Slope of the Land

 What happens when precipitation falls on the ground?

~It can be stored or retained on the surface as ice or snow (water retention).

~It can infiltrate or seep (sink into) the Earth’s surface.

~It can flow over the Earth’s surface as runoff.

~It can be evaporated or transpired .

Infiltration

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

Infiltration

Process of water entry into the soil through the soil surface

Infiltration Capacity

Maximum rate water is absorbed by the soil

Infiltration Rate

Infiltration occurring at less than capacity

It is measured in inches per hour or millimeters per hour.

The rate decreases as the soil becomes saturated.

Cumulative infiltration

Accumulated depth of water infiltrating during given time period

Infiltration Capacity Controlled By:

 Soil Surface Processes>>>Infiltration

 Soil Profile Processes>>> Percolation

Factors Affecting Infiltration

• Slope of the Land • Sorting

• Percolation

• Surface Entry

• Rainfall intensity &

Duration • Shape

• Quality of water

• Degree of saturation

• Porosity • Position of G/W

water table

• Permeability • Vegetation

• Packing • Land Use

Slope of the Land

The steeper the slope (gradient), the less the infiltration or seepage

inverse or indirect relationship

Surface Entry

 If a soil is bare, the impact on raindrops causes in-washing of finer particles and clogs the surface. This retards the infiltration.

Percolation

 For infiltration to continue, water that has entered the soil must be transmitted down by the force of gravity and capillary action. When percolation rate is slow the infiltration rate is bounded by the rate of oercolation.

Degree of Saturation or moisture Presence

The more saturated the loose Earth materials are, the less the infiltration --- inverse or indirect relationship

 Water infiltrates into the ground until it meets the interface between the zone of saturation and the zone of aeration

This interface is the WATER TABLE

The depth of the water table below the surface varies with the amount of infiltration

 Subsurface water below the water table is called GROUND WATER

Rainfall intensity & Duration

 During heavy rainfall, top soil is affected by mechanical compaction and by inwash of the finer materials. This leads to the faster decrease in the rate of infiltration than with low intensity of rainfall.

 Duration of rain affects to the extent that when the same quantity of rain falls in n number of isolated storms instead of a continuous one, the infiltration will be higher in the former case.

Porosity

Porosity is the percentage of open space (pores and cracks) in a material compared to its total volume

Generally: the greater the porosity, the greater the amount of infiltration that can occur

Direct Relationship

Shape

 Well rounded particles have a greater porosity.

 Round particles = more pore space, higher porosity, and more infiltration

 Angular particles = less pore space, less porosity, and less

infiltration

Packing



The closer the particles are packed together, the lower the porosity



Therefore if you have a lower porosity,

infiltration will also be lower.

Sorting

 If all the particles in a material are about the same size, they are said to be sorted

 If the particles are of mixed sizes, they are said to be unsorted

Unsorted = lower porosity because the smaller particles fill in the pore space

Sorted = higher porosity or pore space

Quality of Water

 Water containing silt, salts and other impurities affect the infiltration to the extent they are present. Silts clog the pore spaces retarding infiltration rate considerably.

Position of G/W water table

 Position of ground water table should not be very close to the surface for

infiltration to continue.

Permeability

The ability of a material to allow fluids such as water to pass through it. Larger particles will increase permeability, because pore space is larger.

*Impermeability may be due to tight packing or cementing of particles, which seals off the pores from one another.

Vegetation

Grasses, trees and other plant types capture falling precipitation on leaves and branches, keeping that water from being absorbed into the Earth

If any water gets through the vegetation, the velocity of the water will be reduced and this will give the ground more time to absorb the water

Ground without vegetation usually has high runoff and low infiltration rates

Land Use

Roads, parking lots, and buildings create surfaces that are not longer permeable.

These impermeable surfaces often channel runoff.

Farming, cutting down trees and grazing animals will reduce vegetation and

therefore decrease permeability.

Infiltrometers

Single Ring Double Ring

Infiltration Measurement 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

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.

Horton’s Equation

 Horton: The infiltration capacity decreases

exponentially with time and ultimately reaches a constant rate

 Where f_t is the infiltration rate at time t;

 f₀ is the initial infiltration rate or maximum infiltration rate;

 f_c 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 f_c.

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.

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.

Example: Guessing F

The End