2.10: Measuring Soil Water Content and Matric Potential
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)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. Many methods are available for measuring soil water content. We will only discuss a few of the more proven methods. For more detailed discussions of soil water measuring devices, refer to Evett (2007), Gardner (1986), or Ley (1994). This section does not discuss systems for logging soil water data, transmitting data to the cloud, data storage, or platforms for viewing and interpreting data. These systems make it much easier to incorporate soil water sensor data into day-to-day farm management, and many options are available from industry. A list of questions to consider when selecting a soil water monitoring system has been developed by ITRC (2019).
Given a soil with the following characteristics, calculate the depth to which 4 inches of infiltrated water would penetrate.
| Layer | Depth (in) | θfc | θv |
|---|---|---|---|
| 1 | 0-12 | 0.34 | 0.20 |
| 2 | 12-30 | 0.40 | 0.33 |
| 3 | 30+ | 0.30 | 0.24 |
Solution
Using Equation 2.13:
\(SWD_1 = (0.34-0.20)12\text{ in} = 1.7\text{ in}\)
\(SWD_2 = (0.40-0.33)18\text{ in} = 1.3\text{ in}\)
1.7 in + 1.3 in = 3.0 in is required to fill the first two layers
The remaining water is:
\(4.0 \text{ in} – 3.0 \text{ in} = 1.0 \text{ in}\)
To find the depth penetrated in the third layer (L3) use the same equation, but solve for L3 when SWD3 = 1.0 in:
\(L_3=\dfrac{1.0}{(0.30-0.24)}=16.7\text{ in}\)
The depth from the surface penetrated by a 4-inch application is then 46.7 in:
12 in + 18 in + 16.7 in = 46.7 in
From the information in Example 2.4, calculate the depth of water that was lost by deep percolation if the depth of the crop root zone is 36 inches.
Solution
46.7 in – 36 in = 10.7 in d = (0.30 – 0.24) 10.7 in = 0.6 in