4: Soil Water Flow
- Page ID
- 38735
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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 second chapter we considered the complex and hierarchical spatial organization of the soil solid phase across scales ranging from kilometers to micrometers. We briefly discussed the relevance of these patterns to issues ranging from climate change to agricultural management to pesticide transport. In the third chapter, we focused on two of the most important and dynamic descriptors of the soil's condition, soil water content and soil water potential, and the relationship between those two variables, the soil water retention curve. Now the main aim of this chapter is to build a foundation for accurately understanding soil water flow. But before we can do that, we first need to focus on some of the most fascinating, intricate, and life-giving physical features of soil, the soil pore spaces. We will begin this chapter by focusing on understanding soil pores and pore networks and their significance, and then we will spend the rest of the chapter learning the fundamental physical properties and processes that govern the flow of water in those pore spaces. This chapter's audio overview is available here (link).
- 4.1: Pores and pore networks
- Closely associated with the spatial patterns of the soil solid phase are the complementary spatial patterns of the soil pore network. The soil pores provide vital flow paths for water, oxygen, carbon dioxide, and nutrients without which life in the soil would be impossible. They also provide habitat for a host of living organisms in the soil.
- 4.2: Soil water potential for systems at equilibrium
- Perhaps the most fundamental concept for understanding soil water flow is the fact that differences in soil water potential drive soil water flow. Intuitively, we might assume that water always flows downward through the soil, or perhaps we may assume that water always flows from wetter soil to drier soil. But reality can be surprising, and these intuitions can mislead us.
- 4.3: Poiseuille's Law
- When differences in soil water potential occur, water flows from regions of higher potential to regions of lower potential, unless those regions are separated by an impermeable layer. This knowledge alone is enough for us to determine the direction of soil water flow in any situation where we know or can measure the soil water potentials
- 4.4: Darcy's Law
- where Δψt is the difference in total water potential between two points in a saturated porous media separated by distance, L. The symbol Ks serves to clarify that we are referring to the saturated hydraulic conductivity of the soil, which differs dramatically from the hydraulic conductivity of unsaturated soil as we will soon see. Darcy noted that the hydraulic conductivity depended on the permeability of the porous media.
- 4.5: Factors affecting saturated hydraulic conductivity
- As you may have expected, soil texture strongly influences saturated hydraulic conductivity. Soils dominated by large sand particles tend to have relatively large pore spaces and thus large values of saturated hydraulic conductivity. Soils dominated by small clay particles tend to have relatively small pores spaces and small values of saturated hydraulic conductivity.
- 4.6: Chemical dispersion and flocculation
- In addition to these physical properties, chemical properties of the soil and the solution flowing through the soil can also impact the saturated hydraulic conductivity. These chemical effects arise when soil and solution characteristics promote swelling and chemical dispersion of clay present in the soil.
- 4.7: Darcy's Law for layered soils
- Darcy’s Law for layered soil allows us to estimate water flow rates for layered soils, which we are likely to encounter in the field. However, both forms of Darcy’s Law only apply to saturated water flow. Many times in the field we need to understand or estimate water flow rates when the soil is unsaturated. For example, infiltration into, redistribution through,
- 4.8: Buckingham-Darcy Law
- In 1902, a physicist named Edgar Buckingham was hired by the US Department of Agriculture to work in its newly formed Bureau of Soils [12]. He stayed there for only four years, but his studies during that time led to a conceptual breakthrough that would change the course of soil physics and hydrology. Drawing on earlier work by John Maxwell and Lyman Briggs, Buckingham reasoned that in unsaturated soil the water was attracted to and held by the surfaces of the soil solids by what he called.....
- 4.9: Models for soil hydraulic conductivity
- We sometimes have measurements of soil hydraulic conductivity at saturation and perhaps at one or two water contents below saturation, but we often need a mathematical function to allow calculation of hydraulic conductivity for all other values of water content. For this reason, soil hydraulic conductivity functions have been developed corresponding to each of the soil water retention functions presented in Chapter 3.
- 4.10: Conclusion
- In this chapter, we have considered the complexity of soil pores and pore networks and how they control and complicate the processes of soil water flow. We have learned how to make a simple table to determine the gravitational, pressure, and total water potentials for soil-water systems at equilibrium and during steady, saturated flow
- 4.11: Problem set
- A soil profile is in hydraulic equilibrium with a water table located at a depth of 275 cm below the surface. Draw a sketch representing that soil profile, label the soil surface as point “A” and the water table as point “B”. Create a table showing the gravitational head, pressure head, and total head at points A and B.
- 4.12: References
- Rudiyanto, et al.,Simple functions for describing soil waterretention and the unsaturated hydraulic conductivity fromsaturation to complete dryness.Journal of Hydrology,2020.588: p. 125041 Brooks, R. and A. Corey, Hydraulic Properties of Porous Media. Hydrology Papers, Colorado State University, 1964(March). Campbell, G.S., A Simple Method for Determining Unsaturated Conductivity From Moisture Retention Data. Soil Science, 1974. 117(6): p. 311-314.