2012 22.3 The Quest for Perfect Irrigation: 3,000 Years and Counting
Growing up on a 2000-acre diversified potato farm in western Washington, I always had the job of irrigation. Running two reel big guns in Skagit County around the clock was not glamorous. Getting stuck, setting up and tearing down irrigation pipe, and trying to cover way too much ground with too little infrastructure and manpower created a new name for irrigation forever to remain “irritation” in this little boy’s mind. It wasn’t until much later in life that my fascination with the subject of irrigation evolved. One of my first opportunities on the job as an Agriculture and Natural Resources Specialist with Extension was to travel to Cal Poly San Luis Obispo and learn more about irrigation (thanks to sponsorship from the Washington Red Raspberry Commission). On the trip home I looked out on the sea of reel big guns scattered throughout Skagit County and thought to myself, “There has got to be a better way.”
A bit of irrigation history can shed light on the current systems in use today and options for moving forward. Perennial irrigation was practiced some 3,000 years ago in the Mesopotamian plain. Crops were regularly watered throughout the growing season by coaxing water through a matrix of small channels formed in the fields. Ever since then agriculturalists have been working on improving and expanding the system. The first improvement was to bring water to the field by ditch for flood and furrow irrigation. This worked well for 2,900 years until it was realized that a lot of water was being lost to evaporation. Next, gated pipe was invented: the savior to all irrigation needs. The end.
Wouldn’t that be nice? No, the “Quest for Perfect Irrigation” wouldn’t be complete unless we talked about the invention of the sprinkler. Imagine a mobile water supply system that could be set up in a field to supply crops water. It would work regardless of field elevation gain or losses and it would be mobile. Does that sound too good to be true? It was. Hand lines are still used in the U.S. but they have sent generation of farm kids scurrying for their textbooks with fingers crossed that they would be accepted to college so they would never have to move hand line again. Eventually, metal wheels were added to the hand line and a wheel line was born. Wheel lines are great, except it takes a lot of them to irrigate a field. It is no fun tramping to the center of the field to turn on the engine to move them and, really, who wants to move anything? Hence circles and linear moves were born. These are great irrigation systems as well, however, circles irrigate in a circle and linear moves have efficiencies in the 85% range.
SMALLER IS BETTER
On the progression of efficiency, smaller and more is better than big and few. A typical big gun in Skagit County propels 65% of the water from the gun to the plant. Sprinklers (hand lines, wheel lines, circles, linear moves, and boom type carts) tend to run in the 85% area, leaving room for improvement. Micro sprinklers and drip irrigation use tiny sprinklers to supply water to the plant via plastic tubes. These systems can supply water to the plant with upwards of 95% efficiency. The down side is the sprinkler surfaces (emitters) are so small that they easily plug, so it becomes necessary to filter the water before it passes through the emitter surface. This necessity has started the fourth progression in the irrigation industry: water filtration. To make water clean enough for drip iron bacteria, sand media, and disk filters have been born. All of these filters are somewhat effective if used according to the manufacturer’s recommendations, but this system is a little less than perfect.
After learning more about the differences between irrigation systems, readers may have or want to switch to drip irrigation with the proper filtration system to ensure the longevity of the system. Progression toward efficiency of moving water is not the end of the quest for “perfect” irrigation. An equally important part of irrigation is water scheduling. Just having an efficient irrigation system does not ensure efficient water usage. A farmer needs to know why, how much, and when they need to water – a more scientific approach to irrigation.
When I was a child, my father purchased water-scheduling equipment (tensiometers). I installed and read the tensiometers for the first week or so, but they seemed not to be working correctly so I went back to the kick-the-dirt more-is-better method of irrigation. Luckily for me there is a statute of limitations on poor irrigation management. However, as the world population increases and access to natural resources decreases there is going to be more pressure to ensure proper use of water resources. There are several ways to determine when to turn on irrigation. The ones I will address here are kicking the dirt, soil probe, tensiometers, water marks, pencil and paper, spreadsheet, and irrigation scheduler mobile methods.
It wasn’t that long ago when a farmer’s only method of irrigation scheduling was to go out and kick the soil around. If they could see a darker soil color due to moisture in the first few inches kicked, then they knew to hold off on irrigating for a few days if the plants weren’t stressed. The problem with this system is its accuracy. Based on what I know now, the irrigation I did in my childhood was feast or famine. We typically waited for an 80-degree day to check the field and then applied an inch of water per week until the crop was on its last leg. Then we would scratch our heads over why the crop didn’t do well.
EASY DRINKING – TESTING THE SOIL
It’s helpful for good irrigation practices to think of soil as a sponge that gets placed in water and soaks up all of the liquid until it is complete full. This is the maximum soil available water holding capacity. Each soil and soil type can hold a different amount of water.
Now imagine a plant pulling water out of the soil. Eventually it will become more difficult for the plant to pull water from the soil. This point at which the plant experiences stress is referred to as the Management Allowable Depletion (MAD). An irrigator does not want to drop below this level (unless they are growing grapes or seed; then it is a good thing), as it is when plants have to struggle for water. Past the MAD point, plants will experience stress and eventually the soil will have no more water available for plant uptake. This is referred to as the permanent wilting point, the place where plants decide to give up the ghost. Even though there may be water in the soil, the plant cannot access that moisture.
Another way to test the soil is to use a soil probe. This tool is used to determine how deep the irrigation water is traveling into the soil horizon. They are handy to have, especially with drip irrigation system, to show how large of a wetting pattern is being emitted by the drip system. Alternatively, a dug out cross-section of the irrigated area can be read for wetting pattern.
Tensiometers are also great for indicating when a crop needs to be irrigated. These tools measures how hard a plant’s root system works to uptake water. When installed and serviced correctly, a tensiometer does a great job of letting the irrigator know when it is a good time to irrigate. However, not all farmers have time to service tensiometers during the irrigation season.
Water marks are a nice alternative to a tensiometer. They are placed in the soil and have a water mark that uses electrical conduction and resistance to measure how much moisture is in the soil. The irrigator places sensors in the soil at varying heights according to plant needs. Then the sensors are plugged into a data logger which records these readings in centi-bars from 0-100 ; zero being really wet, 100 extremely dry. In the quest for perfect irrigation, unfortunately, water marks fall too far from the mark. For me they tend to either work or not, and far too often they don’t perform correctly. There are precise manufacturer instructions on making soil slurries (otherwise known as mud) when setting the watermarks. Regardless of how many times I seem to perform this task, about ¼ of my water marks do not function correctly—this is far from perfect.
Some may choose to approach the problem of irrigation timing as if they were tracking the balance in a bank account. The checkbook method allows the irrigator to write out how much water they have received in the form of rain and irrigation and how much water their plants are losing to evapotransporation (ET). There are several of these irrigation checkbook methods available online and all work quite nicely if the user is dedicated to keeping close records.
THE COMPUTER AGE
There have been multiple attempts at electronic spreadsheet irrigation scheduling tools since the advent of computers. From the first version created on DOS to the current Windows 7 systems, there has always been a missing feature: the ability to access relevant and recent weather data in the field. One computer-based system is the Kansched irrigation scheduler which I began using three years ago when I began researching the use of drip irrigation in potatoes.
Gary Clark, Danny Rogers, Mahbub Alam, and others from Kansas State developed Kansched. And it was a major innovation at the time. The user first enters the crop coefficients and rooting depth, and then the daily water received through rain and irrigation. A graph including field capacity, MAD, and permanent wilting point is generated based on these details. All the irrigator has to do is enter the daily rain/irrigation and make sure the crop tracks between field capacity and MAD. This system works well for individuals who like to update their information on a computer daily, but the last time I checked, the majority of farmers farmed out of their pickup trucks more than they do their office.
Dr. Troy R. Peters and his associates at the WSU Irrigated Agriculture Research and Extension Center created a new and improved version of the spreadsheet irrigation scheduling tool referred to as Irrigation Scheduler Mobile. Irrigation Scheduler Mobile takes all of the good qualities from the Kansched model and adds in weather data from any of the 135 weather stations that Washington State University operates in the state. The beauty of this system is that it can be accessed from an iPhone or Android phone. It is almost perfect—but still requires users to enter in irrigation information.
Since the first person began bringing water to their field on the Mesopotamian plain, humans have been trying to invent the perfect irrigation tool. We have progressed from bringing water to the fields in earthen ditches clear up to micro-irrigation that delivers the plant a precise volume of water with the assistance of computer aided programming. I will be the first to admit the industry has come a long way over the past 3,000 years, but if we consider a small boy on the farm day-dreaming of the perfect irrigation system that would work automatically and supply water 100% efficiently to his plants, we still have a long way to go.
Tags: Irrigated, Irrigation, Perennial Irrigation, Sprinkler, Tensiometers, Water, Wheel Lines