Download Lecture 08l Infiltration

Infiltration
Rainwater that soaks into the ground and may reach the
groundwater table.
Base flow: Where Groundwater Meets Surface Water
• Base flow is the portion of stream flow that comes
from subsurface flow: water infiltrated but returned
to the surface in a stream channel.
• Streams run even when it hasn’t rained.
• Base flow is very important for fish, invertebrates,
aquatic plants and other life during dry spells.
Field Tests
24 inch double ring infiltrometer with Mariotte Tubes http://www.hilbec.com/STORMWATER.htm
• Infiltration is measured in the field with bottomless rings. Mariotte
Tubes allow for measurement of liquid flow during the infiltration test by providing
a constant water level in the 24 inch Infiltration Rings
Double-ring Infiltrometer
• Two rings eliminates overestimating the hydraulic conductivity
• Outer ring contributes to lateral flow , so
• Inner ring is contributing mostly to downward flow.
•Water from Mariotte bottles to rings via tap at base of bottles. Ring
water height equals that of the base of the bubble tube.
•When water moves into the soil, reducing the height of ring water to
below that of the bubble tube, more water is fed into the ring.
Estimates 1: Horton’s Equation
• Horton: The infiltration capacity decreases
exponentially with time and ultimately
reaches a constant rate
• Infiltration capacity
• Where ft is the infiltration rate at time t;
• f0 is the initial infiltration rate or maximum
infiltration rate;
• fc is the constant or equilibrium infiltration rate after
the soil has been saturated or minimum infiltration
rate; NOTE e is a number, ~ 2.718
• k is the decay constant specific to the soil.
• the f’s have units in/hr and k is a time constant hr -1
Horton’s Infiltration Model for soil capacity
Infiltration starts at a constant rate, f0, and is decreasing exponentially
with time, t. After some time when the soil saturation level reaches a
critical value, the rate of infiltration will level off to the rate fc.
In a few minutes we will do an example using Horton’s
Equation using an average rectangle estimate to the area under
the curve.
Estimates 2: F index
Infiltration Volume = total rainfall volume – runoff volume as
measured in the rain gages and at the outlet gage, respectively.
F assumes infiltration volume resulted from a constant infiltration
rate. It assumes a high initial infiltration is balanced by a low later
infiltration.
Example: Guessing
F
We will find F in this problem by guessing a value for F , calculating
the total runoff that would result, and comparing our answer to the
known runoff.