2: Soil Water
- Page ID
- 44298
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)- 2.1: Introduction
- Like humans, plants need water to survive. But, from where do plants get their water? Well, the soil beneath your feet is the answer. The soil within the plant root zone serves as a reservoir for storing precipitation and irrigation water for future use by plants. Effective management of an irrigation system requires the understanding and use of the basic concepts of soil water.
- 2.2: Soil Composition
- As Figure 2.1 illustrates, soil is composed of three major components: soil particles, air, and water. The fractions of water and air are contained in the voids between soil particles. The ratio of the volume of pores (voids) to the total (bulk) volume of a soil is the porosity (φ). One way to determine porosity is to measure the volume of a soil that is composed of soil particles and the fraction made up of the pores.
- 2.3: Soil Water Content
- The amount of water in a soil can be expressed in many ways, including a dry soil basis (mass water content), a volumetric basis (volumetric water content),fraction of the available water remaining, and fraction of the available water depleted. With so many different terms, confusion is bound to arise. Irrigation managers must understand all of these terms to interpret soil water status correctly.
- 2.4: Soil Water Potential
- Equation 2.7 ignores the impact of overburden pressure on soil water potential. The gravitational potential is due to the force of gravity pulling downward on the water in the soil. Matric potential is a result of the forces the soil particles place on the water by adhesion and surface tension at the soil-air interface. These combined forces cause capillarity, which is sometimes referred to as soil water tension...
- 2.5: Available Water and the Soil Water Reservoir
- Irrigation managers can view the soil as a reservoir for holding water. Figure 2.8 illustrates the analogy between a reservoir and a soil. Soil without any water would be like an empty reservoir (Figure 2.8a). Pores in the soil, measured as porosity, provide space for the storage of water. When saturated, the entire void space (reservoir) is filled with water as in Figure 2.8b.
- 2.6: Determining Available Water Capacity
- Field capacity may be determined by flooding a small area of land, covering it to suppress evaporation, waiting several days for drainage to become negligible, and then sampling to determine the water content throughout the soil profile. When flooding ceases, the water content falls rapidly as the largest soil pores are quickly drained by gravity.
- 2.7: Tabulated Values of Typical Soil Properties
- Data for soil properties are available from various sources. For example, in the U.S., county-level Soil Survey Reports normally list many of the soil properties described in this chapter.
- 2.8: Infiltration
- What causes water to enter the soil? Two things drive infiltration: capillarity and gravity. During the initial stages of a water application, capillary forces dominate water movement into the soil. Capillary forces work equally in all directions. Thus, capillary forces pulling water into the soil are the same in the horizontal and vertical directions. As time progresses, the capillary forces diminish, and gravity becomes the dominant force.
- 2.9: Storage of Infiltrated Water
- Where does the water go once it has infiltrated into the soil? How deep will it penetrate into the plant root zone? Will it penetrate beyond the root zone? Although an oversimplification, water applied to a soil can be viewed as filling the soil profile in layers as illustrated in Figure 2.14. Even if a soil layer is wetted to saturation, it is assumed that it quickly (in a few days) drains to field capacity. The excess water (excess of FC) from a soil layer drains to the layer immediately benea
- 2.10: Measuring Soil Water Content and Matric Potential
- Measuring soil water content and matric potential is important in irrigation management. Measuring soil water content is useful for determining whether soil water content is being kept within allowable bounds (to be discussed in Chapter 6), when the next irrigation should occur, and how much water the soil can hold without deep percolation.
- 2.10.1: Gravimetric Method
- 2.10.2: Feel and Appearance
- 2.10.3: Neutron Scattering
- 2.10.4: Time Domain Reflectometry
- 2.10.5: Capacitance Probes
- 2.10.6: Tensiometers
- 2.10.7: Electrical Resistance Blocks and Granular Matrix Sensors
- 2.10.8: Thermal Dissipation Blocks
- 2.10.9: Placement of Soil Water Sensors
- 2.10.10: Remote Sensing
- 2.11: Summary
- One of the most important functions of a soil is to serve as a reservoir for storing precipitation and irrigation water for use by plants. Water is stored in the void spaces between soil particles. When the voids are filled with water, the soil is said to be saturated. A saturated soil rapidly drains to a, more or less, constant moisture level called field capacity (FC).
- 2.12: Questions
- An irrigation of 2.5 in of infiltration is followed by 1 in of rainfall infiltration. If a clay loam soil had a 50% depletion of the available water (fd = 0.5) prior to the water application and the root zone depth is 30 in, how much water would deep percolate? If the average count ratio for neutron scattering measurements was 1.0, how much water needs to be infiltrated to bring a silt loam soil to field capacity?
- 2.13: References
- Al-Yaari, A., Wigneron, J. P., Dorigo, W., Colliander, A., Pellarin, T., Hahn, S., Mialon, A., Richaume, P., Fernandez-Moran, R., Fan, L., Kerr, Y. H., & De Lannoy, G. (2019). Assessment and inter-comparison of recently developed/reprocessed microwave satellite soil moisture products using ISMN ground-based measurements. Remote Sens. Environ., 224, 289-303.