Soil and Land Module 5

Soil Hydrology

Water Potential (ψ)

Combined effect of all factors that make water move

Factors affecting ψ

Chemical potential (concentration) - water diffuses from dilute to concentrated solution (osmosis)

Mechanical potential (pressure) - water will flow from high pressure to low pressure (vacuum)

Gravitational potential (height) - water flows downhill

Electrical potential (charge)

These factors sum together to give the overall free energy of a mass of water

Forces restricting soil water movement

Adhesion, Cohesion, Capillarity

Soil Water Potential

More water in soil = more water potential

determined by strength of forces attracting water to soil particles

free water (potential of 0)

Adsorbed water (negative potential)

the lower the soil water potential, the more tightly water is adsorbed to soil particles

consists of sum of matric potential + gravitational potential + osmotic potential

unit mPa, bar = 0.1 mPa = slightly less than 1 atm (14.7 psi)

Types of Soil Water

Saturation

Gravitational water

field capacity (upper storage limit)

Drying

Permanent wilting point (lower storage limit)

hygroscopic water (adhesion water)

available water capacity - can be absorbed by plant roots, between field capacity and wilting point

Water Movement in soil

moves along potential gradients (slope and matric potential of soil)

rate depends on soil permeability (described by hydraulic conductivity)

defined by soil structure and texture

horizon differences influence water movement

Darcy’s Law

combines effect of gradient and hydraulic conductivity

calculates quantity of water (flux) flowing in a saturated soil

used to calculate flow rates in aquifers

hydraulic conductivity diminishes exponentially as soil dries

Water Movement in soil

Rates of water movement defined by soil structure and texture, horizon differences influence water movement too

Gravitational flow (saturated flow) - under force of gravity under saturated conditions

Capillary flow (unsaturated flow) - movement in any direction

Water infiltrates the soil the percolates downward through profile

Groundwater Movement and Aquifer Types

Unconfined Aquifer

Confined Aquifer

Darcy’s Law and Landscape Hydrology

Some water flow through aquifers faster than others

as difference in elevation increases, the rate of flow increases

as the distance increases, the rate of flow decreases