Chapter 4 Infiltration

Infiltration is the process by which precipitation moves downward through the surface of the earth. Infiltration replenishes soil moisture, recharges aquifers, and ultimately supports stream flow during dry periods. The rate of infiltration is influenced by factors 1. Type and extent of vegetal cover. 2. Condition of surface crust. 3. temperature, rainfall intensity. 4. physical properties of the soil and water quality.

Infiltration Cont.… Infiltration is highly dependent to surface; for example, fine materials will seal the surface and makes infiltration rate low even underling soils are highly permeable. For measuring infiltration capacity usually methods of hydrograph analysis and infiltrometers studies are used.

Calculation of Infiltration There has been many studies of intiltration processes.

Horton’s Model The infiltration process was thoroughly studied by Harton in the early 1930, and introduced the equation.

Horton’s infiltration curve and hyetograph The area under the curve indicates for any time interval represents the depth of infiltrated water.

S

Home Work Calculate the infiltration rate and the total depth of infiltrated water after 3 hours of hard rain. Information: f 0 = 7.2 cm hr -1 f c = 0.2 cm hr -1 k = 0.75 hr -1

Green Ampt Method This method is based n Darcy’s law, and now is employed in a very wide range of usages. in original formulation Green and Ampt assumed that soil surface was covered by ponded water of negligible depth. And water infiltrated an homogenous soil with a uniform water content.

Green Ampt Method

Example: Using the Green-Ampt model find the infiltration rate when infiltration capacity, F, reaches to 8 cm and rainfall intensity is 2cm/hr. Given Data IMD = 0.32, S=15.5cm, K= 0.6cm/hr