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MEASURING FLOW in an IRRIGATION DITCH Because You Can’T Control What You Don’T Measure

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MEASURING FLOW in an IRRIGATION DITCH Because You Can’T Control What You Don’T Measure MEASURING FLOW IN AN IRRIGATION DITCH Because you can’t control what you don’t measure Caleb Carter point every second. This is equivalent considerations for determining which ncreased demand for limited water to approximately: type is best. Iamounts is making a contentious • 450 gallons per minute (gpm). Some general guidelines to ob- topic even more so in Wyoming and • 1 acre-inch per hour (1 inch of tain accurate results include: across the West. Using water more water covering 1 acre). • The ditch or canal must have a efficiently is more important than • 2 acre-feet per day (24 hours) (2 shallow grade with a relatively ever. feet of water covering 1 acre). straight segment upstream and uniform cross-section with little Most irrigation water in Wyoming Open ditch flows are typically turbulence. is delivered through canals and measured using a weir, flume, or a ditches. Knowing how much water is submerged orifice installed in a chan- • Weirs require more slope than flowing in a ditch can help determine nel. Water flows over a notch (weir), flumes or submerged orifices. if there is enough water and when to through a channel (flume), or through • The location must not cause ex- turn it off. a submerged hole (submerged cessive sediment loading, debris buildup, or flooding of surround- Measuring flow orifice) of predetermined size, and a depth reading is taken. Flows are de- ing areas. Rate of flow in an irrigation ditch termined using a table or graph. Each is measured in cubic feet per second has advantages and disadvantages Weirs (cfs), corresponding to a cubic foot Weirs are simplest to install and and specific conditions in which it will volume of water (1-foot wide, 1-foot use. They can provide very accurate operate correctly. Cost, ease of mea- high, and 1-foot long) passing a given flow measurements when properly surement, and maintenance are other 16 BARNYARDS & BACKYARDS designed, installed, and maintained. capacity: a flume with a 1-foot There are square, trapezoidal, and throat will measure from 1/3 cfs v-notch weirs, with the v-notch being to 10 cfs. especially good at handling a wide • Can be made from wood, metal, range of flows. or concrete, but they must be Flow is measured using a staff fabricated to precise dimensions gauge (see lower left), placed in the and properly installed to measure channel far enough above the struc- irrigation water accurately. ture so it is unaffected by the curve The trapezoidal flume is another of the water over the weir. The depth style. This structure can be easy to of the water is measured where the install, as its shape conforms well to water surface touches the gauge. size. Flumes can provide accurate the shape of the ditch. Other advan- Weirs do have limitations. They measurement with less drop needed tages include: require enough ditch slope so water between the upstream and down- • Water is not slowed as much, can flow freely over the structure to stream water surface than weirs. which results in very little back- the downstream water surface, and Other advantages include: water and less scouring of a pool, or stilling area, above to slow • Can carry sand/silt through the earthen ditch around the water velocity. Both can be an issue the flume (so less opportunity structure. with silt-laden water or in an earthen for build-up) with no effect on • Does not clog easily. ditch, as the pool may silt up and the accuracy. drop may cause excessive erosion • Flume will operate under greater below the weir. • Requires about one-fourth the submergence (when the down- loss of head (see description be- stream water level is high enough Flumes low*) compared to a weir, mean- to inhibit free flow through the Parshall flumes (below right) are ing the water won’t be slowed as channel) without having to make the most common type of structure much. necessary corrections to deter- used and can provide very accurate • Can obtain accurate measure- mine actual discharge. flow measurements. They have a ments with varying water veloc- A disadvantage is a small change channel of predetermined shape and ity or flow rates. Wide range of in head (*difference in elevation Cipolletti weir Parshall flume SPRING 2017 17 between the water above the orifice Submerged orifices typically cost and the water below the orifice), or less than weirs and can fit into small- reading on the measuring gauge, can er spaces. A meter gate, a type of cause drastic changes to flow mea- submerged orifice, can be used to let surements. Accurate depth readings water from a main ditch into a field, are critical to obtain accurate flow as the gate can be opened/closed or measurements. partially opened to set a specific flow. A flume should be in a straight A disadvantage of the submerged section of the ditch where flow is un- orifice is the potential for clogging if impeded in its approach to the struc- water is carrying lots of silt or debris. ture. Flow should be smooth with no Most irrigation systems work turbulence. The measuring device is Submerged orifice most efficiently when run at correct mounted to the side of the flume, at capacity. Accurately measuring water the top and bottom of the structure. • An orifice with smooth, flowing to the field can assure maxi- sharp edges and of accurate mum efficiency and help determine Submerged orifice dimensions. how long to run an irrigation set, if A submerged orifice measures • A provision for measuring the you are meeting crop water needs, or water flowing through an opening head. if there is a leak or other issue. of predetermined size. The opening To measure head, two stakes are Ensure all structures are installed must be fully submerged to be an ac- set, with the tops at the same el- and maintained properly, including curate measure of water flow in the evation – one above the orifice, one cleaning out any debris buildup each ditch. This works well with very little below. Measurements are made from spring. Take time to learn accurate change in head, or in flat ditches. the top of each stake to the water sur- management and measurement tech- The essential components of a sub- face. Stakes should be placed where niques for any structures in use. This merged orifice include: the water is calm. The head, in feet, is will help you to make the best use • A smooth, vertical face of suffi- then compared to an appropriate chart of one of Wyoming’s most valuable cient size. for that orifice to determine flow. resources. Installation, measurement information Proper installation and measure- ment using structures described here is beyond the scope of this article. Please refer to the University of Wyoming Extension publica- tion Irrigation Water Measurement: Irrigation Ditches and Pipelines, Bulletin 538R that can purchased online for $7.50 at http://bit.ly/wa- termeasure or check with your local extension office. Parshall flume in action There is no device yet (that we know of) that measures the flow of information from someone but if there were, Caleb Carter would move the needle high. A University of Wyoming Extension educator, he is based in Goshen County and serves southeast Wyoming. He can be reached at (307) 532-2436 or [email protected]. 18 BARNYARDS & BACKYARDS.
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