Infiltration : Objectives Understand the three processes of infiltration Understand the difference between rate (flux) and volume of infiltration Identify physical and biological factors that affect infiltration rates and volumes Learn about methods for measuring infiltration Understand why infiltration varies in space and time Identify management effects on infiltration rates and volumes

What is infiltration? The movement of water through the air-soil interface It is one of the things that can happen to precipitation that reaches the soil surface Infiltration is the actual rate at which water is entering the soil at any given time (SCSA, 1976)

Processes of infiltration Entry through the soil surface (infiltration) Storage in the soil profile (soil moisture) Transmission through the soil profile- (percolation)

Infiltration........... Precipitation reaching the ground may infiltrate. This is the process of moving from the atmosphere into the soil. Infiltration may be regarded as either a rate or a total. For example: the soil can infiltrate 1.2 inches/hour. Alternatively, we could say the soil has a total infiltration capacity of 3 inches. Note that in both cases the units are Length or length per time!

Infiltration, cont........... Infiltration is nearly impossible to measure directly - as we would disturb the sample in doing so. We can infer infiltration in a variety of ways (to be discussed at a later point). The exact point at which the atmosphere ends and the soil beings is very difficult to define and generally we are not concerned with this fine detail! In other words, we mostly want to know how much of the precipitation actually enters the soil.

Percolation..... Once the water infiltrates into the ground, the downward movement of water through the soil profile may begin.

Percolation..... The percolating water may move as a saturated front - under the influence of gravity… Or, it may move as unsaturated flow mostly due to capillary forces.

Percolation….the point The vertical percolation of the water into various levels or zones allows for storage in the subsurface – . This stored subsurface water is held and released as either evaporation, transpiration, or as streamflow eventually reaching the watershed outlet.

Infiltration nomenclature i = intensity of rainfall (rate) (length/time) f = infiltration rate- measure of hydraulic conductivity (length/time) F = infiltrated volume (Length3) or depth (L) If i < f what happens? If i > f what happens?

Infiltration Infiltration is the actual rate at which water is entering the soil at any given time(SCSA, 1976).

What factors affect infiltration? Flow influences Head (ponding) Viscosity (function of temperature) Water quality Soil chemistry Soil and water temperature Air entrapment

What factors affect infiltration? Soil surface conditions Land use Vegetation cover Roughness and slope Cracking and crusting Surface sealing, swelling

What factors affect infiltration? Hydrophobicity Dryness Heat Plant chemicals Aromatic oils Other chemicals Fire

What factors affect infiltration? Subsurface conditions Soil Hydrologic group (A B C D) Texture Porosity Depth Shrink and swell Layering Spatial variability Structure

What factors affect infiltration? Subsurface conditions Root system Water table depth Subsurface drainage Water release relationship Hydraulic conductivity

Factors that affect surface and subsurface conditions that affect infiltration Mechanical processes, plowing, Frost- freeze-thaw cycles Litter layer, organic matter Compaction Antecedent soil water condition Chemical activity Biological activity Microbial activity

Soil Type Effects on Infiltration Sand soils have the highest infiltration rates Clay soils have the lowest infiltration rates. High organic matter improves infiltration rates. Taylor & Ashcroft, 1972

Effect of Initial Water Content Soil Cover Effects

How do we measure infiltration? Single ring infiltrometer Constant head (ponded depth) Results tend to be higher than that due to rainfall Point scale

Infiltrometer ring http://ocw.mit.edu/NR/rdonlyres/Civil-and-Environmental-Engineering/1-72 Fall-2004/9227A886-A1FE-408F-BFED-1665F5E5B6B8/0/1_72_lecture_13.pdf

How do we measure infiltration? Rainfall simulators Needle drip systems Stand pipes Sprinkler nozzles Rotating boom All measure input of water and output of water (runoff)- difference is the amount infiltrated Plot scale Need lots of water, vehicles, plot boundaries

How do we measure infiltration? Average infiltration method Small basins or plots Use storms with bursts of rain Compute the amount of rain in the burst Separate the runoff volume due to the burst Difference is infiltrated volume

How do we measure infiltration? Soil surveys Usually report infiltration ranges for various soil types Example rates Sand 124 mm/hr Sandy loam 50 mm/hr Loam 13.2 mm/hr Silt loam 1.05 mm/hr Light clay 0.44 mm/hr

Management effects on infiltration Compaction or alteration of soil surface and vegetative cover Grazing, skidding logs, recreational use, vehicles, plowing Even low ground pressure skidders can increase bulk density by up to 45% at a depth of 15 cm – frequent travel over wet soils

Management effects on infiltration Compaction or alteration of soil surface and vegetative cover Grazing, cropping and logging Changes interception, organic matter layers, rooting depths, ground cover

Infiltration rate over time Infiltration rate i(t), cm/hr Infinite at time t = 0? i(t) can’t exceed precipitation rate Zero at time t = ∞? Soil Physics 2010

Infiltration rate over time Infiltration rate i(t), cm/hr Infiltration rate can be either soil-limited or rain-limited i(t) can’t exceed precipitation rate Not really. Soil behind (above) the wetting front isn’t 100% saturated. Some people write ic instead. As t → ∞, i(t) → Ks Soil Physics 2010

Single-ring Double-ring Measuring infiltration: ring infiltrometer Falling head method: Pour in water, wait for steady flow, then measure water depth over time. Constant head method: Maintain a constant water level, and measure how much water that requires over time. Single-ring Double-ring Soil Physics 2010

Measuring infiltration There is less effect of the ring size on the results when using the double-ring: Maintain equal depths, but only measure flow into inner ring. Outer ring will supply most of the horizontal flow, so inner ring gives mainly vertical Water is applied to the soil surface at a positive pressure Soil Physics 2010

Double Ring Infiltrometer Measure rate of fall in inner ring Infilration http://www.alwi.com/wastewater.php

Estimating infiltration at the scale of a catchment (watershed): Measure baseflow before rainfall Measure rainfall Measure streamflow Estimate runoff by baseflow separation Estimate: Infiltration = rainfall - runoff Soil Physics 2010

These models have 2 main purposes: Infiltration models Green & Ampt (1911) Horton (1930) Kostiakov (1932) Philip (1957) There are many others, but we won’t study them. These models have 2 main purposes: Explain the observed infiltration patterns Predict future infiltration Soil Physics 2010

Infiltration by Horton’s method

No theory: this is purely empirical Kostiakov’s model t i(t) with i : infiltration rate, L/T t : time, T B, n : fitting parameters usually n ≈ 1/2 No theory: this is purely empirical No physical interpretation of B and n. Note that i(0) = ∞, and i(∞) = 0. Frequently this model fits the data better than more physically-based models. Soil Physics 2010

No physical interpretation of b. Note that i(0) = ∞, and i(∞) = ic. Green & Ampt’s model t i(t) ic with i : infiltration rate, L/T ic : final i : i(∞), L/T t : time, T b : fitting parameter I : cumulative infiltration, L No physical interpretation of b. Note that i(0) = ∞, and i(∞) = ic. Assumes all flow is saturated flow Soil Physics 2010

Philip’s model t i(t) ic with i : infiltration rate, L/T ic : final i : i(∞), L/T t : time, T s : sorptivity, L/T0.5 Exact solution of Richards’ equation, with additional assumptions Infinite series, but only 1st 2 terms used Doesn’t work well at short times Sorptivity isn’t used much outside of Australia (J. R. Philip was Australian) Soil Physics 2010

Infiltration into a soil water column 1 2

Green-Ampt method Relative saturation Incremental change in soil water content as wetting front passes

Green-Ampt Parameter Definitions Relative saturation Incremental change in soil water content as wetting front passes

Math for Green Ampt

Green- Ampt Parameters