6.4.2: Simplified Checkbook Method
- Page ID
- 44417
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)A major limitation to the checkbook accounting procedure has been the lack of reliable real time ET data. This has largely been overcome with the advent of automated weather station networks. Weather data from these stations are used to calculate crop ET on a continuous basis. Now with the easier availability of ET data, the biggest problem is estimation of effective irrigation depths. Water measurement is key. Once the water is measured, effective depths can be determined using the techniques described in Chapter 5.
The checkbook accounting procedure requires daily record keeping. This has slowed its acceptance. Computer software eliminates the need to manually perform the daily calculations, but keeping records is still necessary except when all sensors are electronic and data can be transmitted from remote locations directly to computers or hand-held smart devices.
One way that irrigators apply the ET data without daily recording is to simply irrigate when the effective depth has been consumed (Equation 6.9). For example, if ET is 0.25 inches per day and the effective depth is 0.75 inches per application, irrigation must be applied every 3 days. Thus, the water manager is reacting to the amount of water applied and ET. To adjust for rainfall, irrigation can be delayed in accordance with how long it will take to consume the rainfall. If a 0.5- inch rainfall occurs, the irrigation schedule should be delayed 2 days (assuming that you weren’t behind with irrigation before the rainfall occurred and all of the rainfall was effective in satisfying ET).
Another simple checkbook accounting approach is to adjust the effective depth of application according to the amount of ET and rainfall that has occurred over some pre-established time interval. Suppose weekly irrigations are desired. Then the ET and effective precipitation is summed for the time interval. The effective irrigation needed is then equal to the accumulated ET minus the accumulated effective precipitation or
\(d_e =\Sigma ET-\Sigma P_e\) (6.26)
A drawback to the two “simpler” checkbook accounting approaches discussed above is that the water applications lag behind the time of the water use. On soils with low AWC and/or for plants with shallow root zones, the lag may cause some water stress before water is applied.
The daily ET for a potato field and the effective precipitation (Pe) for a week period is given below. How much effective irrigation water is needed to make up the balance between ET and rainfall?Given:
| Day | ET (in) | Pe (in) |
| Sunday | 0.20 | -- |
| Monday | 0.30 | 0.50 |
| Tuesday | 0.15 | -- |
| Wednesday | 0.25 | 0.20 |
| Thursday | 0.20 | -- |
| Friday | 0.25 | -- |
| Saturday | 0.25 | -- |
| \(\text{Total }(\Sigma)\) | 1.60 | 0.70 |
Find: Effective irrigation required
Solution
\(d_e =\Sigma ET-\Sigma P_e\)
\(d_e = 1.60-0.70=0.90 \text{ in}\)