FAQ - Field Scale Irrigation Systems & Management

Q: What are the benefits of soil-moisture monitoring?
A: Soil moisture monitoring using soil probes, tensiometers, gypsum blocks, neutron access tubes or TDR helps the producer avoid over-applying or under-applying irrigation water during application and throughout the growing season. In order to monitor soil moisture, it is important to know soil type and soil texture to better understand available water capacity for the plant or crop and field capacity.

Q: When is the best time in the irrigation season to postpone or cancel a scheduled irrigation?
A: If it is necessary to forfeit, postpone, or cancel a scheduled irrigation, the best time for that to happen will be dictated by management objectives and knowledge of availability of water later in the irrigation season. From an agronomic perspective, the growth stages in which many crops are least sensitive to moisture stress or deficit irrigation are during emergence and early season growth and immediately before maturity. This is often particularly true for forage crops. For seed-producing crops, vegetable and fruit crops, the period of greatest sensitivity to deficit irrigation is likely to be between the middle and latter part of the season, i.e., just before maturity and harvest. Thus, for these crops, forfeit, postponement, or cancellation of a scheduled irrigation would be best early in the growing season. The other consideration that should be made is the availability of water during latter parts of the growing period. If it is known or predicted that water will be in short supply during the latter part of the growing season, then it may be best to irrigate as long as possible and allow circumstances to dictate when the skipped irrigation will occur.

Q: What are some of the factors which can influence field scale water use efficiency and which irrigation managers can have some control or influence over?
A: Factors which can influence field scale water use efficiency include: poor maintenance of irrigation systems, limited knowledge of water savings and conservation practices, lack of economic incentives to implement water saving technologies, lack of clear understanding of plant-soil water relations, and lack of user-friendly knowledge of plant water use and efficiency and impacts of irrigation management on water resources.

Q: What is the one field scale irrigation management practice which can have the most influence on improving water use efficiency in irrigated fields?
A: Numerous studies have shown that science and data-based irrigation scheduling, such as ET-based, soil water based scheduling, or water balance budgeting, is one of the single-most influential factors affecting field scale water use efficiency. Precision irrigation scheduling can significantly increase water use efficiency. However, adoption of such 'best management' practices is often dependent, to a large degree, on incentives, especially at the individual or farmer level. One study using a spreadsheet-based irrigation scheduling tool demonstrated the potential to facilitate up to 20% savings in annual irrigation water requirements in many cases as a result of irrigation scheduling. This can translate into an immediate energy savings and operational cost reduction for irrigators pumping water.

Q: What are some estimates of reasonable expectations of reducing irrigation water applications and improving water use efficiency through improved design, management and maintenance of irrigation systems?
A: One study in South Africa documented the potential to reduce irrigation water applied by up to 20% per year and to increase crop yields significantly by making improvements to design of irrigation systems and altering water allocation plans and arrangements to provide positive incentives for the adoption of state-of-the-art irrigation tools and guidelines. This type of yield response won't necessarily hold true for all crops, since some crops can have a very flat (non-significant) response to additional water.

Q: What are the principal factors which can affect sprinkler irrigation systems performance at the field level?
A: Factors which can affect irrigation system performance at the field level include: 1) the uniformity with which water is applied, the magnitude of the water applications relative to soil water storage capacity and crop stress thresholds, and the rate at which water is applied relative to soil infiltration characteristics; 2) the amount of water lost through wind drift to non-cultivated areas, evaporation from bare soil surfaces, and evaporation of water when it is sprayed from the irrigation system sprinkler heads; 3) runoff, and 4) management, both in terms of the level to which irrigation systems are operated and maintained and via the implementation or non-implementation of appropriate irrigation strategies (irrigation scheduling). Relatively speaking, wind drift and evaporation from irrigation systems represent 'marginal' losses which can be minimized by sprinkler package selection.

Q: Are there specific types of measurements that can and should be made to help evaluate irrigation system performance?
A: A suite of measurements can be completed to obtain an overall system performance evaluation. This suite of measurements is often referred to calculate an Irrigation Engineering Performance Indices (IEPIs). One of the most important measurements is either the Christiansen Uniformity Coefficient (CU) or the distribution uniformity (DU), which are performance indices that describe how uniformly or non-uniformly water is being applied to the field. The CU or DU of applied water can have significant effects on irrigation performance because even if the timing and average amount of water applications is well matched to crop water demand and soil water storage capacity, non-uniformity results in some areas receiving relatively higher water applications and other areas receiving relatively lower water applications. Excessive runoff and deep percolation losses are likely on the areas receiving the relatively higher water applications and reductions in crop yield can be expected on the areas receiving the relatively lower water applications. Depending on how well an area is drained, reductions in crop yields can also occur on the areas receiving excess water.

Q: What are some of the other measurements that should be completed, besides distribution uniformity, to assess field scale irrigation systems performance; and how can all of the results of measurements be evaluated collectively to evaluate system performance?
A: Numerous aspects of sprinkler irrigation system hardware should be checked, including the bowls of a pump through to the performance of emitters in the field. Measurements including operating pressures along the system, nozzle wear, emitter flow rates and power consumption are all aimed at ensuring that the system hardware is performing according to design specifications and accepted standards. If pressure regulators are being utilized, nozzle pressure needs to be taken to determine if the regulators are working correctly. Systems simulation models can be used to translate this type of data/information into associated impacts on crop yields and water use and then relative profit margin. It’s important to recognize that nozzle wear is extremely difficult to measure. Very small changes in the diameter of the nozzle can have major changes in output of the nozzle.

Q: Is it true that field scale irrigation systems management must include allowances for some water loss?
A: No irrigation system will apply water without some waste or losses. Thus, some water losses are expected and accepted in proper irrigation system design, installation, and management. However, excessive waste may be caused by poor irrigation system design, improper installation, poor management, and equipment failures. Waste may occur as non-uniform water applications, excessive applications, surface runoff or subsurface (lateral) flow from the irrigated area, canal seepage, percolation below the root zone, evaporation from the irrigation distribution system, leakage from defective pipe connections, or other losses. Some losses are intentionally calculated into the application. Leaching must occur in soils that have high salt contents. This leaching prevents the increase in salts that would render the land unusable.

Q: What is meant by the term 'irrigation efficiency'?
A: Seasonal irrigation efficiency is a measure of the effectiveness of an irrigation system to deliver water to afield, or the effectiveness of an irrigation system to increase crop yields. Irrigation efficiency may be expressed as the ratio of the volume of water used or available for use in crop production to the volume pumped or delivered for use. Likewise, irrigation efficiency may he expressed as the ratio of crop yield or increase in yield over non-irrigated production to the volume of irrigation water used. Irrigation efficiencies thus provide a basis for the comparison of irrigation systems from the standpoint of water beneficially used (or conversely, water wasted) and from the standpoint of yield per unit of water used.

Q: What are some of the factors which affect irrigation efficiency?
A: Irrigation efficiencies vary with the type of irrigation system and with other factors such as soil, crop, and climate characteristics, as well as with the level of maintenance and management of the irrigation system. The type of irrigation system used and the intended level of irrigation efficiency will partially depend on the availability and value of water for irrigation, as well as labor. Thus, economic factors influence the irrigation efficiency sought or obtained in a specific production system.

Q: What are some of the factors that influence efficiency of pressurized sprinkler irrigation systems?
A: Sprinkler irrigation systems can lose water due to evaporation and wind drift, although these losses are usually not the most significant losses. Some water is lost by interception on vegetation, soil, mulch, and other surfaces during irrigation. However, much of this intercepted water is not lost and can compensate for a portion of the plant transpiration by evaporating directly from the plant canopy and other surfaces, thus cooling the canopy and reducing transpiration. Application efficiencies will be reduced if water falls between widely spaced plants or outside the crop root zone, as in the cases of container nurseries or young citrus production systems, or water which is shed away from the crop root zone as in the case of plastic mulched bed production systems.

Q: I've heard it said that irrigation water use efficiency can be increased by irrigating at night. Is this true?
A: It is generally true that in many cases, irrigation water use efficiency can be increased by irrigating at night or during cool, cloudy, and windless conditions, but these gains are generally small. Benefit that can be gained from night-time irrigation depends on characteristics of crop production systems. And, there are exceptions to the case of improved irrigation water use efficiency from night-time irrigation. For example, some crops may suffer from increased disease due to night-time irrigation, while other crops may require irrigations more frequently than once per day or may require cooling by irrigation during peak water use periods of the day. Other issues related to night only irrigation relate to the capacity of the system. Since irrigation at night is less than 50% of the total hours, capacity must be doubled or more. This increase in capacity can cause issues with increased runoff.

Q: What are some of the factors that contribute to non-uniform irrigation water applications?
A: With sprinkler irrigation, non-uniform water application occurs when sprinklers are not properly selected or are not properly matched to the sprinkler spacing and operating pressure used. Non-uniformity also occurs if pressure losses within the irrigation system are excessive (due either to friction losses or elevation changes). These issues can be compensated for by the use of pressure regulators as well as correct sizing of pipes. Other causes of non-uniformity such as clogged nozzles or enlarged nozzles from abrasion by pumping sand or normal wear (nozzles life is estimated at 10,000 hours) also reduce application efficiencies. With flood or furrow irrigation, non-uniform water application occurs because of soil variability, lack of uniformity of water supply to individual furrows, and losses of water due to infiltration and evaporation as water moves across irrigated fields. Many of these factors are generally beyond the control of the individual irrigation operator. Neither with sprinkler systems nor flood and furrow systems is it possible to apply water with perfect uniformity because of friction losses, elevation changes, manufacturing variation in components, and other factors.

Q: What are some of the factors that should be considered in attempting to optimize furrow and flood irrigation systems at the field scale?
A: Water distribution in furrow-irrigated fields is governed by soil properties. Often the most significant challenge to evaluating the efficiency of furrow irrigation at the field scale level is either spatial or temporal variability, i.e., how soil properties and water deliveries vary throughout furrow and flood-irrigated fields and over time during a single irrigation event or throughout the irrigation season. Researchers have developed 'whole field' simulation models, such as IrriProbe, which utilizes estimates and measures of infiltration rates at various positions in irrigated furrows, to evaluate true irrigation performance taking into account the inter-furrow variability in infiltration and advancement rate across irrigated fields.

Q: What is 'surge' or 'surge-furrow' irrigation and how can this be used to improve field scale irrigation efficiency?
A: Surge or surge-furrow irrigation is a combination computerized x mechanized method of furrow irrigating which is used to improve the uniformity of water entering the soil down a row in a furrow irrigation system. Water is introduced to one area of the irrigated field for a certain or period of time duration, then switched to a different irrigated area, then returned to the original area. Surge valves automatically switch the irrigation water between lateral lines, on a timed sequence. Switching back and forth is continued until the entire length of the furrow is watered. By pulsing, or surging, the water advances down the furrow faster than it would with the constant flow in a conventional furrow irrigation system, thus improving the uniformity of application of water throughout each irrigated furrow. By decreasing the time needed to advance to the end of the furrow, deep percolation is reduced. This is particularly true in coarse-textured soils. Irrigators have reported the surge systems averaged between 20 percent and 30 percent reduction in water use per irrigation, depending on soil type and system flow rate.

Q: Are there other technologies besides surge irrigation, to improve field scale uniformity of furrow-irrigated systems?
A: A relatively new advance in irrigation water delivery and water spreading is 'lay flow' irrigation tubing. Computer programs have now been developed to calculate the needed gradient of the crown end of a field to match energy losses within the pipeline to equalize furrow flow streams. The program selects hole sizes to help make existing systems operate more efficiently. Uniform furrow flow streams result in water conservation (from 1 to 10 inches per acre per year), reduced potential of surface water contamination through reduced irrigation tail water (from 1 to 6 inches per acre per year), and increased yields.

Q: What are some of the 'secondary' benefits of optimizing field-scale irrigation water management?
A: Improving the efficiency of irrigation systems can reduce water loss due to deep percolation and runoff. Consequently, improving irrigation efficiency can reduce the amount of water and agricultural chemicals entering groundwater and surface draining systems.

Q: Water table management is currently being researched as a tool in the arsenal of tools to be used for field scale irrigation systems and their management. What does this refer to and how does it work?
A: Water table management involves management of water supply and drainage systems at the field level, whereby the water table can be lowered below the root zone during wet periods (conventional drainage), raised during dry periods (subirrigation), and maintained during transition (controlled drainage). Water supply is controlled by inlet structure management while water table level is controlled through management of inflow to field scale drainage structures. The design of drainage and water table management systems needs to be tailored to the soils, crops, and climatological conditions. Computer simulation models such as DRAINMOD, SWARTRE, and WATRCOM can be used to relate water management system design to soil and climatic properties.

Q: What factors affect overall irrigation system efficiency and how are these factors used to determine overall irrigation efficiency?
A: Factors which affect overall irrigation system efficiency include reservoir storage, water conveyance, and application efficiency. The overall irrigation efficiency is calculated by multiplying together the efficiencies of the components affecting overall efficiency. An example might help best illustrate this: for an irrigation system which is using water from an open reservoir with a 60% (0.60) storage efficiency, conveying it using an open channel from which 1/5 is lost in transit (0.80 or 80% conveyance efficiency), and which is using the flood method of water application, which is 50% (0.50)efficient, the overall irrigation system efficiency would be 0.60 x 0.80 x 0.50=0.24 or 24%. In a system like this, the water supply would need to be 4 times the crop irrigation requirement because only 24% of the water collected in the reservoir would be effectively used.

Q: What might be an optimistic expectation of the maximum overall irrigation system efficiency that could be achieved – under ideal conditions?
A: An irrigation system with a high efficiency might consist of water supplied by groundwater pumping, pumped water conveyed in a pipeline rather than being stored, and irrigation using a buried drip system. In this example, for instance, an irrigation system that pumps water from groundwater and conveys the water in a pipeline directly to the field irrigation system without leaks, and applies it with a drip type of microirrigation system which has an application efficiency of 85% would produce an irrigation efficiency of 1.00 x 1.00 x 0.85 = 0.85 or 85%. This type of system would only need to pump 18% (1.0/0.85 = 1.18 or 118%) more water than the crop irrigation requirement because losses did not occur in storage or conveyance, and the application efficiency was increased.

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