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  • NutraDrip Irrigation

Irrigation 101: When and How Much to Irrigate



Introduction 

Some of the frequent questions we get asked are  

  1. How do I know when to irrigate? 

  1. How much water do I put on when I do irrigate? 

In this post, we will give a high level overview of the answers to these questions, and give you some resources to help you navigate these questions on your farm.  

There are many different factors to consider when irrigating your crops; a few starting points to consider: understand what your system capabilities are, what your well or water supply is capable of, what your soil holding capabilities are, and what is the crop using.  


System capacity and Water Supply 

When sub surface drip systems are designed, most systems are designed around some kind of constraint with water supply; rarely does a grower have all the water they want. Ideally, an SDI (subsurface drip irrigation) system is designed to use 5 gallons per minute per acre. That will give us an application rate of 0.25 inches per day over the entire field. This is an ideal scenario. Why? Evapotranspiration, often abbreviated as ET, is the rate at which water is leaving your field. There are many factors that play into it, and the number changes every single day.



There are two components to ET: evaporation and transpiration. Evaporation is the water leaving the soil, and transpiration is moisture leaving the plants. Plants are similar to humans in that they “sweat” or bring liquid up through the roots into the plant, the moisture leaves the plant, and they transpire. This is a healthy process, as plants need to do this in order to photosynthesize and create energy. At the beginning of the season, you may be using 5/100ths of water every day just from evapotranspiration coming off the field. As the season progresses, the ET increases. The long term average ET is 0.25 of an inch. That is what makes the application of 0.25 an inch of water per day the sweet spot for irrigation systems. On hot, dry, windy days, the ET can get over 0.25 inches, even up to 0.5 inch per day.  

If we have an irrigation system that can replace what is leaving each day (around 0.25 inch), then we can wait until the field starts drying out, because we can keep up each day. If an irrigation system only has the capacity to apply two gallons per minute per acre, the grower can only apply 0.10 inch per day. This means they drip system isn’t able to supply what the plant needs most of the time, so the grower has to rely on the water holding capacity of the soil in the field, also called field capacity.  


Soil Capacity 

Think of your soil as a reservoir. Picture a 36 inch soil column; in that column, we will be able to hold anywhere from one inch per foot, up to 3-4 inches per foot, depending on the soil type. On the low end of that range, a field will only hold 3 inches of water; on the high end of that range, soil may hold 12 inches of water. This is important to note, because if you are a grower that has 12 inches of water holding capacity, and you can apply 0.25 inches of water each day, you have a lot of buffer room to manage your system. The closer you get to the 3 inch capacity in your soil, the lower your water capacity, the less wiggle room you have in managing your system.  

Imagine the soil capacity as a bucket; if you have a lower amount of water in that bucket, you need to keep it as full as possible at all times. If you get into two or three weeks with no rain coming, and your ET is 0.25-0.3 of an inch, the water line in that bucket will really start dropping and the crop will start showing stress.  

Resources  

Now that we have a basic understanding of when and how much to irrigate, there are a few resources that are extremely helpful in this decision making.  

One resource that can be very valuable is finding out what the ET is in your area. For example, the NRCS office out of Holdrege, NE sends out a daily text with ET and other information for the previous day. They also send out a newsletter that contains relevant information for those who are looking to manage their irrigation and crops. (More information for Nebraska: TriBasin District, CropWatch from UNL, or contact Curtis Scheele at the NRCS office in Holdrege if you wish to sign up for the newsletter at curtis.scheele@usda.gov). This varies widely based on the area you are in, so making sure the information is local is important. We would recommend reaching out to your local NRCS office to see what resources are available to you, either email newsletters, text, or other websites with reliable data.  

Two other tools that provide a lot of data that can help drive your decisions are Irriwatch (a vitual soil moisture tool) and soil moisture probes.  


Irriwatch

Irriwatch is a thermal based satellite that gives us actual ET every day. Each day between noon and 2 pm, a satellite flies over the field to take the temperature of the crop and calculates the ET every day. They also estimate the water holding capacity based on the radiance and temperature they get off the soil, and how well the plant is transpiring. They essentially use the plant as a sensor to validate or give a model information about how much water they think is in the soil. It’s a very low-cost option to look at your whole field. We at NutraDrip have had experience with it and feel it provides high value for the cost. You can read more about Irriwatch HERE. 


Soil Moisture Probes

Soil moisture probes are the second data set we encourage you to use. This means you have a sensor in your soil “bucket” and know exactly where that water level is every single day. There are many options out there. The one we are currently recommending is GroGuru. It is a more permanent probe that goes in the ground and stays there for seven years. You get seven years of continual data. This is helpful, as you can look on your phone on May 1st and see it is already dry. Maybe you need to start irrigating before your crop comes up? Do you need to fill up the subsoil, so you have that reservoir of water to pull from later in the season? We have more in depth information on monitoring your root zone HERE.  


Conclusion 

The answer to the big frequently asked questions “when and how much do I water?” lies in looking at your irrigation system design, the amount of water you can apply per hour, the capacity of your water supply, how much moisture your soil can hold, and how much water is being lost through evapotranspiration each day. There isn’t a “one size fits all” answer. If you look at your design, and it says you can apply 0.02 inches per hour, that means it needs to run for 10 hours to apply 0.2 inches of water for that zone or set of zones. Knowing this data, along with watching your ET and the soil moisture helps you decide the best schedule to effectively irrigate and making sure the crop has the best day it can every single day.

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