Infiltration : Objectives

1. Infiltration : Objectives • Understand the three processes of infiltration • Understand the difference between rate (flux) and volume of infiltration • Identify physical and biological factors that affect infiltration rates and volumes • Learn about methods for measuring infiltration • Understand why infiltration varies in space and time • Identify management effects on infiltration rates and volumes

2. What is infiltration? The movement of water through the air-soil interface It is one of the things that can happen to precipitation that reaches the soil surface Infiltration is the actual rate at which water is entering the soil at any given time (SCSA, 1976)

3. Processes of infiltration • Entry through the soil surface (infiltration) • Storage in the soil profile (soil moisture) • Transmission through the soil profile- (percolation)

4. Infiltration........... Precipitation reaching the ground may infiltrate. This is the process of moving from the atmosphere into the soil. Infiltration may be regarded as either a rate or a total. For example: the soil can infiltrate 1.2 inches/hour. Alternatively, we could say the soil has a total infiltration capacity of 3 inches. Note that in both cases the units are Length or length per time!

5. Infiltration, cont........... Infiltration is nearly impossible to measure directly - as we would disturb the sample in doing so. We can infer infiltration in a variety of ways (to be discussed at a later point). The exact point at which the atmosphere ends and the soil beings is very difficult to define and generally we are not concerned with this fine detail! In other words, we mostly want to know how much of the precipitation actually enters the soil.

6. Percolation..... Once the water infiltrates into the ground, the downward movement of water through the soil profile may begin.

7. Percolation..... The percolating water may move as a saturated front - under the influence of gravity… Or, it may move as unsaturated flow mostly due to capillary forces.

8. Percolation….the point • The vertical percolation of the water into various levels or zones allows for storage in the subsurface – . • This stored subsurface water is held and released as either evaporation, transpiration, or as streamflow eventually reaching the watershed outlet.

9. Infiltration nomenclature • i = intensity of rainfall (rate) (length/time) • f = infiltration rate- measure of hydraulic conductivity (length/time) • F = infiltrated volume (Length3) or depth (L) If i < f what happens? If i > f what happens?

10. Infiltration • Infiltration is the actual rate at which water is entering the soil at any given time(SCSA, 1976).

11. What factors affect infiltration? • Flow influences • Head (ponding) • Viscosity (function of temperature) • Water quality • Soil chemistry • Soil and water temperature • Air entrapment

12. What factors affect infiltration? • Soil surface conditions • Land use • Vegetation cover • Roughness and slope • Cracking and crusting • Surface sealing, swelling

13. What factors affect infiltration? • Hydrophobicity • Dryness • Heat • Plant chemicals • Aromatic oils • Other chemicals • Fire

14. What factors affect infiltration? • Subsurface conditions • Soil • Hydrologic group (A B C D) • Texture • Porosity • Depth • Shrink and swell • Layering • Spatial variability • Structure

15. What factors affect infiltration? • Subsurface conditions • Root system • Water table depth • Subsurface drainage • Water release relationship • Hydraulic conductivity

16. Factors that affect surface and subsurface conditions that affect infiltration • Mechanical processes, plowing, • Frost- freeze-thaw cycles • Litter layer, organic matter • Compaction • Antecedent soil water condition • Chemical activity • Biological activity • Microbial activity

17. Taylor & Ashcroft, 1972 Soil Type Effects on Infiltration • Sand soils have the highest infiltration rates • Clay soils have the lowest infiltration rates. • High organic matter improves infiltration rates.

18. Effect of Initial Water Content Soil Cover Effects

19. How do we measure infiltration? • Single ring infiltrometer • Constant head (ponded depth) • Results tend to be higher than that due to rainfall • Method described in lab notes • Point scale

20. Infiltrometer ring http://ocw.mit.edu/NR/rdonlyres/Civil-and-Environmental-Engineering/1-72 Fall-2004/9227A886-A1FE-408F-BFED-1665F5E5B6B8/0/1_72_lecture_13.pdf

21. How do we measure infiltration? • Rainfall simulators • Needle drip systems • Stand pipes • Sprinkler nozzles • Rotating boom • All measure input of water and output of water (runoff)- difference is the amount infiltrated • Plot scale • Need lots of water, vehicles, plot boundaries

22. How do we measure infiltration? • Average infiltration method • Small basins or plots • Use storms with bursts of rain • Compute the amount of rain in the burst • Separate the runoff volume due to the burst • Difference is infiltrated volume

23. How do we measure infiltration? • Soil surveys • Usually report infiltration ranges for various soil types • Example rates • Sand 124 mm/hr • Sandy loam 50 mm/hr • Loam 13.2 mm/hr • Silt loam 1.05 mm/hr • Light clay 0.44 mm/hr

24. Management effects on infiltration • Compaction or alteration of soil surface and vegetative cover • Grazing, skidding logs, recreational use, vehicles, plowing • Even low ground pressure skidders can increase bulk density by up to 45% at a depth of 15 cm – frequent travel over wet soils

25. Management effects on infiltration • Compaction or alteration of soil surface and vegetative cover • Grazing, cropping and logging • Changes interception, organic matter layers, rooting depths, ground cover