Irrigation in the Pacific Northwest

FAQs for Agricultural Irrigators

How much water does my crop need?
When do I turn my water on?
How long do I leave it on?
What is evapotranspiration (ET)? How can I find it?
Water's cheap. Why should I care about irrigation management?
How much will it cost if I don’t do irrigation scheduling correctly?
What does it mean to winterize my irrigation system and how do I do it?
How much is too much to pay a consultant to do irrigation scheduling for me?
What are the environmental and health consequences of poor irrigation management?
What's irrigation water really worth?
I'm considering putting in a new irrigation system. What are my options? Which should I use?
Which is the most efficient irrigation system?
What is uniformity? Why is it important?
How much water am I putting on?
Why measure soil moisture or plant water status?
It’s a water short year and I don’t have enough water. What should I do?


Select another FAQ or tutorial.


How much water does my crop need?


Daily water use by a crop is very dependent upon the weather. Plants will use a lot more water during the long hot days of summer than they will during the cool, cloudy and humid days of spring or fall. The water use of a potato crop in one area of the state may be vastly different from the water use in another part of the state because of differences in the local climates between the two areas. Crop water use also depends on the developmental stage of the crop. The average monthly water use of most economically important crops in Washington State can be found in the Washington Irrigation Guide. These numbers can be used to design irrigation systems and to create very rudimentary irrigation schedules. Crop water use of a reference crop (usually full-grown alfalfa, or grass) can be estimated using a mathematical equation. This equation uses temperature, relative humidity, wind speed, day length, cloudiness, heat flux from the soil, altitude, and latitude of the reference site. This data is often available from weather stations located around the state. This reference crop water use is then multiplied by a crop coefficient that takes into account crop differences and the growth stage of that crop. The Washington State government funds a network of weather stations (AgWeatherNet) in various locations throughout the state that collect the necessary data and archives it in a database. Crop water use is calculated from the weather data from each of these stations, and crop coefficients are applied for many key crops and are available on the Web free of cost.


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When do I turn my water on?


To achieve maximum yields, irrigate before the crop sees any water stress. Different crops have different drought tolerances. Some crops are very sensitive to water stress (leaf vegetables) while others are less sensitive (millet, sorghum). Timing irrigations by using water stress signals in the crops means that you will always be too late. Yield potential was already sacrificed. A better method is to monitor the soil water content using soil moisture sensors and/or using a data-based irrigation scheduling method.


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How long do I leave it on?


Leave the water on long enough to replace the soil water deficit. In other words you want to fill the soil in the plant’s root zone back up to field capacity. If you are using some sort of data-based irrigation scheduling method, or if you are using a soil moisture sensor that can show soil water content (as opposed to soil water potential) then you will have a good idea of how much water to apply. It is vitally important to know what your application rate is. You can calculate your water application rate in a general sense, or for sprinklers, or for drip irrigation. If you know how much water needs to be applied then divide the amount of water to apply by the application rate to get the irrigation system run time. For example if you have a soil water deficit of 1.5 inches in the root zone (3 ft deep) of a corn field and the application rate is 0.12 inches per hour then leave the water on for 1.5 / 0.12 = 12.5 hours.

Most people who change water want to do it on a regular basis. For example, I personally am not interested in changing my water every 7.5 hours. It is better for me to change the water at 8:00 AM and 8:00 PM, or every 12 hours. When I calculate run time, I will use 11.5 hours because I have the water off for 30 minutes while I move the sets. So, in the above example I won’t completely refill my root zone. I will apply 11.5 x 0.12 = 1.38 inches. This is OK as long as I get back with the water before the crop sees any stress again.


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What is evapotranspiration (ET)? How can I find it?


Evapotranspiration is the combination of the evaporation of water from the soil surface and the transpiration of water through the plant’s leaves. Combined they are called evapotranspiration. This is crop water use. ET can be estimated from weather data. AgWeatherNet has weather stations located around the state to measure the weather parameters necessary to calculate ET. The math is done for you online.


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Water's cheap. Why should I care about irrigation management?


Because poor management can lead to weak crop yields, damage the environment, make messes on our property, incite plant diseases, wash out valuable nutrients in the soil, and because pumping water is not free.

Water is still the best fertilizer. Nothing has so clear of an effect on crop yield and quality as water management. Poor irrigation management costs more in opportunities lost than most people realize.

Over irrigation washes out chemicals and fertilizers that we paid good money for off of the fields and into streams and water bodies where they cause problems for fish, wildlife, and others who use the water. Non-point source water quality problems are closely tied to water quantity problems. Water is the primary vehicle that carries pollutants into our streams and water bodies.


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How much will it cost if I don’t do irrigation scheduling correctly?


Poor irrigation scheduling means you applied too much water or too little water. You can actually do both in the same season if your timing is poor. This results in lost yield, inferior product quality and/or wasted water and energy. Lost yield opportunity and lost premiums for higher quality crops will hurt the pocketbook the most. The dollar amount of how much your yield loss is crop and market dependent. If you pay energy costs to pump your irrigation water then you probably paid a higher energy bill than you needed to. You may also leach expensive fertilizers and pesticides out of your soil negating the hoped for effects of these products on your fields. Irrigation scheduling is worth doing right!


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What does it mean to winterize my irrigation system, and how do I do it?


When water freezes, it expands. Although it doesn’t expand very much, it expands with an absolutely huge amount of force. It will break concrete, burst steel pipes and break just about anything else that it is contained in. The less flexible the container (pipe, valve, fitting), the more likely it is to break. If don’t enjoy doing lots of digging to replace lots of piping and valves and anything else that water might be in, you should definitely consider doing what you can to remove any water that is inside of pipes that will freeze. Frost will penetrate the ground different depths depending on the season and the climate. If the pipeline is below the frost line, then you can leave water in the pipes. If not, you should remove the water. This is winterizing...removing water from irrigation lines and bringing in anything else that you don’t want to leave out all winter. Removing water can be done by opening a valve in a low spot (a place that the water will drain to by gravity) and let the water out on the ground, or some people blow the lines out with high capacity air compressors.


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How much is too much to pay a consultant to do irrigation scheduling for me?


How good are you at doing it yourself? Do you have the time to do it right? How much will it cost you if you don’t do it right? How sensitive is the crop yield and quality to water stress or overirrigation? The answers to these questions motivate many growers to hire consultants to do irrigation scheduling for high value crops such as potatoes, onions, tree fruit, and grapes.


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What are the environmental and health consequences of poor irrigation management?


Water is the primary vehicle that moves fertilizers and pesticides off of fields where they are both needed and desirable into groundwater and streams where they are not. The best way to control the flow of these non-point-source pollutants off of fields is good irrigation management.


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What's irrigation water really worth?


More than you pay. In a dry climate like we have here in eastern Washington, water rights will increase your property values by as much as 5 to 10 times what they are worth without water rights. Many studies have been done to determine the cost effectiveness of saving money by applying less water to crops. It very rarely pans out. In other words the water returns much more value to growers than they pay for the water. Water is our earth’s most precious resource. It is a gift that not only makes life possible, it makes life enjoyable.


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I'm considering putting in a new irrigation system. What are my options? Which should I use?


Like the answer to so many questions… it depends. It depends in part on the following: your water source and when and how much you can get, the value of the crop being grown, the degree of irrigation control required to profitably produce the crop, the soils, the field slope and topography, existing laws and restrictions, and probably most importantly the system cost. To get a good idea of what is economical and practical in your area, looking at what your neighbors are doing is a good start.

In general there are three different classes of irrigation: surface, sprinkle, and drip.

Surface irrigation is defined as any irrigation system that uses the surface of the soil to transport the water across the field. The most common surface irrigation method in Washington is furrow (rill) irrigation.

Sprinkle irrigation uses water that flows through pipes under pressure and exits through a nozzle to the atmosphere at high velocities and travels through the air to be distributed across the soil surface. The most common sprinkle irrigation systems in Washington are center pivots, wheel-lines, hand-lines, and big guns.

Drip irrigation water also travels through pipes or tubes under pressure, but the water is delivered near each plant and exits through emitters at very low flow rates into the soil. Drip lines can be buried, laid right on the ground, or suspended from trellis wires as is commonly done with perennial crops.


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Which is the most efficient irrigation system?


As a general rule, buried drip. Irrigation efficiency is defined as the amount of water stored in the root zone of the crop for later use divided by the amount of water that flowed onto the field. Typically surface irrigation is the least efficient, followed by sprinkle, and drip and trickle irrigation is the most efficient. However, efficiency greatly depends on management and characteristics of a particular system.


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What is uniformity? Why is it important?


Uniformity is the ability of an irrigation system to apply water evenly across the soil surface. If an irrigation system has poor uniformity some plants won’t get enough water, some will get too much, and excessive water will leach below the root zone and be lost. In order to adequately irrigate all plants in a field with an irrigation system that has poor uniformity, excess water must be applied. This is wasteful and expensive.


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How much water am I putting on?


Good question! To manage irrigation effectively, it is very important to know your application rate! This is how much water is being applied in a given time period (inches/hour). If you know some details about your irrigation system, you can calculate the water application for drip irrigation, and for sprinklers.

The most accurate way of determining this application rate is to measure it. For sprinkler irrigation this can be done by putting a straight sided can (catch can) underneath the sprinkler for a given time period and measuring the depth of application in the can. Since no irrigation system applies water perfectly uniformly, it is often a good idea to put out several cans in different areas where the sprinkler throws water and take an average.

There are calculators for various irrigation systems including center pivots, drip, hand-line and wheel line, and solid set sprinklers.


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Why measure soil moisture or plant water status?


Why do you have a gas gauge on your car? The best way to know how much water is left in the soil for your plants to use without running out and causing stress and resulting yield reductions is to measure it.


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It’s a water short year and I don’t have enough water. What should I do?


When you don’t have enough water to fully irrigate the crops you would like to grow, you have several options: (1) improve your efficiency, (2) plant crops that use less water, (3) take the yield hit on all of your crops, or (4) don’t water some acreage at all in order to fully irrigate your remaining acreage.

Obviously the most attractive choice is to improve your efficiency. The cheapest way to do this is by careful and watchful irrigation management. Measure your soil moisture. Only irrigate when necessary and put on just the right amount such that no water is wasted. Irrigation efficiencies can also be improved by converting to a more efficient irrigation system. This almost always involves large capital investments. There is cost-share money available from the NRCS if converting from surface irrigation to sprinkle or drip.

Some crops require more water than others. In a water short year, you can change your plants from a heavy water user like corn, potatoes or alfalfa to a crop that uses less water such as wheat, safflower, sorghum, or rapeseed. Alternatively you could plant a crop that has a much shorter growing season like peas, carrots, green onions, spinach or some similar crop. These different crops require different equipment sets and the costs of either retooling, or hiring this work done would have to be considered.

If you are growing a perennial crop such as tree fruit, grapes, or berries, changing crops is generally not an option. Not watering some acreage in favor of fully watering other acreage is also not a good option as those trees, vines, or bushes may die and a valuable investment will be lost. However, if you are growing an annual crop and you are fairly confident at the beginning of the season that there will not be enough water throughout the year, it is usually more economical to plant less acreage and fully water the acreage that you plant. This is because plants use a certain amount of water in creating the roots, stems and leaves required to capture water, nutrients and sunlight and use them to produce the desired fruit or grain. If less acreage is planted, then the water used to otherwise build up the basis of this water and sunlight processing system may be better put to use creating larger yields in existing fields. It is also usually economical because the machinery and energy (fuel) costs of harvesting and transporting 50 acres of wheat at 100 bushels/acre (5,000 bushels) is less than harvesting and transporting 100 acres of wheat at 50 bushels/acre (5,000 bushels). Even though the same amount of grain is hauled to the elevator for both scenarios, in the latter the combine covered twice the distance using more fuel and taking more wear and tear.


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