12.2.5: Uniformity Issues
- Page ID
- 44646
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Poor uniformity is evidenced by plant water stress in areas receiving less water. The problem can be due to pressure distribution, but other factors are possible. One issue is stretching the spacing between laterals or sprinklers along the lateral. When the spacing is excessive for prevailing wind conditions the overlap is inadequate to provide uniformity. Poor uniformity may also arise from worn sprinklers and nozzles. The sprinkler bearing may be worn causing the sprinkler to rotate slowly or stick in locations during rotation. Bearings can be replaced but it is often best to replace the entire sprinkler with this amount of wear. Wear of brass straight-bore nozzles can be evaluated by matching the diameter to a drill bit of that size. If the nozzles are worn significantly, they can be replaced very economically; however, sprinklers should be checked to ensure that they should not also be replaced.
The diameter of coverage of some sprinklers can be increased by inserting straightening vanes. The straightening vane shown in Figure 11.2 decreases turbulence and increases the diameter of coverage which may provide the coverage needed for acceptable uniformity. The sprinkler jet with a vane does not breakup as quickly as sprinklers without vanes. This also provides more throw and helps fight wind effects; however, vanes may lead to poorer distributions about an individual sprinkler resulting from a doughnut pattern. Straightening vanes are inexpensive and easily installed, so vanes can be evaluated for a few sprinklers. If vanes do not improve performance, then other alternatives should be considered.
The spacing between lateral sets is often limited by the length of mainline pipe. Aluminum pipe is commonly available in either 20-, 30- or 40-ft lengths. This dictates the width of sets and ultimately the uniformity. Operators can adapt to this problem by offsetting the lateral each set. Suppose that mainline joints are 30 ft long and two joints are used for a set width of 60 feet. For odd numbered irrigations, the lateral could be placed at locations of 30, 90, 150, etc., feet from the field boundary. The lateral is then placed between these setting for even numbered irrigations or at locations of 0, 60, 120, 180, etc. feet from the field boundary. Offsets place laterals halfway between the previous set and the cumulative uniformity of water application generally improves. Offsetting may cause issues when the first and/or last set is along the field boundary or where lanes are required for tow-line systems in tall crops.