INFILTRATION PPT
- 2. 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
- 3. 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 t dftF 0 )()( )(tf dt tdF tf )( )(
- 5. 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
- 6. Hortonian Infiltration Recall Richard’s Equation Assume K and D are constants, not a function of or z Solve for moisture diffusion at surface K z D zt z K z D t 2 2 0 2 2 z D t kt cc effftf )()( 0 f0 initial infiltration rate, fc is constant rate and k is decay constant
- 7. Hortonian Infiltration 0 0.5 1 1.5 2 2.5 3 3.5 0 0.5 1 1.5 2 Time Infiltrationrate,f k1 k3 k2 k1 < k2 < k3 fc f0
- 8. 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 K z D zt KtSttF 2/1 )( KSttf 2/1 2 1 )(
- 9. Green – Ampt Infiltration Wetted Zone Wetting Front Ponded Water Ground Surface Dry Soil 0h L n i z LLtF i )()( dt dL dt dF f zh K z Kf f z h Kqz MoistureSoilInitial FrontWettingtoDepth i L
- 10. Green – Ampt Infiltration (Cont.) Apply finite difference to the derivative, between Ground surface Wetting front K z Kf Wetted Zone Wetting Front Ground Surface Dry Soil L i z 0,0 z fLz , K L KK z KK z Kf f 0 0 F L LtF )( 1 F Kf f K z Kf
- 11. 1 L K dt dL f 1 F Kf f dt dL f Green – Ampt Infiltration (Cont.) LtF )( Wetted Zone Wetting Front Ground Surface Dry Soil L i z L dL dLdt K f f CLLt K ff )ln( Integrate Evaluate the constant of integration )ln( ffC 0@0 tL )ln( L LKt f f f
- 12. Green – Ampt Infiltration (Cont.) )ln( L LKt f f f )1ln( f f F KtF 1 F Kf f Wetted Zone Wetting Front Ground Surface Dry Soil L i z Nonlinear equation, requiring iterative solution.
- 13. Soil Parameters Green-Ampt model requires Hydraulic conductivity, Porosity, Wetting Front Suction Head Soil Class Porosity Effective Porosity Wetting Front Suction Head Hydraulic Conductivity n e K (cm) (cm/h) Sand 0.437 0.417 4.95 11.78 Loam 0.463 0.434 9.89 0.34 Clay 0.475 0.385 31.63 0.03 re n ees )1( e r es Effective saturation Effective porosity
- 14. Ponding time Elapsed time between the time rainfall begins and the time water begins to pond on the soil surface (tp)
- 15. Ponding Time Up to the time of ponding, all rainfall has infiltrated (i = rainfall rate) if ptiF * 1 F Kf f 1 * p f ti Ki )( Kii Kt f p Potential Infiltration Actual Infiltration Rainfall Accumulated Rainfall Infiltration Time Time Infiltrationrate,f Cumulative Infiltration,F i pt pp tiF *
- 16. Example Silty-Loam soil, 30% effective saturation, rainfall 5 cm/hr intensity 30.0 /65.0 7.16 486.0 e e s hrcmK cm 340.0)486.0)(3.01()1( ees 340.0*7.16 hr17.0 ))(65.00.5(0.5 68.5 65.0 )( KiKii Kt f p