Soil Physics 2010

1. Outline • Announcements • More tension infiltrometers • More infiltration Soil Physics 2010

2. Announcements • One more review session in G217 Agronomy, today, noon – 2:00 pm. • Homework 5 due Wednesday after Spring Break • Homework 5 is now posted. There is a pdf file, and an Excel file. Soil Physics 2010

3. Measuring infiltration Steady infiltration at a given tension y gives estimate of K(y) Tension infiltrometer (developed in part here at ISU; patent held by Ankeny, Horton, & Kaspar) Bubble tower Reservoir Water is applied to the soil surface at a negative pressure Soil Physics 2010

4. How do you supply water under tension? Dh Pulling air down is like pulling water up Air inlet Mariotte bottle Air coming out of this hose has a suction Dh: That’s what is needed to pull air down, and bubbles out of the inlet tube. Air outlet Soil Physics 2010

5. Mariotte bottle, part 2 Dh q B Air inlet This supplies water at a constant tension – as used in the hydraulic diffusivity experiment. Mariotte bottle Soil Physics 2010

6. Mariotte bottle, part 3 Dh Air inlet This is a useful way to supply water at a constant head… or to maintain water at a constant height. Mariotte bottle Soil Physics 2010

7. Mariotte bottle, part 4 Dh1 Change h by moving end of tube up or down. Water exits tube at h = –Dh1 + Dh2 Dh2 Supply Mariotte Control Mariotte Soil Physics 2010

8. Tension infiltrometer Reservoir Bubble tower Mariotte control bottle (“bubble tower”) and supply bottle (“reservoir”) in a single portable unit. Big reservoir, small Dh control Fewer holes in big reservoir Soil Physics 2010

9. The point of the tension infiltrometer The tension infiltrometer allows field measurement of a few points on the K(h) curve, near saturation where it changes fastest. Reservoir Bubble tower Soil Physics 2010

10. Why is the wetting front sharp? K(q) q q • Need less gradient when q is near qs. Need big gradient at low q. • If it weren’t, it would become sharp. • At the front, capillary forces dominate. Behind the front, gravity rules. L z qi Soil Physics 2010

11. Why is the wetting front sharp? q Behind the front, ym near zero. Water in large pores can stay in large pores → high K. z At the front, ym is strong. Water in large pores is pulled into smaller pores → K drops. 3) At the front, capillary forces dominate. Behind the front, gravity rules. qi Soil Physics 2010

12. More about infiltration Specifically the Green & Ampt model q ym= 0 at saturation L Gradient Potential difference from surface to wetting front: L + ym z Distance from surface to wetting front: L qi yminitial condition Soil Physics 2010

13. Green & Ampt model q L Gradient L + ym z L qi As I (Sinfiltration) increases, gradient decreases. Soil Physics 2010

14. Green & Ampt model q z qi Soil Physics 2010

15. Philips model Recall that horizontal infiltration can be modeled as a diffusion process, with x(t) t½ But for vertical infiltration, the gradient is always at least 1, so x(t) → Ks at large t. So for vertical infiltration, the short-time infiltration rate is i(t) t½ , but the long-term behavior is more like i(t) t. Soil Physics 2010

16. J. R. Philips on infiltration Infinite series solution, with the first 2 terms dominating: Early time: diffusion term dominates Late time: constant term dominates – conceptually like Green & Ampt Soil Physics 2010