CE 394K.2 Hydrology Infiltration Reading AH Sec 5.1 to 5.5 Some slides were prepared by Venkatesh Merwade Slides 2-6 come from

Neutron scattering (attenuation)Neutron scattering (attenuation) –Measures volumetric water content (  v ) –Attenuation of high-energy neutrons by hydrogen nucleus –Advantages: samples a relatively large soil spheresamples a relatively large soil sphere repeatedly sample same site and several depthsrepeatedly sample same site and several depths accurateaccurate –Disadvantages: high cost instrumenthigh cost instrument radioactive licensing and safetyradioactive licensing and safety not reliable for shallow measurements near the soil surfacenot reliable for shallow measurements near the soil surface Dielectric constantDielectric constant –A soil’s dielectric constant is dependent on soil moisture –Time domain reflectometry (TDR) –Frequency domain reflectometry (FDR) –Primarily used for research purposes at this time Soil Water Measurement

Soil Water Measurement Neutron Attenuation

Tensiometers –Measure soil water potential (tension) –Practical operating range is about 0 to 0.75 bar of tension (this can be a limitation on medium- and fine-textured soils) Electrical resistance blocks –Measure soil water potential (tension) –Tend to work better at higher tensions (lower water contents) Thermal dissipation blocks –Measure soil water potential (tension) –Require individual calibration Soil Water Measurement

Tensiometer for Measuring Soil Water Potential Porous Ceramic Tip Vacuum Gauge (0-100 centibar) Water Reservoir Variable Tube Length (12 in- 48 in) Based on Root Zone Depth

Electrical Resistance Blocks & Meters

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 

Richard’s Equation Recall –Darcy’s Law –Total head So Darcy becomes Continuity becomes Soil water diffusivity

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

Infiltration into a horizontal soil column x 0 Θ = Θ n for t = 0, x > 0 Θ = Θ o for x = 0, t > 0 Equation: Boundary conditions

Measurement of Diffusivity by Evaporation from Soil Cores Air stream q = soil water flux = evaporation rate q x

Measurement of Diffusivity by Evaporation from Soil Cores

Numerical Solution of Richard’s Equation (Ernest To)

Implicit Numerical Solution of Richard’s Equation x (i) t (j) i-1 i i+1 j j -1

Implicit Numerical Solution of Richard’s Equation

Matrix solution of the equations

Θ

f

F

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

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

Infiltrometers Single RingDouble Ring

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