Soil Physics 2010 Outline Announcements More tension infiltrometers More infiltration

Soil Physics 2010 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. Announcements

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

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

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

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

Soil Physics 2010 Mariotte bottle, part 4 h1h1 Water exits tube at h = –  h 1 +  h 2 Control Mariotte Supply Mariotte h2h2 Change h by moving end of tube up or down.

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

Reservoir Bubble tower Soil Physics 2010 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.

ii Soil Physics 2010 Why is the wetting front sharp? z  L K()K()  1)Need less gradient when  is near  s. Need big gradient at low . 2)If it weren’t, it would become sharp. 3)At the front, capillary forces dominate. Behind the front, gravity rules.

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

ii Soil Physics 2010 More about infiltration Specifically the Green & Ampt model z   m initial condition  m = 0 at saturation L Potential difference from surface to wetting front: Distance from surface to wetting front: L +  m L Gradient

Soil Physics 2010 Green & Ampt model ii z  L L +  m L Gradient As I (  infiltration) increases, gradient decreases.

Soil Physics 2010 Green & Ampt model ii z 

Soil Physics 2010 Philips 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.

Soil Physics 2010 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