f(t), F(t) ?
- majority of rainfall will infiltrate unless rainfall intensity is very high or the soil is already saturated
- f(t) = infiltration rate (cm/s)
- F(t) = cumulative infiltration (cm)
Primary factors behind infiltration? Other related factors?
gravity and capillary suction are main 2
other related factors:
- soil types and soil hydraulic properties
- land surface conditions
- antecedent moisture conditions (dry condition leads to high f(t) b/c of high capillary suction and vice versa)
- storm intensity, depth, duration
Soil hydraulic properties? how do they relate to hydraulic conductivity? what are hydraulic properties based on?
hydraulic conductivity = k, soils ability to transmit water
isotropic (kx = ky)
ansiotropic (kx =/ ky)
homogeneous (same soil)
heterogeneous (diff soil)
based on hydraulic conductivity and water-retention(soils ability to store and release water)
- define total porosity?
- define soil moisture content?
- saturated soil moisture content?
- 5 hydraulic conductivity of soil
- effective porosity
- capillary suction of soil
- relative conductivity
- capillary suction of the soil at the wetting front
1.phi = volume of void / total volume
2.theta = voluime of water / total volume
0 <= theta <= phi
- when theta = phi
- 5 K(theta) [cm/hr]
4.effective porosity n or theta(e) = phi - theta(r)
[theta r = residual theta)
- pitchfork (theta) [cm]
- K(theta) / K(theta(saturated))
- pitchfork [cm]
conditions for an ideal infiltration event?
-under homogeneous soil property, uniform soil moisture content (theta(i)), uniform rainfall intensity i, initially soil is relatively dry, theta (i) «_space;theta(s), and so K(theta) is low
steps of an ideal infiltration event 1-5
- for water to move down the soil column, a wetting front needs to build up so that there is a net capillary suction below the front because theta(i) «_space;theta(s). In the beginning, potential gradient f(t) is high because the wetting front is virtually at the soil surface
- initally, infiltration capacity is higher than rainfall intensity i, but f(1) f(2) f(3) and f(4) equal the rainfall intesntiy b/c infiltration rate cannot be larger than the rainfall supply rate
- as the wetting front moves down the soil column, the potential gradient decreases, soil moisture level changes which means infiltration capacity decreases until it becomes equal to i, at point 4 where the land surface is saturated
- after point 4, f becomes equal to its infiltration capacity which continues to decline until it becomes k(theta(s)), i.e when the soil column is fully or almost fully saturated
- at point 4 and beyond, surface runoff begins b/c f<i></i>
Green ampt’s infiltration model
- based on darcy’s law and continuity eqn
- requires soil hydraulic parameters:
soil moisture = theta(i) [initia;] and theta(s) [saturated]
hydraulic conductivity K(theta(s))
capillary suction - pitchfork
assumptions in green ampt’s infiltration model (continuity eqn) 1-5
- wetting front is a sharp and dividing line between theta(i) and theta(s)
- soil surface is ponded to a negligible depth ho
- theta(i) is uniform throughout
- soil moisture increases from theta(i) to theta(s) as soil is wetted
- pitchfork is negative and constant
because of assumption #4,
F(t) = H(theta(s) - theta(i))
horton’s infiltration model
assumes that infiltration is an exponential decay
f(t) = fc + (fo - fc)e^-Kt
- fc = final infiltration capacity in equilibrium
- fo = initial infiltration rate
- K = recession cosntant
at t=infinity, f(infinity) = fc
at t=0, f(0) = fc + fo - fc = fo
double ring infiltrometer
- used to determine K, fc, fo for hortons infiltration model
- comprises of metal rings
- water is applied to both the inner and outer rings but measurements are taken from the inner ring with a constant head device and the rate of water supply is recorded until a steady infiltration rate is observed
- the outer ring serves to eliminate the effect of lateral spreading of water
- may yield infiltration capacities that are higher than infiltration measured from natural rainfall
CN related to??
4 obvious watershed properties
- soil groups
- landcover complex
- land treatment and management of hydrologic conditions (HC)
- antecedent soil moisture conditions (AMC)
land treatment and management of hydrologic conditions (HC). wats poor/good HC?
poor HC less than 50% coverage of native pasture due to heavy grazing
good HC is more than 75% coverage of native pasture and light grazing
AMC ??
3 amc
dry (condition 1)
average (condition 2)
wet (condition 3)
difference between green ampt and horton infiltration models and the curve number method?
in green ampt and horton: no deprassion storage Ia considered and soil layer is assumed semi infinite or bottomless which is untrue in real life
-both are considered in CN method
When is capilarry suction 0
when theta = thetaS
What to know for NRCS method:
Based off 4 watershed properties:
- soil groups
A B C D - landcover complex
- Hydrologic conditions (HC)
- poor HC if less than 50% coverage of native pasture and heavy grazing
- good HC is more than 75% coverage of native pasture and light grazing - Associated moisture condition (AMC)
- condition 1: dry
- condition 2: average
- condition 3: wet
When does infiltration rate approach 0 in NRCS method
as S approaches S’, as S’ is the storage volume at infiltration
