1. Soil Physics 2010 Outline Announcements Recharge Soil Structure (pores)

2. Soil Physics 2010  Homework 5 is posted on the course website. There is a pdf, and an Excel spreadsheet. Homework 5 due Wednesday after Spring Break I can do a review session over Break if there is interest. Quiz! Announcements

3. Soil Physics 2010 Quiz 1) Sketch a graph of infiltration rate over time, with infiltration limited by the precipitation rate early, and by the soil later. Label the axes. time, T i(t), L/T Rainfall- limited Soil- limited

4. Soil Physics 2010 Quiz 2) In the diagram below, water is flowing through the flexible tube. (a) Describe the force(s) that cause the water to flow. (b) Describe the force(s) that oppose the water flowing. The matric potential in the soil pulls the water into the soil, so the water flows due to capillary forces. In order for water to leave the Mariotte bottle, air must be pulled down into the water a distance  h. This is buoyancy or gravitational force.

5. Soil Physics 2010 Philip model Recall that horizontal infiltration can be modeled as a diffusion process, with x(t)  t ½ But for vertical infiltration, the gradient is always at least 1, so x(t) → K s at large t. So for vertical infiltration, the short-time infiltration rate is i(t)  t ½, but the long-term behavior is more like i(t)  t.

6. Soil Physics 2010 J. R. Philip on infiltration Infinite series solution, with the first 2 terms dominating: Early time: 1 st term dominates Late time: 2 nd term dominates – conceptually like Green & Ampt What’s this?

7. Soil Physics 2010 Illustrating Philip’s concept Sorptivity is like suction at the wetting front. s: sorptivity, with units L T -0.5 (like the Boltzmann variable B) I(t) Sorptivity mostly shows up in Australian research.

8. Soil Physics 2010 Illustrating Philip’s concept Infiltration from a circular pond of zero depth: Capillary forces are small Gravity always dominates s is small Coarse soil (e.g., sand) t = 0 t = 1 t = 2 t = 3

9. Soil Physics 2010 Illustrating Philip’s concept Infiltration from a circular pond of zero depth: Capillary forces are large Gravity is important only as t → ∞ s is large Fine soil (e.g., clay) t = 0 t = 1 t = 2 t = 3

10. Soil Physics 2010 Illustrating Philip’s concept Infiltration from a circular pond of zero depth: Capillary forces dominate early Gravity more important over time s is intermediate Medium soil (e.g., loam) t = 0 t = 1 t = 2 t = 3

11. Infiltration Soil Physics 2010 Infiltration’s friends and relations Precipitation Percolation Groundwater flow Groundwater Recharge Runoff

12. Soil Physics 2010 Infiltration’s friends and relations Evaporation Groundwater flow Groundwater Recharge Then the rain stops. Baseflow Redistribution

13. Soil Physics 2010 z After the rain stops: Redistribution Conservation of mass – blue area must remain constant Surface soil changes from wettest to driest part of the profile with “new” water – even without evaporation.  For shallow wetting of the profile

14. Soil Physics 2010 After saturating a deep, fine-textured soil: z  Lines represent time doubling: ½ day, 1 day, 2 days, 4 days, etc. For t in days, k can be quite small, e.g. k ≈ 0.0001 Averaged over some depth, we can expect something like

15. Soil Physics 2010 After saturating a deep, coarse soil: z  This thought-experiment is like the “long column” method for measuring K(  ). With several measurements in depth and time, and without evaporation, we could work out K(  ) and h(  ).

16. Recharge Evaporation Groundwater flow Groundwater Recharge Baseflow Redistribution Soil Physics 2010 courtesy of AJ

17. Soil Physics 2010 Soil Structure (Pores) Pore structure is about Transport, so it can be useful to examine other transportation networks.

18. Soil Physics 2010 Soil Structure (Pores)

19. Soil Physics 2010 Soil Structure (Pores) A soil acquires structure (pores and particles both) over time. How? Why one structure versus another?