FAQs - IRRIGATION WATER DELIVERY & CONVEYANCE SYSTEMS
Q: What are some common devices used to measure flow?
A: Different methods to measure water flow for closed pipelines include propeller meters, orifice, venture, or differential pressure meters, and magnetic flux meters. Devices used to measure flow in open channels include weirs and flumes, stage discharge rating tables, and area/point velocity measurements. Ultrasonic devices, or travel time methods, may be used for both open and closed channels. Consideration in using open and closed meter as well as indirect methods requires attention to installation at each site. Improper installation can lead to inaccurate measurements. Important to note: volumetric measurements do not directly save water but they do provide information about costs associated with water use and efficiency of irrigation systems.
Q: What does the term 'reservoir storage efficiency' mean?
A: Reservoir storage efficiency is the ratio of the volume of irrigation water available from an irrigation reservoir to the volume of water delivered to the reservoir. This ratio is normally less than 1.0 because of seepage, evaporation, and transpiration losses.
Q: What are the principal factors that influence reservoir storage efficiency?
A: The principal factors that influence reservoir storage efficiency are seepage losses, evaporation, and transpiration losses. The amount of seepage loss will strongly depend on the properties of the materials from which the reservoir is constructed. Seepage losses may be reduced by lining reservoirs with impermeable soils (typically clays) or man made liners such as plastic sheets. Metal, plastic, or fiberglass tanks may be used as reservoirs to eliminate seepage losses, but the cost of tanks is often prohibitive for the volumes of water required for irrigation. Evaporation losses can be eliminated by covering the water surfaces, but this is not practical except for tanks or small reservoirs. Evaporation losses from field scale reservoirs can be reduced by designing reservoirs with smaller surface areas and greater depths. Transpiration losses from a reservoir occur as a result of vegetative growth in and around the reservoir. These losses can be reduced by preventing or minimizing growth in and near the reservoir. Vegetative growth along the shoreline can be reduced by minimizing shallow water areas. Some vegetation, especially grasses, will normally be required to stabilize the soil embankments and prevent sediment transport into reservoirs.
Q: What does the term 'water conveyance efficiency' mean?
A: Water conveyance efficiency is the ratio of the volume of water delivered for irrigation to the volume of water placed in the conveyance system. This ratio is normally less than 1.0 for open channel conveyance systems, but it may be approximately 1.0 for pipeline conveyance systems.
Q: What are the principal factors that influence water conveyance efficiency?
A: The principal factors that influence water conveyance efficiency of open channel conveyance systems, i.e., ditches, canals, streams, are seepage, evaporation, and transpiration. These losses can be reduced by using lined channels and controlling vegetative growth. Some evaporation losses will be unavoidable. Seepage and other losses are avoided in pipelined conveyance systems because leakage is minimal from well-designed and well-managed pipelines.
Q: Are there effective approaches or methods for addressing and/or reducing seepage and conveyance losses from open ditches?
A: Most strategies for reducing seepage losses include installing a liner in existing ditches. The lining materials include geotextiles, synthetic membranes, compacted earth, various putties, and concrete. Among the most popular liners are geotextiles, synthetic membranes, and concrete. The use of liners has met with mixed success. One study indicated that, while performance was improved from lining two secondary canals, it was not enough to justify costs. Other studies have indicated that lining does increase overall system efficiency.
Q: I need to make a presentation to our irrigation district water users about water conveyance components. Where can I find user-friendly information?
A: There's a nice power point presentation prepared by the NRCS from 2005 on Irrigation Water Conveyance at http://irrigationtoolbox.com/Powerpoints/Chapter2/IrrigationWaterConveyance.ppt. The object of the presentation is to present an overview of the various features of irrigation water conveyance systems and how these features may affect on-farm water management.
Q: What is the 'conceptual' framework within which irrigation water delivery systems should be evaluated?
A: There is reasonable agreement in the scientific literature that irrigation water delivery systems should be evaluated on the dimensions of adequacy, timeliness, and equity. Water quality may be an additional important dimension in some systems. Indicators of these three characteristics – adequacy, timeliness, and equity, should provide sufficient information to answer three specific questions: 1) to what extent does the quantity of water provided suffice for the growth needs of the crops that are being irrigated; 2) does the timing of water deliveries match the growth needs of the crops and the expectations of the irrigator; and 3) is the water being distributed fairly among the multiple users of the system?
Q: What are some the challenges of assessing 'adequacy' of delivery of irrigation water to individual water users?
A: A significant challenge to assessing 'adequacy' of delivery of irrigation water to individuals irrigators is the diversity of irrigation circumstances among various irrigators – including specific types of irrigation systems, crops being irrigated, crop growth stage, soil types and soil variability. Other challenges include how to assess and account for ‘effective’ rainfall, seepage and percolation losses on-farm, and needs for salt-leaching among differing fields and soils with differing irrigation histories and water table conditions.
Q: Numerous reports and references can be found in the science literature explaining the components of irrigation efficiency, conveyance assessments and sources of water losses in irrigation systems. I'm looking for an 'easy-to-read' summary of various efficiencies and sources of water losses in irrigation systems.
A: Kansas State University Research and Extension Irrigation Engineers have prepared a concise summary of 'Efficiencies and water losses of irrigation systems'. This six-page summary includes concise definitions, explanations, and illustrations of some of the principal contributors to irrigation water losses. The fact sheet is available at: http://cursos.puc.cl/unimit_agl_014-1/almacen/1222620929_lgurovic_sec4_pos0.pdf