6.1: Introduction
- Page ID
- 44411
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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 Chapter 4 we discussed plant water use (ET). In Chapter 2 we discussed how the soil-plant root zone serves as a reservoir. But how should we manage this reservoir? The ET demand of a crop is supplied from three sources of water: (1) rainfall that occurs during the growing season, (2) precipitation that was stored in the plant root zone during the dormant or off season and (3) irrigation. For example, in central Nebraska, the growing season ET for corn averages about 26 inches per year. Approximately 12 inches of this water would come from growing season rainfall, 4 inches from the stored soil water, and the balance 10 inches from irrigation (net). Good managers of irrigation water strive to maximize the use of precipitation while minimizing deep percolation. Proper irrigation scheduling will help reach these goals. Irrigation scheduling includes determining how often to apply water and how much water to apply. Irrigation scheduling is imperative for good water management. In this chapter we discuss irrigation scheduling and how system efficiency, available water capacity, plant root zone, and evapotranspiration affect the frequency and amount of water application, i.e., we build on what was covered in Chapters 2, 3, and 4. In practice irrigation scheduling is often based on the irrigator’s personal experience, plant appearance, watching the neighbor, or just simply irrigating whenever water is available. Sometimes the irrigation controller is set and never adjusted for rainfall or changes in ET. As water resources become limited and as more concern is raised about the effects of irrigation on water quality, the need for improved methods for scheduling will increase. Three general approaches or philosophies for scheduling irrigations are:
• maintain soil water content within desired limits,
• use plant status indicators to signal the need for water, and
• irrigate according to calendar date or other fixed schedule.
The soil water approach uses the plant root zone as a reservoir for storing and releasing water, as we discussed in Chapter 2. This reservoir can be managed using irrigation scheduling. The soil water status can be determined by checkbook accounting or by direct measurement of soil water. Checkbook accounting uses estimates of ET, rainfall, and irrigation to calculate the soil water level. Plant status indicators range from simply observing the stage of plant development to more sophisticated methods, such as measuring plant leaf temperature and plant water potential. Fixed schedules are often associated with irrigation water supply districts that lack the flexibility to deliver water on demand. No one scheduling philosophy is correct in itself. The emphasis in this chapter is on managing the soil water reservoir, but it also includes a short discussion on plant status indicators. It is likely that in the future a combination of soil water maintenance and plant status will be the most appropriate choice.