LECTURE 4

Lecture 4

Infiltration

Fetter 3.1

What processes occur to precipitation?

Interception - Rainfall intercepted by vegetation

Evaporation/Transpiration

Runoff

Infiltration

When rain hits a dry soil, surface effects between the soil particles and water exert a tension that draws moisture into soil è Infiltration

As the capillary forces diminish with increased soil-moisture content, the infiltration capacity decreases

Infiltration Capacity

the maximum rate at which infiltration can occur under specific moisture conditions.

function of soil moisture content

Infiltration capacity (L/T) or (in/hr) or (cm/hr)

Figure 3.1

Figure 3.1 Decreasing infiltration capacity of an initially dry soil as the soil-mixture

content of the surface layer increases.

f_o = Initial infiltration capacity

f_p = Infiltration capacity

f_c = Equilibrium infiltration capacity

If precipitation rate (L/T) < f_c (L/T), then all rain infiltrates

If precipitation rate > f_p then (or soil frozen),
	1) Depression storage

	If Depression storage filled, then Horton overland flow

Figure 3.3

Figure 3.3 Incremental precipitation rate and its dissociation into amounts of infiltration,

depression storage, and overland flow. Infiltration begins when the precipitation does.

Overland flow does not begin until the depression storage is exhausted. Overland flow

continues past the termination of precipitation. Infiltration will continue as long as there

is any water in the depression storage-usually past the period of overland flow.

Now move to detail Soil Moisture and Ground water

Water Table

The theoretical surface in a subsurface water body at which the pore water pressure is equal to the pressure of the atmosphere. It is generally the level to which water rises in a well also penetrating a few feet into the zone of saturation. All of the spaces (pores) of the material are saturated with water.

Subsurface diagram

Capillary Fringe

A zone above the water table. It is millimeters to 10’s of feet thick zone in which the pores are nearly saturated with water due to surface tension of the air-water interface and the molecular attraction of the liquid and solid phases -- capillarity. The thickness of the capillary fringe depends on the size of the pores that form the capillary fringe. In a coarse material, like gravel, it may be only a few mm thick. In fine-grained material, clay, silt, it may be several meters thick. Because of the differences in pore sizes in an area, the top of the capillary fringe is often an irregular surface.

Rise of water in capillary tube

The surface of the capillary fringe is subject to fluctuations in the water table. A

pumping stress may lower it while a precipitation event may raise it.

ENV 302 - Lectures