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How to Use an Irrigation Calculator for a High Tunnel


I’m Joe Hannan with Iowa State University Extension and Outreach. I’m a commercial horticulture field specialist working with fruit and vegetable producers across central and western Iowa. One of my main emphasis is working with farmers to produce crops and high tunnels. And today I’m going to show you how to use the drip irrigation calculator to determine how much and when to irrigate. We start by going to iowa produce dot-org to get the calculator. We’re going to go on the production tab and just scroll down until we get to the drip irrigation calculator. This is an excel file that we’re going to want to download to our computer. Once we’ve got the calculator – download it. We’ve got a couple different things going on here. We’ve got a basic drip irrigation calculator and we’ve got an advanced drip irrigation calculator. For anybody out of the state of Iowa you’re going to want to use the basic drip irrigation calculator as it uses general soil types. Where if you’re in the state of Iowa you can actually go in and select your actual soil type that your high tunnel sitting on and get more specific runtimes or irrigation amount of water to apply. So starting with the basic drip irrigation calculator with our high tunnel. We want to know how long is our high tunnel. So typically we’ve got a 30 x 70 or 30 x 90 foot long high tunnel. So we’re going to say our rows are 85 feet long. We’re going to be growing tomatoes on a 30-foot wide high tunnels. We’re going to put in six rows and these are not quite at full rooting depth. We’re going to say the root zone is about 18 inches deep and we’re going to use a loam type soil. So the crop reading zone will see that I selected 18 inches. This is going to vary from crop to crop and growth stage crop. So when we got new transplants out in the field we’re going to be using that shallow root zone of about six inches as the plants grow and mature. Or we’re gonna adjust this root depth up to 12 inches 18 inches even up to 24 inches for very deep-rooted tomato or cucumber crops. And make sure that you go back through and adjust that root zone as a crop matures. So the calculator spits out a couple different pieces of information for us here. It pops up what the tensiometer trigger point or when we turn on that irrigation system. And so with the lonely type soil that’s 20-30 centi bars. Remember that you’re going to have two tensiometers for each of your irrigation zones. One at about halfway down the rroot zone and that’s your trigger potentiometer. This is the one that corresponds 20-30 centi bars. This is the one that you use to determine when to turn on that irrigation system. The bottom tensiometer the one at the bottom of the root zone is the one that tells you if you’re putting on enough water or not. Once the tensiometer reaches the system we turn on the irrigation system. We’re going to need to apply 474 gallons of water to that crop to irrigate those tomatoes. Now if we want to know how long to actually run that irrigation system, we also need to know what the emitter spacing is, what the emitter flow rate is. So here we’re going to use a drip tape that has an emitter spacing of 12 inches and we’re going to use an emitter flow rate of point 25 gallons per hour. So in order to apply that 474 gallons of water to our six rows of tomatoes in the high tunnel we need to run the irrigation system for three hours and 43 minutes. So the emitter spacing and the emitter flow rate those are the numbers that you get from the drip tape. So the tape can have 6 12 18 24 inch emitters facing and variable flow rate. You can have flow rates as well as .25 – .5 – .75. One gallon per hour. So look at your packaging to see what your actual interfacing flow rates are and plug those directly into these boxes here to get that runtime. Once we’ve got these gallons to apply and the hours and minutes for our irrigation system we want to go back through and actually make sure that this is the correct amount of water. So go back and watch that bottom tensiometer to make sure that it is constantly foot sitting at about two to three or four centi bars. If it gets too wet that reading is going to be zero centi bars and we need to kind of back off that irrigation runtime little bit. And if it’s too high for that bottom tensiometer starting to read 5 , 6 up towards 10 centi bars. Then we want to turn up that irrigation runtime a little bit so the calculator will get your close but you still need to kind of go out and do some fine-tuning based on your tensiometer readings. Those are the basic procedures for using the drip irrigation calculator. Again this is a good estimate for how much water and how much runtime you need to run that irrigation system. You’re going to need to go back out to the field and check that the actual runtime is correct to make small adjustments as the season goes on based on that bottom tensiometer. I frequently get asked what can I grow in high tunnel and the answer is you can grow anything in a high tunnel. High tunnels are perfect environments for growing plants.

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