6.2: Plant Response to ET and Soil Water
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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 relationship between crop yield and transpiration and ET is illustrated in Figure 6.1. As illustrated in Figure 6.1b, once soil water evaporation is satisfied, there is a linear increase in yield as evapotranspiration increases until maximum yield is reached. In this book most of the discussion relating to crop or forage production will center on managing water for maximum, or near maximum, yield. More advanced books and papers discuss deficit or limited irrigation, where yield is reduced because less irrigation water is applied than necessary to meet full crop water requirements (English et al., 1990, Trout et al., 2020). With deficit irrigation, ET is less than the crop ET necessary for maximum yield. Figure 6.2 shows the relationship between growth or yield and fr (fraction of available soil water remaining). If soil moisture is maintained above certain limits, maximum, or near maximum, yield is achieved. The minimum fraction of available water remaining that should occur to avoid plant stress and a yield reduction is the critical fraction remaining (frc) a term presented in Chapter 4. When the available soil water is maintained equal to or above frc, maximum yields are attainable because the plants are able to extract adequate water from the soil.
Figure 6.1. Relationship between yield, T, and ET.
Figure 6.2. Relationship between available water remaining and yield (adopted from Stegman, 1983).
| Crop Group | 0.08 Maximum ET (in/day) | 0.12 Maximum ET (in/day) | 0.16 Maximum ET (in/day) | 0.20 Maximum ET (in/day) | 0.24 Maximum ET (in/day) | 0.28 Maximum ET (in/day) | 0.31 Maximum ET (in/day) | 0.35 Maximum ET (in/day) | 0.39 Maximum ET (in/day) |
|---|---|---|---|---|---|---|---|---|---|
| 1: onion, pepper, potato | 0.50[a] | 0.43 | 0.35 | 0.30 | 0.25 | 0.23 | 0.20 | 0.20 | 0.18 |
| 2: banana, cabbage, pea, tomato | 0.68 | 0.58 | 0.45 | 0.40 | 0.35 | 0.33 | 0.28 | 0.25 | 0.23 |
| 3: alfalfa, bean, citrus, groundnut, pineapple, sunflower, watermelon, wheat | 0.80 | 0.70 | 0.60 | 0.5 | 0.45 | 0.43 | 0.38 | 0.35 | 0.30 |
| 4: cotton, sorghum, olive, grape, safflower, corn, soybean, sugarbeet, tobacco | 0.88 | 0.80 | 0.70 | 0.60 | 0.55 | 0.50 | 0.45 |
0.43 |
0.40 |
| [a] fdc to Maintain Maximum Evapotranspiration Rates | |||||||||
The frc is related to fdc, the maximum allowable fraction depletion of the available soil moisture, by the following equation:
fdc = 1 – frc (6.1)
fdc is dependent on the plant species and genotype and on weather conditions. Weather influences the maximum ET each day. According to Doorenbos and Kassam (1979), fdc ranges from 0.18 to 0.88, depending upon how plants respond to soil water deficits and on the maximum ET for a given day. Data for various conditions are given in Table 6.1. From the table you can see that for corn with a maximum evapotranspiration of 0.28 inches per day, fdc is 0.5 and frc is 0.50 (from Equation 6.1). A crop, such as onions, grown under the same environment or weather conditions, will have a fdc of about 0.23 (frc = 0.77). Thus, the criteria for management depends on the crop and the environmental conditions. If the weather is relatively cool (low ET), a high percentage of the soil water can be depleted before stress occurs. Conversely, on hot days (high ET), less soil water depletion is allowed before plants undergo stress.
A common level of fdc is 0.50. This is an average value and can be used where more appropriate data, such as that shown in Table 6.1, is not available.
The above discussion has implied that the management goal is to produce maximum (or near maximum) yield or biomass. This may not be the case for landscaping plants and turfgrass. With these plants the goal is satisfactory plant appearance and/or adequate functional quality. To maintain a high-quality golf green will require more water than is required to satisfy the needs of a low-maintenance utility turf.
The management objective must be defined for any irrigation scheduling procedure. Possible objectives include:
- maximum yield or biomass production,
- maximum economic return,
- functional value of the plants (e.g., an athletic field),
- aesthetic value (i.e., keeping the plants healthy), and
- maintaining plant life.