GEG 3102 Lecture Notes - Lecture 6: Permanent Wilting Point, Infiltrometer, Soil Texture
17 Oct 2016
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6 – Soil Water
09/28/2016
Infiltration:
- The downward movement of water from the (very) surface down into the soil
- Influenced by ground cover, as well as soil texture, structure and water content
- Although clay has a higher porosity, water flows better through sand
- Soil composition: pore space = 40-60% (air or water), mineral = 45%, organic = 5%
- Wilting point: After this point (if more water is lost) plants will begin to wilt
• Too hard to extract the water that is present
- Splash: when a raindrop hits the ground and disturbs the surface
• Splash erosion
• Has the potential the modify the ground surface, which modifies infiltration
- Infiltration capacity:
• The maximum rate at which water can infiltrate a soil (for a constant rainfall
intensity)
• Is initially equal to rainfall intensity, until ponding begins
• Time to ponding is very important as once the water begins to pond it can begin to
flow (leading to erosion)
• Exponential decline in the ponded infiltration rate (Horton)
• Ponding occurs when the precipitation exceeds the ability of the soil to take water,
leading to runoff
- Measuring infiltration rate:
• Double ring infiltrometer: widely used, but cannot simulate raindrops
• Rain simulator: better simulation of raindrops, but only for low velocities
Transmission of soil water:
- Occurs in the vadose zone (unsaturated)
• Above the water table
• Volume changes with fluctuations in the height of the water table
- Types of soil water:
• Hygroscopic: Thin film around soil grains
- Very hard to separate from soil (heat to 105° for 24 hours)
• Capillary: Held by cohesion and adhesion to and between particles
- Remains while in field capacity
• Gravitational: Free water that moves due to gravity
- Released when the (sand) is at fields capacity
- Clay: less than 2 microns; Silt: 2 – 63 microns; Sand: 63 – 2000 microns
• 1000 microns = 1 mm
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Document Summary
Influenced by ground cover, as well as soil texture, structure and water content. The downward movement of water from the (very) surface down into the soil. Although clay has a higher porosity, water flows better through sand. Soil composition: pore space = 40-60% (air or water), mineral = 45%, organic = 5% Wilting point: after this point (if more water is lost) plants will begin to wilt: too hard to extract the water that is present. Splash: when a raindrop hits the ground and disturbs the surface: splash erosion, has the potential the modify the ground surface, which modifies infiltration. Infiltration capacity: the maximum rate at which water can infiltrate a soil (for a constant rainfall intensity) Measuring infiltration rate: double ring infiltrometer: widely used, but cannot simulate raindrops, rain simulator: better simulation of raindrops, but only for low velocities.
