2.6: Determining Available Water Capacity
- Page ID
- 44331
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)The values of θfc and θwp of a soil used to calculate AWC, can be determined by field and laboratory methods. Discussion of the various techniques to measure these variables is beyond the scope of this book. The reader should refer to Bruce and Luxmore (1986) and Klute (1986) and references therein for detailed information. Relatively simple experiments for approximating these variables are explained below.
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. After the rate of drainage slows in 1 to 3 days, the water content remains nearly constant. This is field capacity. At this time, the soil should be sampled for water content. As a rule of thumb, 1 day of drainage will generally be adequate for sandy soils, 2 days for silt loam soils, and 3 days for silty clay loam soils. A simpler field method of determining field capacity is to take soil samples at intervals following a thorough irrigation or rain in a fallow field. When θv remains nearly constant the value is θfc.
The water content at WP can be determined by measurements in areas where the available soil water has been exhausted. In this case, an area that experiences severe water stress would be a good location to take a soil sample. The sample could be analyzed for θv at that time to determine the θwp throughout the soil profile.
If field capacity is known, θwp can be estimated by subtracting AWC from θfc. Suppose the θfc is 0.30 in/in and the AWC is 0.18 in/in. Wilting point, θwp is then 0.30 minus 0.18 or 0.12 in/in. Often, AWC is tabulated in soil survey reports and textbooks.
Generally in irrigation management, the same value of θwp is used throughout the root zone for calculating water requirements. At the same time, we use root zones shallower than what is explored by plant roots. This creates a margin of safety, and to some extent, accounts for the fact that the permanent wilting point in the upper portion of the root zone is often higher than in the lower portion. This simplification makes water balance calculations much easier and has worked well in scheduling and designing irrigation systems.