CE 394K.2 Hydrology Infiltration Reading for Today: AH Sec 4.3 and 4.4 Reading for Thurs: AH Sec 5.1 to 5.5 Subsequent slides prepared by Venkatesh Merwade

Darcy’s Law K = hydraulic conductivity q = specific discharge V = q/n = average velocity through the area

Definitions solid Pore with air Pore with water Element of soil, V (Saturated) Element of soil, V (Unsaturated)

Infiltration General –Process of water penetrating from ground into soil –Factors affecting Condition of soil surface, vegetative cover, soil properties, hydraulic conductivity, antecedent soil moisture –Four zones Saturated, transmission, wetting, and wetting front depth Wetting Zone Transmission Zone Transition Zone Saturation Zone Wetting Front 

Infiltration Infiltration rate –Rate at which water enters the soil at the surface (in/hr or cm/hr) Cumulative infiltration –Accumulated depth of water infiltrating during given time period

Infiltration Methods Horton and Phillips –Infiltration models developed as approximate solutions of an exact theory (Richard’s Equation) Green – Ampt –Infiltration model developed from an approximate theory to an exact solution

Hortonian Infiltration Recall Richard’s Equation –Assume K and D are constants, not a function of  or z Solve for moisture diffusion at surface f 0 initial infiltration rate, f c is constant rate and k is decay constant

Hortonian Infiltration

Philips Equation Recall Richard’s Equation –Assume K and D are functions of , not z Solution –Two terms represent effects of Suction head Gravity head S – Sorptivity –Function of soil suction potential –Found from experiment

Green – Ampt Infiltration Wetted Zone Wetting Front Ponded Water Ground Surface Dry Soil

Green – Ampt Infiltration (Cont.) Apply finite difference to the derivative, between –Ground surface –Wetting front Wetted Zone Wetting Front Ground Surface Dry Soil

Green – Ampt Infiltration (Cont.) Wetted Zone Wetting Front Ground Surface Dry Soil Integrate Evaluate the constant of integration

Green – Ampt Infiltration (Cont.) Wetted Zone Wetting Front Ground Surface Dry Soil See: Nonlinear equation, requiring iterative solution.

Soil Parameters Green-Ampt model requires –Hydraulic conductivity, Porosity, Wetting Front Suction Head –Brooks and Corey Effective saturation Effective porosity

Ponding time Elapsed time between the time rainfall begins and the time water begins to pond on the soil surface (t p )

Ponding Time Up to the time of ponding, all rainfall has infiltrated (i = rainfall rate) Potential Infiltration Actual Infiltration Rainfall Accumulated Rainfall Infiltration Time Infiltration rate, f Cumulative Infiltration, F

Example Silty-Loam soil, 30% effective saturation, rainfall 5 cm/hr intensity