Lecture Panel 10 Flashcards
Infiltration
movement of water from the soil surface into the soil
-inputs from precipitation and snowmelt
Redistribution
Movement of infiltrated water within the unsaturated zone
Exfiltration
Evaporation of water vapour from the upper layers of the soil (output)
Vegetative Uptake
Soil water is absorbed by plant roots and subsequently released during transpiration
Interflow
Soil water is carried down slope within the unsaturated zone
Percolation and groundwater recharge
Soil flows downward through porous soil into the saturated zone and creates ground water recharge
Capillary rise
Movement of water upward from the saturated zone to the unsaturated zone due to surface tension
Why is it important to understand infiltration and redistribution?
to determine the water budget
What is Darcy’s Law
Tells you discharge (Q) from a tube of saturated soil Q=-KAdh/dl A= area K= hydraulic conductivity dh= change in head dl= change in length
Specific discharge
q=Q/A=-Kdh/dl
can say this is apparent velocity (v=L/t) of saturated flow
Anisotropic unsaturated flow
flow rate differs depending on direction, due to grain shape and orientation
How are hydraulic conductivity and water content related?
directly proportional - but the rates differ by soil texture
Darcy’s law for unsaturated flow
Pressure head and connectivity are functions of moisture:
q = -K*(1+dpsi/dz)
Infiltration rate
rate water enters soil
infiltration capacity
maximum rate at which water can enter soil
Water input rate
precipitation rate
Time to ponding
time at which input rate exceeds infiltration capacity
5 moisture zones during infiltration
- saturated zone
- transition zone
- transmission zone
- wetting zone
- wetting front
saturation zone
during infiltration - near the surface, extending up to 1.5 cm down
transition zone
during infiltration - about 5cm thick and located below the saturated zone. rapid decrease in water content
transmission zone
during infiltration - water content varies slowly with depth and time
wetting zone
during infiltration - where sharp decrease in water content is observed
wetting front
during infiltration - region of very steep moisture gradient. this represents the limit of moisture penetration into the soil
infiltration and runoff over time during storm
infiltration starts out very fast and efficient, then can’t hold anything more due to saturation of the top layers. runoff is reciprocal
Factors controlling infiltration
- water input rate
- hydraulic conductivity of the surface
- -soil organic layers
- -frost
- -swelling or drying
- -rain compaction
- -in-washing of sediment
- -human modification
- initial water content
- -wetter soils have higher unsaturated hydraulic conductivity but will also fill up faster
- slopes
- soil chemistry
- water chemistry
- -K at 30C almost 2x at 0
Rings for infiltration rate measurement
ring in soil, water in ring at constant head
- we keep water level constant, and since we are just barely avoiding runoff we know we are at max infiltration rate
- issues are ring deforms soil, not nature conditions
- can use two rings to stop lateral spread
Rainfall simulator
device for measuring infiltration with attempted realism
Hortons early infiltration insight
infiltration begins at some rate of, and exponentially decreases until it reaches some constant rate, fc
