6: Infiltration
- Page ID
- 38737
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)In the prior chapter, we considered water inputs to the soil and the ways in which those water inputs can be altered due to interception. We also looked at the process of raindrop impact on the soil surface and its potentially harmful effects. Now, we are prepared to go-with-the-flow and consider the next step in the soil water balance-the process of infiltration. Infiltration is simply the process by which water enters the soil profile. Infiltration was the focus of perhaps one of the first and certainly one of the most influential soil physics teaching videos ever produced. That classic video produced at Washington State University in 1959 by Walter H. Gardner and his associate, J.C. Hsieh, inspired modern remake created at the University of Gembloux in Belgium and available here [website]Please take the time now to watch that video, which will help you better visualize and understand key features of the infiltration process. Please also take a few minutes to listen to the audio overview for this chapter [website].
The infiltration rate is the volume of water flowing into the soil per unit of surface area per unit time. As seen in the video above, infiltration rates typically decrease over time during the course of an infiltration event, approaching a relatively constant and low rate if the infiltration event is prolonged. You can see in Fig. 6-1how the infiltration rates initially declined sharply then approached a fairly constant level during ponded infiltration into a silty clay loam soil in Hawaii [1].The fundamental cause of this decrease in infiltration rates over time was one of the main scientific mysteries in the early days of soil physics and hydrology. This mystery was studied in detail by an American engineer named Robert Horton one of the most prominent early hydrologists.
- 6.1: Horton infiltration model
- The main deficiencies of this infiltration model are that the parameters are not clearly related to measurable physical properties of the soil and, more importantly, that the model’s conceptual framework fails to include the most fundamental reason that infiltration rates decrease over time. That fundamental discovery had, in fact, been made decades before but apparently was not widely accepted in Horton’s time.
- 6.2: Green-Ampt infiltration model
- This model works best when a relatively sharp or distinct wetting front exists throughout the infiltration process. Such a distinct wetting front is more likely to occur in coarse-textured soils than in fine-textured soils and in initially dry soil than in initially wet soil. The model also works best when the soil texture is homogeneous throughout the wetted region and when air-entrapment, surface crusting, and soil swelling do not substantially influence the infiltration process.
- 6.3: Infiltration for a constant rainfall rate
- The Green-Ampt model described in the prior section is only valid for ponded infiltration. It does not provide infiltration predictions for the case when water is supplied to the soil surface, by rain or sprinkler irrigation, at a rate below the soil’s infiltration capacity. In 1971, engineers Russell Mein and Curtis Larson at the University of Minnesota made an important breakthrough by modifying the Green-Ampt model to account for the case of infiltration under constant-rate rainfall
- 6.4: Infiltration measurements
- The outer ring of the double-ring infiltrometer is intended to reduce lateral flow from the inner ring, allowing the data from the inner ring to be analyzed as if the flow were one-dimensional. When proper field procedures and analytical techniques are applied, both the single- and double-ring infiltrometers provide similar estimates of hydraulic conductivity
- 6.5: Problem set
- For that same soil with the same initial condition, consider the infiltration process for rainfall at a constant rate of 1.5 cm h-1. Use the Mein and Larson modification of the Green-Ampt model to answer the following:
- 6.6: References
- Ahuja, L.R., S.A. El-Swaify, and A. Rahman, Measuring Hydrologic Properties of Soil with a Double-ring Infiltrometer and Multiple-depth Tensiometers1. Soil Science Society of America Journal, 1976. 40(4): p. 494-499. Horton, R.E., The role of infiltration in the hydrologic cycle. Eos, Transactions American Geophysical Union, 1933. 14(1): p. 446-460. Horton, R.E., Analysis of runoff‐plat experiments with varying infiltration‐capacity. Eos, Transactions American Geophysical Union, 1939. 20(4): p.