Measuring and Conserving Irrigation Water


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Measuring and Conserving Irrigation Water

  1. 1. Measuring and Conserving ATTRA Irrigation Water A Publication of ATTRA - National Sustainable Agriculture Information Service • 1-800-346-9140 • www.attra.ncat.orgBy Mike Morris and This publication describes how to find the net water application rate for any irrigation system. It fur-Vicki Lynne ther explains how to calculate the number of hours the system should be operated, describes severalNCAT Energy ways to measure flowing water in an open channel or pipeline, and offers suggestions for irrigatingSpecialists with limited water supplies.© 2006 NCATContents Introduction TIntroduction ..................... 1 his publicationFinding Net Water explains severalApplication in Inches ways to mea-per Set ................................ 1 sure flowing irriga-Measuring Flows ............ 6 tion water. Legions ofReferences ........................ 9 irrigators make crop-Further Resources .......... 9 watering decisions byAppendix: Finding guesswork, imitation offlow rate with abucket test ...................... 10 the neighbors, or “the way it’s always been done.” If this sounds familiar, this publica- tion is for you. A basic understanding of your system’s capac- NCAT photo ity to deliver water is a very powerful piece of knowledge, one that allows you to take track crop water use (evapotranspiration) a more scientific approach to the irrigation as the season goes by. Your local Natural process, achieve greater control, and begin Resources Conservation Service (NRCS), conserving water without compromising Extension, or soil and water conservation crop yields. First and foremost, all irriga- district office should be able to assist you. tors need to know their system’s net water application rate, measured in inches per Finding Net Water ApplicationATTRA—National Sustainable hour or inches per irrigation. in Inches per SetAgriculture Information Serviceis managed by the National Cen- Not covered in this publication are some Some sprinkler systems, and most sur-ter for Appropriate Technology equally basic skills essential to irrigation face irrigation systems, apply water in one(NCAT) and is funded under agrant from the United States management. All irrigators should know location for a period of time before beingDepartment of Agriculture’s general irrigation guidelines for the crops turned off or moved to another area of theRural Business-Cooperative Ser-vice. Visit the NCAT Web site they grow. All irrigators should know how field. This period is often called a set or set( to check soil moisture levels. (Please refer time. Other sprinkler systems, such as cen-html) for more informa-tion on our sustainable to the ATTRA publication Soil Moisture ter pivots and linear-move systems, moveagriculture projects. Monitoring.) Most irrigators should also more or less continuously. Their set time is
  2. 2. Table 1 shows attainable system effi- ciencies for some irrigation systems: the percentage of water that actually enters and remains in the root zone. These are only average values for well-managed and well-maintained systems. Soil texture will heavily influence how your system’s efficiency compares to the ranges below. In sit- uations where true application effi - ciency is hard to estimate, measured distribution uniformity (DU) may be the most accurate “system efficiency” value to use in the formulas and cal- culations below. DU measures how uniformly water infi ltrates into the soil in various parts of a field. Talk to your local NRCS office or soil and water conservation district for help measuring your DU. NCAT photo considered to be the period needed to cover the entire irrigated area. Table 1. Attainable Irrigation System Application Good irrigation management requires that Efficiencies you know how much water the irrigation system delivers to a crop’s roots over a given period of time. You can then adjust the System Type Efficiency (%) duration or frequency of your sets so the amount of water you put into the soil equals Surface Systems the amount that plants have used up. Level border 60-80 Furrow 60-80 Finding System Efficiency Surge 65-80 In any irrigation system, some water is Graded border 55-75 lost or fails (for a variety of reasons) to Corrugate 40-55 become available to the plant roots. For Wild Flood 25-40 example, water is lost to deep percolation, wind drift, runoff, and evaporation during Sprinkler Systems application. The term net water application Linear move 75-90 refers to the amount of water (in inches of Center pivot (low pressure) 75-90 depth) that your irrigation system actually Fixed solid set 70-85 delivers to the crop root zone during a set. Center pivot (high pressure) 65-80 To calculate net water application, you start Hand move or side roll laterals 60-75 with the gross amount of water applied and Traveling gun 60-70 multiply it by a system efficiency. Stationary gun 50-60 Micro-irrigation systems Gross water applied × system efficiency = net water applied Surface/subsurface drip 85-95 Micro spray or mist 85-90Page 2 ATTRA Measuring and Conserving Irrigation Water
  3. 3. A Shortcut Method for Most Figure 1. Spacings on Hand Move SprinklersSprinkler SystemsTo estimate net water application in inchesper set for most sprinkler systems (but notpivots), you can simply use the following twotables. Use Table 2 to convert nozzle sizeand pressure to gallons per minute (gpm).Once you’ve determined the flow rate ingpm, use Table 3 (along with the correctspacing for your system) to fi nd your sys-tem’s gross water application in inches perhour. Then multiply this number by your Table 2. Nozzle discharge (gpm)system efficiency and set duration to fi ndnet water application. Nozzle Nozzle Pressure, psi SizeFor example, consider a side roll system (inch) 30 40 50 60 70with new 9/64-inch nozzles and 40 pounds G 3/32 1.4 1.7 1.9 2.0 2.1per square inch (psi) operating pressure, ood irriga- 1/8 2.6 3.0 3.3 3.5 3.8with 40-foot by 40-foot sprinkler spacing tion man-(see Figure 1), and an 11-hour set time 9/64 3.3 3.7 4.2 4.5 4.9 agementoperating at 65 percent system efficiency. 5/32 3.9 4.5 5.0 5.4 5.8 requires that you 11/64 4.7 5.4 6.0 6.6 7.1From Table 2, find the 9/64-inch nozzle on know how much 3/16 5.5 6.3 7.0 7.7 8.3the left and read across to the figure under water the irrigation40 psi. The number is 3.7 gpm. 13/64 6.4 7.4 8.2 9.0 9.7 7/32 7.4 8.6 9.6 0.5 11.3 system delivers to aThen, using Table 3, fi nd the 40-foot by crop’s roots over a40-foot spacing on the left and read across Caution: The values in Table 2 are based on new given period of the 3 gpm and 4 gpm columns. Since 3.7 nozzles. Flow from worn nozzles can vary significantly from these values.gpm is a little more than halfway between,estimate the gross water application at0.22 inches per hour. Multiply 0.22 by 11hours—the set duration—and by 0.65—the You can estimate net water application insystem efficiency percentage. inches per set for any irrigation system if you know the system’s flow rate, irrigated Net water application: 0.22 × 11 × 0.65 area, and system efficiency. The calculation = 1.6 inches per set. is described in the following pages.Table 3. Water Application — Inches per Hour Sprinkler gpm/Sprinkler Spacing 2 3 4 5 6 7 8 9 10 11 12 15 18 20 25 30 x 30 0.21 0.32 0.43 30 x 50 0.25 0.32 0.38 0.44 0.51 0.57 0.64 0.70 0.76 40 x 40 0.18 0.24 0.30 0.36 0.42 0.48 0.54 40 x 60 0.20 0.24 0.28 0.32 0.36 0.40 0.44 0.48 0.60 0.72 0.80 50 x 60 0.19 0.22 0.26 0.29 0.32 0.35 0.39 0.48 0.58 0.64 60 x 60 0.21 0.24 0.27 0.29 0.32 0.40 0.48 0.53 0.67 60 x 80 0.20 0.22 0.24 0.30 0.36 0.40 ATTRA Page 3
  4. 4. Finding Flow Rate use the total design gpm of your system for a flow estimate. Be aware, however, that It is sometimes difficult to estimate flow design numbers do not take into account rates for surface irrigation systems, but increased flow (sometimes significant) due several methods are possible. to nozzle wear and pressure variations. • D itch or pipel i ne: Mea su re Divide the design gpm by the number f low rates with one of the f low of operating sprinklers on your sprinkler measuring devices described later system to fi nd the average gpm per sprin- in this publication. kler. For pivots, use the design gpm for • Furrow systems: Measure f low the entire pivot. with one of the portable measur- A more accurate way to find the flow rate for ing devices available. Or catch the your sprinkler system—especially if it’s an flow to a single furrow in a bucket older system—is to conduct a simple bucket of known capacity, and measure the test. (See Appendix.) time it takes to fi ll the bucket. If you know the total flow into a furrow system, you can divide this amount Finding Irrigated Area by the number of furrows to find the For most surface irrigation systems, the irri- flow rate into each furrow. gated area is simply the entire area of the field. For furrow systems, you can use the • Siphon tubes: Look up the f low length and spacing of the furrows to esti- rate in a siphon tube head-discharge mate irrigated area. (See example below.) chart. For most sprinkler systems, fi nd the area To calculate flow rate for most sprinkler in square feet watered by one sprinkler, systems, you’ll need to know the aver- usually described as the distance, in feet, age gpm per sprinkler. For center pivots, between sprinklers on the line multiplied by flow varies along the length of lateral pipe, the distance, in feet, between mainline riser and you’ll need to know the gpm for the valves. Typical areas are 30 feet by 50 feet entire pivot. or 40 feet by 60 feet. (See Figure 1 for an If your sprinkler system is relatively new example.) For pivots, use the entire swept and the nozzles have little wear, you can area in acres and multiply by 43,560. (The conversion of acres to square feet.) Calculating Net Water Application per Set To calculate net water application in inches per set for any irrigation system, use the following general formula: Net water application (inches) = set time (hours) x flow rate (gpm) x 96.3 x system efficiency irrigated area (sq. ft.) Note: 96.3 is a conversion factor. NCAT photoPage 4 ATTRA Measuring and Conserving Irrigation Water
  5. 5. Wheel line, hand line, end tow: Use Wild flood: Use total area floodedthe average flow rate (gpm) for one nozzle. and total flow. Example: Example: 12-hour set, 8 gpm per sprinkler, 40’x 60’ Seven 24-hour days, 800 gpm flow, spacing, 65% system efficiency 40-acre field, 20% system efficiency 12 x 8 x 96.3 x 0.65 =2.5 inches net water 7 × 24 × 800 × 96.3 × 0.20 =1.5 inches net 40 x 60 application per set 40 x 43,560 water applicationCenter pivot system: Use the total Graded furrows: Use furrow lengthpivot flow. and spacing. Example: Example: 11-hour set, 10 gpm flow per furrow, 50-hour rotation, 900 gpm, 130-acre field 660-foot-long × 3-foot-wide furrows, (= 5,662,800 ft²), 75% system efficiency 50% system efficiency 50 x 900 x 96.3 x .75 =0.6 inches net water 11 × 10 × 96.3 × 0.50 =2.7 inches net 5,662,800 application per set 3 × 660 water application Note: The wheel line example above uses the fl ow for one nozzle divided by the area covered by one nozzle. The pivot formula uses the flow for the entire pivot divided by the area covered by the entire pivot. The result is the same—inches per set.Stationary big gun sprinkler: Useflow per sprinkler. Example: 10-hour set, 78 gpm, 120’x 120’ spacing, 50% system efficiency 10 x 78 x 96.3 x 0.50 =2.6 inches net water application per set 120 x 120Traveling big gun sprinkler: Usethe following formula: Net water gpm x 1.6 x efficiency (%) application = SxW where W = width between travel lanes in feet, and S = travel speed in feet per minute (fpm). Example: 300 feet between travel lanes, 0.4 fpm travel speed, 400 gpm, 60% system efficiency 400 x 1.6 x 0.6 =3.2 inches net water 0.4 X 300 application NCAT ATTRA Page 5
  6. 6. Determining the Correct typically include one or more staff gages (also spelled gauges), installed either on Set Time the structure itself or in an adjacent still- Use the same basic formula above to deter- ing well, to determine water depth. (A staff mine how long it takes to apply a desired gage is simply a vertical staff—resembling amount of water by inserting the amount an ordinary yardstick—marked with num- of water you want to apply for “net water bers that indicate water depth.) Using a application.” table for the specific size and type of struc- ture, you look up the measured depth and Set time hours = convert it to a flow rate. More expensive, net water application (inches) x irrigated area (sq. ft.) but more accurate, electronic measuring flow rate (gpm) x 96.3 x system efficiency devices are now available to offer continu- ous flow measurement and recording. Example: Wheel line, 8 gpm per sprinkler, Most open channel flow measuring devices you want to apply 1.2 inches, work only under limited conditions. A 40’x 60’ spacing, 65% system efficiency. structure suitable for a river or large canalM ost open will not be the best choice for a small ditch. 1.2 x 40 x 60 = 5.8 hours. Only a few of the more common methods to channel 8 x 96.3 x .65 Round up to 6 hours measure flows on smaller canals, ditches, flow mea- and farm turnouts are considered here. Asuring devices work comprehensive guide is the U.S. Bureau ofonly under limited For surface irrigation systems, it may be Reclamation’s Water Measurement Manual,conditions. easier to use the following extremely handy available on the Internet at formula: pmts/hydraulics_lab/pubs/wmm/index.htm. To ensure accuracy, the measuring site Set time hours = must meet a number of conditions: gross water application (inches) x area irrigated (acres) • The ditch or canal must have a shal- flow rate (cfs) low grade with a relatively straight Surface Irrigation Example: upstream segment and uniform Graded borders, 1.2 cfs flow, 10 acre field, cross-section, little turbulence, and desired gross water application 1.5 inches. quiet flow. Correct set time (hours)= 1.5 x 10 = 12.5 hours • Weirs require more slope than 1.2 flumes or submerged orifices. • The location must not cause sedi- ment loading, debris buildup, or flooding of surrounding areas. Measuring Flows In selecting a water measurement struc- You can measure f lowing water in an ture, consider cost, accuracy, ease of mea- open channel or pipeline using one of surement, construction and maintenance the methods below. You can also mea- requirements, and state or local water laws sure water with a simple bucket test at the and regulations. point where sprinklers apply it to the field Weirs are easy to construct, install, and (See Appendix). use, but require enough ditch slope so that water can fall freely from the structure to Measuring Open Channel Flow the downstream water surface. In the case Open channel methods generally rely on of rectangular and trapezoidal weirs, water a structure such as a weir, flume, or orifice flows through a sharp-edged rectangular installed in the channel. These structures or trapezoidal notch. In the case of v-notchPage 6 ATTRA Measuring and Conserving Irrigation Water
  7. 7. weirs, water flows through a sharp-edged Ramp flumes (also known as modified broad-(usually) 90 degree-angled notch. This crested weirs) are accurate, cost less to buildweir is especially good at handling a wide than most other devices, and are simplerrange of flows. to construct. Metal Strip 90¡ Center Line 3H Side View Downstream End Ditch Bottom Figure 4. Ramp FlumeFigure 2. V-Notch Weir Submerged orifices are often used where ditch slope is insufficient for weirs. TheyFlumes are more complex structures than generally cost less than weirs and can fit intoweirs, and include a constricted throat sec- limited spaces, but are susceptible to trashtion that requires careful construction and build-up. Water flowing through an orificeinstallation. Flumes are used where ditch is discharged below the downstream waterand canal grades are relatively flat. They surface. For thiscan still be relatively accurate even when dev ice to besubmerged. accurate, it must be submerged.Parshall flumes—one of the most common H1 The meter gate, H2types—require only about a quarter of a type of sub-the ditch grade needed for weirs and can merged orifice,accommodate a wide range of flows. Cut- can be used forthroat flumes are a “throatless” variation on farm turnouts.the Parshall flume, resulting in simplified The gate canconstruction. be used to mea- sure flow, closed to shut off flow, or positioned at various settings to reduce or Figure 5. Submerged Orifice increase flow. Top View Choosing, installing, and using weirs, flumes, and submerged orifices is not dif- ficult. In order to get accurate results, though, you need to install these devices properly, maintain them regularly, use proper measuring techniques, and use the right calibration curves and tables. Your Side View local NRCS, Extension, or soil and waterFigure 3. Parshall Flume conservation district office may be able to assist ATTRA Page 7
  8. 8. Measuring Pipeline Flow • Don’t over-irrigate. Learn the water- holding capacity of your soils, their Flow in a pipeline can be measured very allowable depletion, and the effective accurately if the correct measuring device is root depth and critical growth stages installed and used properly. These devices of your crops. need periodic calibration and maintenance, and water must be relatively (in some cases, • Focus irrigation on critical growth very) clean. Accurate measurement requires stages. Depending on the crop, full pipe flow. you’ll usually see one of two types of responses to drought stress:Related ATTRA Pipeline flow is measured by either intrusivePublications devices (located inside the pipe or inserted 1. Seed crops, cereals, and oilseeds are most sensitive to drought through the pipe wall) or external devices.Drought Resistant Soil stress during flowering or seed Intrusive flow meters include Venturi, noz- formation, and relatively insen-Drought Resource zle, and orifice plate meters that measureGuide sitive during early vegetative flow through a constriction within the pipe. growth. Irrigate enough at theEnergy Saving Tips None have moving parts. They require onset of seed formation to carryfor Irrigators little maintenance in clean water and are the crop through seed fill.The Montana Irriga- installed directly in the pipeline.tor’s Pocket Guide 2. Perennial crops grown pri- Propeller meters use a multi-blade propeller marily for forage, and some rootProtecting Water positioned inside the pipe. Propeller meters crops, are relatively insensi-Quality on Organic can pass some debris, but even moderate tive to moderate drought stressFarms amounts can foul the blades. for short periods throughoutSoil Moisture Monitor- Pitot tubes are inserted into the side of a the growing season. They caning: Low-Cost Tools recover from stress periods withand Methods pipe. They require drilling a hole through the pipe (allowing the insertion of the tube), little reduction in yield. FocusSustainable Irrigation making them less convenient and common on irrigating during periods of(PowerPoint than propeller meters. maximum growth.presentation) Non-intrusive (external) flow meters send • Irrigate early in the season. Fill theSustainable Soil ultrasonic or acoustic waves through a pipe root zone to field capacity before hotManagement and take very accurate measurements of weather starts. Doppler shift or transit time to calculate • Leave room in the soil for precipita- flow rate. These devices are clamped onto tion. Crop residue and cover crops the outside of the pipe wall (nothing is phys- help capture snow and rain and ically inserted into the pipe) which makes reduce evaporation. them extremely quick and convenient to • Aim for optimum rather than max- use. Several types of ultrasonic meters are imum yield (i.e., the greatest yield currently on the mar- with the least input). ket. They are costly, in the range of $1,500 to • Plant drought-tolerant crops or quick- $10,000, although prices maturing crops that require most of are dropping, and some their water early in the season. training is required for • Reduce the amount of land you irri- accurate measurements. gate and use the saved water on the remainder, or reduce the amount of water you apply over the whole irri- Irrigating with gated area. Limited Water • Irrigate every other furrow, switch- Supplies ing at each irrigation. You’ll still get When water supplies water to one side of each row, gener- NCAT photo are short: ally using far less water.Page 8 ATTRA Measuring and Conserving Irrigation Water
  9. 9. References Production Services/Distribution Kansas State UniversityNCAT Resources 26 Umberger Hall Manhattan, KS 66506-3404Drought Introduction. 2005. By Barbara Bellows. 785-532-5830PowerPoint presentation. 22 p. An introduction to droughts, impacts on soils, crops, Selection and Use of Water Meters for Irrigation and animals with an overview of management prac- Water Measurement. 2003. By Melissa C. Baum, tices to lessen the impacts of drought. Michael D. Dukes, and Dorota Z. Haman. University of Florida, Institute of Food and Agricultural Sciences,Drought Resistant Soil. 2002. By Gainesville, FL.Preston Sullivan. ATTRA Publication, 169. 7 p. Descriptions, use, maintenance, and comparison of Explains how organic matter can dramatically various flow-measuring devices. increase the soil’s water storage capacity and how ground cover increases the water infiltration rate while How Good Is Your Water Measurement? 1999. By lowering soil water evaporation. Both practices reduce Robert Hill. University of Utah Cooperative the severity of drought and the need for irrigation. Extension Service, Utah State University, Logan, UT. Resource Guide. 2005. Open channel and pipeline devices, typical prices, andBy Barbara Bellows. 29 p. sources of equipment. Phone 435-797-1000 to a hard copy. A comprehensive list of text and Web-base resources, including links to climate monitoring information, drought-tolerant plant lists, and drought disaster relief Further Resources programs. State-specific drought information and U.S. Bureau of Reclamation. 2001. Water Mea- drought programs are provided at the end of this surement Manual. U.S. Department of the Interior, document. Washington, DC. 317 p.The Montana Irrigator’s Pocket Guide. 2003. By USDA-Natural Resources Conservation Service. 1997.Mike Morris, Vicki Lynne, Nancy Matheson, and Al NRCS Irrigation Guide. Natural Resources Conser-Kurki. National Center for Appropriate Technology, vation Service, Washington, DC. 702 p.Butte, Montana. 161 p. A take-to-the-field reference to help irrigators save 652.html energy, water, and money, it includes guidelines for water management, equipment maintenance, and handy conversions and formulas. Get a free printed Web Sites copy by calling 800-346-9140 (toll-free). Irrigation Engineering Publications From the University of Nebraska Institute ofSustainable Irrigation. 2004. By Agriculture and Natural Resources.Barbara Bellows. PowerPoint presentation. 46 p. Explains sustainable irrigation practices and methods Dozens of publications on irrigation management and for protecting soil and water quality. hardware.Other Publications Irrigation Training and Research CenterIrrigation Water Measurement as From the BioResource and Agricultural Engineer-a Management Tool. 2002. By Danny H. Rogers, ing Department at Cal Poly State University, San LuisGary Clark, and Mahbub Alam. Kansas State Univer- Obispo.sity Agricultural Experiment Station and Cooperative www.itrc.orgExtension Service, Manhattan, KS. Extensive collection of publications, databases, and links. Tensiometer installation, use, and troubleshooting. Request a hard copy ATTRA Page 9
  10. 10. Water Conserve – Water Conservation Portal For pivots:A project of Ecological Internet, Inc. Step 1. Measure the flow of one sprinkler in each of nozzle diameters along the pivot using the method A water conservation portal and Internet search tool described for hand move, side roll, or linear move that provides access to reviewed water conservation systems above. news and information. Includes news stories, discussion groups, and links. Seconds to Average gpm/ # of sprinklers Total gpm fill bucket sprinkler in each set in each setWaterightFrom the Center for Irrigation Technology at California 300 ÷ sec = gpm x = gpmState University, Fresno. 300 ÷ sec = gpm x = A “multi-function, educational resource for irrigation 300 ÷ sec = gpm x = gpm water management.” Includes an energy use/cost Total Flow = gpm calculator, guidelines for estimating fuel require- ments, options for reducing energy use and costs, and discussions of various other energy-related topics. Step 2. Estimate flow from the end gun using end gun pressure and nozzle diameter from the table below.Appendix: Finding Flow Rate = gpmwith a Bucket TestFor hand move, side roll, or linear move systems:Measure the flow of one sprinkler per lateral, Table 4. Estimated End Gun Flow in GPMsituated on relatively level ground. The selected sprinklershould be about one-third of the way down the length Diameter of end gun nozzle (inches)of the lateral from the mainline. Use a hose to direct PSI ½ ¾ 1 1½ 2the flow into a five-gallon bucket. Using a stopwatch, 10 23.6 53.2 94.4 212 378estimate the time it takes, in seconds, to fi ll the bucket.For greater accuracy, take more than one reading per 20 33.4 75.3 134 300 534sprinkler and average the times. Repeat for the other 30 40.9 92.2 164 368 654sprinklers on other laterals. 40 47.2 106 189 425 755 50 52.8 119 211 485 845 Number of seconds to fill bucket = seconds Avg gpm/sprinkler = 300 ÷ the number of seconds Step 3. Add the results from Step 1 and Step 2 above to calculate total flow for pivot. 300 ÷ seconds = gpm Total flow per hand line or wheel line = gpm = Avg gpm/sprinkler x # of sprinklers = gpmPage 10 ATTRA Measuring and Conserving Irrigation Water
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  12. 12. Measuring and Conserving Irrigation Water By Mike Morris and Vicki Lynne NCAT Energy Specialists © 2006 NCAT Paul Driscoll, Editor Amy Smith, Production This publication is available on the Web at: or IP 280 Slot 280 Version 040306Page 12 ATTRA