Infiltration

1. Infiltration Rainwater that soaks into the ground and may reach the groundwater table.

2. 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.

3. Field Tests • 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 24 inch double ring infiltrometer with Mariotte Tubes http://www.hilbec.com/STORMWATER.htm

4. 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.

5. 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

6. 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.

7. In a few minutes we will do an example using Horton’s Equationusing an average rectangle estimate to the area under the curve.

8. 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.

9. Example: GuessingF 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.