Water Flow and Infiltration
Soil Science Department, NCSU
Water is the most essential component for biological activities in the soil. Water is also the main carrier for the transport of pollutants from land-based waste disposal systems (e.g., septic systems) into ground water and surface water resources. While the water flow generally refers to the movement of water within the soil body, infiltration has been defined as the entry of water into the soil Examples are the infiltration of rainwater into the soil, seepage into the soil at the bottom of ponds, and lagoons, and infiltration of wastewater from the trenches of a septic system into the soil. Water movement in the soil is governed by Darcy’s law, relating the flux (the rate at which water moves through a unit cross sectional area within the soil body) to the soil hydraulic conductivity and the hydraulic gradient along the flow path. Although Darcy’s law was originally developed for saturated flow, it also applies to the water movement in the vadose or unsaturated zones of soils. Assessment of the rate of water flow between two points in a body of soil generally requires the knowledge of the hydraulic gradient along the two points and the average value of the hydraulic conductivity for the soil water content or pressure head between the points. For saturated conditions, the gradient along the flow path is determined by measuring the pressure head at the two points by piezometers and measuring the saturated hydraulic conductivity in situ or in the laboratory using intact or repacked samples. For the unsaturated zone, pressure head at the two points under consideration can be measured by tensiometer or be estimated from soil water characteristic or water release curve using measured soil water content. The unsaturated hydraulic conductivity must be measured directly, or be calculated from a mathematical relationship based on soil water content or pressure head.
Whether we are considering a saturated or unsaturated zone, water moves through the pores between soil particles (referred to as interparticle pores), through planar voids between soil peds, and through macropores (generally defined as large tubular pores that are visible with a naked eye) created by plant roots and animals. We refer to the matrix flow as the flow of water through the interparticle pores and to preferential flow or macropore flow as the flow of water through wide planar voids and tubular channels. Depending on the distribution of the total hydraulic head in the soil, soil water can move in all directions. Under areal application, such as uniform rainfall or spray irrigation, water movement in the vadose zone is generally in vertical direction. Above a shallow water table, on the other hand, water can move upward from the water table due to evaporation at the soil surface. While water flow from line or strip sources is two- dimensional, movement of water from point and isc type sources occurs in all three directions. In the saturated zone below a water table, water flow can be in any direction, but is generally considered to by horizontal toward a well or drainage channel. As for infiltration, the initial infiltration rate depends on the antecede soil water content, but the final infiltration rate is the rate of entry of water into the soil under steady-state conditions. This rate is generally considered to be numerically equal to the saturated hydraulic conductivity of the saturated layer at the soil surface. Infiltration rate is commonly measured using a single- or double-cylinder infiltrometer.