Create Presentation
Download Presentation

Download Presentation

Surface Water Hydrology: Infiltration – Green and Ampt Method

Download Presentation
## Surface Water Hydrology: Infiltration – Green and Ampt Method

- - - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - - -

**Surface Water Hydrology: Infiltration – Green and Ampt**Method Nick Fang, Ph.D., P.E. 2013**http://techalive.mtu.edu/meec/module01/Thehydrologiccycle.htm**http://techalive.mtu.edu/meec/module01/Thehydrologiccycle.htm**Runoff Generated from a Watershed**• DEM • Watershed • Precipitation • Infiltration • Runoff • Tributary • Stream flow**Introduction**• Infiltration is a key component to hydrological process. • Mathematical Methods have been developed for computing and simulating infiltration: Green-Ampt Model (1911) Philip’s method for Richards equation (1957) Finite difference method for Richards equation • Computer Models have been developed to simulate the infiltration procedure. HEC-HMS (Green-Ampt) MIKE-SHE and HYDRUS (Richards, Finite difference method) • Important factors: Rainfall Intensity Soil Properties**Some of the precipitation that falls on land seeps into the**ground where it is stored in aquifer and is transported to streams and lakes by subsurface flow. • The amount of infiltration is influenced by the permeability and moisture content of the soil, the presence of vegetation and the volume and intensity of precipitation. • Large particles have large spaces between them and let more water in and large rain events can lead to more infiltration on the amount of water stored in the aquifer.**Infiltration Process**• The process of rain water entry into the soil. • Early: capillary force dominates. • Later stage: gravitational force dominates = infiltration rate = Ks. • Ponding = Infiltration rate decreases with time (rainfall duration).**Let us consider a simple system**• Homogenous soil • 1-D vertical system • Uniform initial conditions • Constant rainfall rate (i)**1**2 Infiltration into a Soil Water Column (Darcy’s Law) Flow Equation Mass Equation f: infiltration rate (depth/time); F: Cumulative infiltration depth (depth); K(ө): Vertical hydraulic conductivity (depth/time); z: Depth below surface (depth); h: Potential or head (depth); ө: Volumetric moisture content; L: Soil column depth (depth). Ψ: Suction Head – pull the water downward (-depth)**Green – Ampt Infiltration Method**Relative saturation Incremental change in soil water content as wetting front passes**Math for Green-Ampt (1911) Infiltration Theory with time**variable dependent on rainfall intensity (i) Green-Ampt model calculates cumulative infiltration by assuming water flow into a vertical soil profile like a piston flow Parameters required: K: Vertical hydraulic conductivity (depth/time); : Suction head (negative depth); by Chow et al. (1988) : Moisture content difference at two levels.**Theoretical Observations**• Infiltration rate approaches Ks. • Wetting front assumes a constant shape when the infiltration rate approaches a constant value.**3 Conditional Infiltration Scenarios**Case 1: i < Ks. = Runoff will never occur and all rainfall will infiltrate regardless of the duration; then f = i (curve A) . Case 2: Ks < i < f. = The time to ponding varies for different rainfall intensities; then f = i until F = its = Fs; following Equation (curve B-C) Case 3: i > f. = Runoff can occur. Ponding Time As long as i > Ks, infiltration rate (f) asymptotically approaches Ks.**i**More Detailed Information for Case 1 Case 1: i < Ks**i**More Detailed Information for Case 2 Case 2: Ks < i < f**Soil Classification**USDA particle size classification < 0.002mm is clay 0.002 – 0.5mm is silt 0.5mm – 10mm is sand > 10mm is gravel http://www.soils.agri.umn.edu/academics/classes/soil2125/doc/s2chp1.htm**USDA Soil Texture Triangle**A soil with 30% sand, 40% clay and 30% silt is a Clay Loam % Silt = 100 - %Sand - %Clay**Variation of Saturated Hydraulic Conductivity with Soil**Texture K Se**Figure 1. 3-D plot of infiltration rates for sandy soil**conditions. Figure 2. 3-D plot of infiltration rates for clayey soil conditions. Typical Relationships for Green- Ampt Variables in 3–D http://www.ocrwm.doe.gov/ym_repository/studies/sitechar/infiltration.shtml**Double Ring Infiltrometer**Measure rate of fall in inner ring Infiltration http://www.alwi.com/wastewater.php**Application of KBr tracer to estimate infiltration rates at**an unsaturated-zone well installation.**Example 1**This formula has been transformed from its original one in order to solve for Fs.**Confused?**Or B A When Ψ is given positive, Formula A will be used. When Ψ is given negative, Formula B will be used.