadose, unsaturated or aeration zone: located below the surface where the soil pores contain both air and water.

Root zone: located at the top of the unsaturated zone where the roots of plants can reach the soil water.

Interflow: under some conditions (slope, soil characteristics) water can flow laterally in the unsaturated zone.

Groundwater recharge: excess water in the unsaturated zone can migrate downward by gravity recharging the grounwater zone.

Intermediate zone: pores are filled with capillary water which can be uptaken by the roots depending on some conditions (soil texture, distance between roots and water table).

Groundwater: water stored in the saturation zone. In this zone water moves as groundwater flow through the soil layers in a horizontal direction where the pressure gradient is the driving force of the flow.

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