Soil Physics 2010

1. Infiltration 1-3 Jake’s lectures Toby’s & Jake’s notes combined Soil Physics 2010

2. Infiltration Infiltration is a surface process: water (e.g., rain) moves from above to below the soil surface. As water moves in, air must either (1) escape, or (2) be compressed below the infiltrating water. Generally air escapes through larger pores, which slows the infiltration. Sometimes you see bubbles in puddles while it’s raining. (Note that Richards’ equation does not account for air flow: it is only about water flow.) Soil Physics 2010

3. Infiltration • Infiltration is important for: • Soil water recharge (e.g., for plants) • Groundwater recharge • River baseflow • Erosion (water that doesn’t infiltrate runs off) • Flooding • Contaminant movement Soil Physics 2010

4. Soil properties • Soil properties important to infiltration: • Texture, pore size distribution • Hydraulic conductivity • Structure (including macropores for air escape) • Antecedent (initial) water content • Wettability • Layering Infiltration is driven by both gravity and matric potential Drier soil has greater matric potential pulling water in, and more porosity available to hold that water. Soil Physics 2010

5. Soil management affects infiltration • Less infiltration • Roads, roofs, etc. • Compaction • Tillage (frequently) • Bare soil surface • More infiltration • Good soil structure • Plants, root channels • Tillage (occasionally) • Organic matter • Drained Soil Physics 2010

6. Infiltration rate over time Infiltration rate i(t), cm/hr time • Why this decrease? • At short times: • Air escapes more easily • Greater hydraulic gradient Soil Physics 2010

7. Infiltration rate over time Infiltration rate i(t), cm/hr time Infinite at time t = 0? i(t) can’t exceed precipitation rate Zero at time t = ∞? Soil Physics 2010

8. Infiltration rate over time Infiltration rate i(t), cm/hr time Infiltration rate can be either soil-limited or rain-limited i(t) can’t exceed precipitation rate Not really. Soil behind (above) the wetting front isn’t 100% saturated. Some people write ic instead. As t → ∞, i(t) → Ks Soil Physics 2010

9. The wetting profile q saturated Initial volume wetness qi transmission zone Note: not saturated! wetting front depth Why is the wetting front sharp? (discussed further in next file) Soil Physics 2010

10. Measuring infiltration: ring infiltrometer Falling head method: Pour in water, wait for steady flow, then measure water depth over time. Constant head method: Maintain a constant water level, and measure how much water that requires over time. Single-ring Double-ring Soil Physics 2010

11. Measuring infiltration • There is less effect of the ring size on the results when using the double-ring: • Maintain equal depths, but only measure flow into inner ring. • Outer ring will supply most of the horizontal flow, so inner ring gives mainly vertical Water is applied to the soil surface at a positive pressure Soil Physics 2010

12. Measuring infiltration: the tension infiltrometer (Developed in part here at ISU. Patent holders are Ankeny, Horton, and Kaspar) Water is applied to the soil surface at a negative pressure Bubble tower Reservoir Steady infiltration at a given tension y gives estimate of K(y) Soil Physics 2010

13. Estimating infiltration at the scale of a catchment (watershed): • Measure baseflow before rainfall • Measure rainfall • Measure streamflow • Estimate runoff by baseflow separation • Estimate: Infiltration = rainfall - runoff Soil Physics 2010

14. Infiltration models • Green & Ampt (1911) • Kostiakov (1932) • Philip (1957) There are many others, but we won’t study them. These models have 2 main purposes: Explain the observed infiltration patterns Predict future infiltration Soil Physics 2010

15. Kostiakov’s model i(t) t with i : infiltration rate, L/T t : time, T B, n : fitting parameters usually n ≈ 1/2 No theory: this is purely empirical No physical interpretation of B and n. Note that i(0) = ∞, and i(∞) = 0. Frequently this model fits the data better than more physically-based models. Soil Physics 2010

16. Green & Ampt’s model i(t) ic t with i : infiltration rate, L/T ic : final i : i(∞), L/T t : time, T b : fitting parameter I : cumulative infiltration, L No physical interpretation of b. Note that i(0) = ∞, and i(∞) = ic. Assumes all flow is saturated flow Soil Physics 2010

17. Philip’s model i(t) ic t with i : infiltration rate, L/T ic : final i : i(∞), L/T t : time, T s : sorptivity, L/T0.5 Exact solution of Richards’ equation, with additional assumptions Infinite series, but only 1st 2 terms used Doesn’t work well at short times Sorptivity isn’t used much outside of Australia (J. R. Philip was Australian) Soil Physics 2010