Lecture ERS 482/682 (Fall 2002) Infiltration ERS 482/682 Small Watershed Hydrology

Lecture ERS 482/682 (Fall 2002) Definitions infiltration: –process by which water enters the soil surface infiltration rate, f(t): –rate at which water enters the soil surface water-input rate, w(t): –rate at which water arrives at the soil surface infiltration capacity, f*(t): –maximum rate at which infiltration can occur depth of ponding, H(t): –depth of water standing on the surface [L T -1 ] [L]

Lecture ERS 482/682 (Fall 2002) Definitions percolation –downward movement of water through the soil hydraulic conductivity, K h : –rate at which water moves through a porous medium under a unit potential-energy gradient sorptivity, S p : –rate at which water will be drawn into an unsaturated soil in the absence of gravity forces soil-water pressure or matric potential,  : –water pressure (tension) head in a soil air-entry tension,  ae : –pressure head when significant volumes of air begin to appear in soil pores; occurs at the capillary fringe (i.e., height of the tension-saturated zone) [L T -1 ] [L T -1/2 ] [L] [L]

Lecture ERS 482/682 (Fall 2002) Why is infiltration important?

Lecture ERS 482/682 (Fall 2002) Why is infiltration important? Determines availability of water for overland flow –Flood prediction

Lecture ERS 482/682 (Fall 2002) Why is infiltration important? Determines availability of water for overland flow –Flood prediction –Irrigation plans

Lecture ERS 482/682 (Fall 2002) Why is infiltration important? Determines availability of water for overland flow –Flood prediction –Irrigation plans –Runoff pollution Determines how much water goes into the soil –Groundwater estimates –Water availability for plants

Lecture ERS 482/682 (Fall 2002) Infiltration conditions No ponding: Soil column

Lecture ERS 482/682 (Fall 2002) Infiltration conditions No ponding: Soil column Saturation from above:

Lecture ERS 482/682 (Fall 2002) Infiltration conditions No ponding: Soil column Saturation from above: Saturation from below:

Lecture ERS 482/682 (Fall 2002) Figure 5.2: Manning (1987) Capillarity: –Sorptivity –Matric potential Gravity: –Percolation –Hydraulic conductivity

Lecture ERS 482/682 (Fall 2002) What are models? Models are representations of the real world conceptualization A model is a conceptualization of a system essential characteristics that retains the essential characteristics of specific purpose that system for a specific purpose.

Lecture ERS 482/682 (Fall 2002) Assumptions for most infiltration models Water moves vertically Homogeneous soil Soil volume > pore size Moving water is liquid only Water movement not affected by –Airflow in soil pores –Temperature –Osmotic gradients

Lecture ERS 482/682 (Fall 2002) Infiltration models Horton Kostiakov Green-Ampt Philip Others

Lecture ERS 482/682 (Fall 2002) Time, t Infiltration rate, f(t) What we want to quantify… t sat w

Lecture ERS 482/682 (Fall 2002) Time, t Infiltration rate, f(t) What we want to quantify… K* h t sat w

Lecture ERS 482/682 (Fall 2002) Time, t Infiltration rate, f(t) What we want to quantify… K* h t sat,1 t sat,2 t sat,3 Runoff f(t)<f*(t) f(t)=f*(t)

Lecture ERS 482/682 (Fall 2002) What we want to quantify… Figure 4.2: Brooks et al. (1991)

Lecture ERS 482/682 (Fall 2002) Horton model decreasingInfiltration rate resembles a decreasing exponential function: Exponential function: where e = …

Lecture ERS 482/682 (Fall 2002) Horton model Time, t Infiltration rate, f(t) fcfcfcfc f0f0f0f0 f(t) = f c + (f 0 – f c )e -kt x x x x x x x x

Lecture ERS 482/682 (Fall 2002) Kostiakov model Time, t Infiltration rate, f(t) f(t) = K k t - 

Lecture ERS 482/682 (Fall 2002) Water Soil column Dry soil  =  0 Wet soil  =  Green-Ampt model Based on –Darcy’s law (Eq. 6-8b) Capillary suction at wetting front wettingfront zf(t)zf(t) H(t)H(t)

Lecture ERS 482/682 (Fall 2002) Green-Ampt model Figure 8.10: Hornberger et al.(1998)  zf(t)zf(t)

Lecture ERS 482/682 (Fall 2002) Initially (before rain)  =  0, H(t) = 0,  f = 0 Green-Ampt model Soil column Dry soil  =  0

Lecture ERS 482/682 (Fall 2002) Green-Ampt model If w < K* h : H(t) = 0  Soil column Dry soil  =  0 Kh()Kh() w = rainfall rate Storage  until t = t w  >  0 time when rain stops

Lecture ERS 482/682 (Fall 2002) Green-Ampt model If w > K* h : H(t) = 0  Soil column Dry soil  =  0 up to K* h K h (  )  up to K* h Storage  until t=t p  = = time when ponding starts

Lecture ERS 482/682 (Fall 2002) Green-Ampt model If w > K* h : Soil column Dry soil  =  0 for t>t p == Equation 6-40 (error in book)

Lecture ERS 482/682 (Fall 2002) Green-Ampt model If w > K* h : Soil column Dry soil  =  0 for t>t p == Equation 6-42 Volume infiltrated zf(t)zf(t) Change in water content H(t) ~ 0 rate

Lecture ERS 482/682 (Fall 2002) Green-Ampt model Difficulties with model –Need to know Porosity,  Initial water content,  0 K* h  f See Examples 6-6 and 6-7measure Table 6-1 Equation 6-46 with Table 6-1

Lecture ERS 482/682 (Fall 2002) Philip model For t>t p Soil column Dry soil  =  0 == Volume infiltrated where t = time since ponding began S p = sorptivity K p = hydraulic conductivity

Lecture ERS 482/682 (Fall 2002) Philip model Works after ponding only Used for characterizing spatial variability of infiltrometer measurements Soil column Dry soil  =  0 ==

Lecture ERS 482/682 (Fall 2002) Other models Richard’s equation –Physically-based –Numerically intensive Morel-Seytoux and Khanji model –Includes viscous resistance Smith-Parlange model –Account for different rates of changing hydraulic conductivity with water content

Lecture ERS 482/682 (Fall 2002) Measuring infiltration Flooding (ring) infiltrometers –Single ring –Double ring Rainfall- runoff plot infiltrometers

Lecture ERS 482/682 (Fall 2002) Ring infiltrometers Bouwer (1986) Cylinder infiltration True infiltration Water-entry pressure head  0.5  ae

Lecture ERS 482/682 (Fall 2002) Estimating infiltration parameters Box 6-2 and Example 6-9 Time, t (hr) f(t) (cm hr -1 ) ponding begins; determined in Example 6-7 Data from Example 6-8

Lecture ERS 482/682 (Fall 2002) Estimating infiltration parameters Box 6-2 and Example 6-9 Time, t (hr) t’ (hr) f(t) (cm hr -1 )

Lecture ERS 482/682 (Fall 2002) Estimating infiltration parameters Box 6-2 and Example 6-9 Time, t (hr) t’ (hr) f(t) (cm hr -1 ) Least squares approach: Find the parameters that provide the ‘best fit’ of the model to the observed data ‘best fit’ occurs when sum of the squared differences between measured and modeled values is minimized

Lecture ERS 482/682 (Fall 2002) Estimating infiltration parameters Box 6-2 and Example 6-9 Time, t (hr) t’ (hr) f(t) (cm hr -1 ) Equations 6B2-8 and 6B2-9 note error in book!

Lecture ERS 482/682 (Fall 2002) Estimating infiltration parameters Box 6-2 and Example 6-9 Time, t (hr) t’ (hr) f(t) (cm hr -1 ) sumsumsumsum

Lecture ERS 482/682 (Fall 2002) Variability of infiltration Factors that affect infiltration rate –Water-input rate or depth of ponding –Hydraulic conductivity at the surface Organic surface layers Frost Swelling-drying Inwashing of fine sediment Anthropogenic modification

Lecture ERS 482/682 (Fall 2002) Variability of infiltration Factors that affect infiltration rate –Water-input rate or depth of ponding –Hydraulic conductivity at the surface Organic surface layers Frost Swelling-drying Inwashing of fine sediment Anthropogenic modification

Lecture ERS 482/682 (Fall 2002) Variability of infiltration Factors that affect infiltration rate –Water content of surface pores –Surface slope and roughness –Chemical characteristics of soil hydrophobicity –Physical/chemical properties of water Figure 4.5: Brooks et al. (1991)

Lecture ERS 482/682 (Fall 2002) Point  watershed??? Manley (1977) approach Fraction of watershed area Infiltration capacity K* + h w Rainfall rate Infiltration

Lecture ERS 482/682 (Fall 2002) Point  watershed??? Areal-weighted averages Philip equation: Measure at several locations Calculate area-weighted average of S p and K p Areal-weighted average of infiltration

Lecture ERS 482/682 (Fall 2002) Point  watershed??? Divide watershed into subareas –Soil properties –Initial conditions –Etc. Calculate areally-weighted infiltration

Lecture ERS 482/682 (Fall 2002) Example: Incline Creek Watershed Objective: determine which data collection techniques are best for quantifying spatial variations in surface infiltration –Used Philip equation Sullivan et al. 1996

Lecture ERS 482/682 (Fall 2002) Watershed size: 7.2 km 2

Lecture ERS 482/682 (Fall 2002) Performed 50 tests with disk permeameter

Lecture ERS 482/682 (Fall 2002) Performed 50 tests with disk permeameter Sites were selected based on: Accessibility Minimal surface disturbance Macropores were absent Tried to pick sites that represented different soil types and vegetative cover

Lecture ERS 482/682 (Fall 2002) Created GIS coverages Soil types Vegetative groupings Used field method to determine average areal % of vegetation classification per disk- permeameter test Calculated weighted values for K s based on average areal % vegetation cover

Lecture ERS 482/682 (Fall 2002) The infiltration rates were used to estimate time to ponding and runoff potential