By: Kirk Williams, Lecturer-Texas Tech University
Drip irrigation systems are a critical part of vineyard infrastructure. Drip irrigation systems need maintenance to deliver water efficiently and uniformly over a long period of time. Where drip irrigation systems are used to deliver fertilizers and pesticides distribution uniformity becomes even more critical. A distribution uniformity target of at least 85% is critical to producing high quality grapes and achieving good water use efficiencies. Fall and the dormant season is a good time to examine your drip irrigation, measure distribution uniformity and see what maintenance or improvements need to be done.
Routine maintenance activities should include flushing drip lines and laterals to clear the system of contaminants. Flushing of the lateral lines and drip lines is especially critical if you have had to fix large leaks that occur once the water has gone through the filtration system. Severe leaks need to be fixed immediately but post-harvest time is a great time to fix all of the small leaks that may be occurring in your drip irrigation system. While flushing, pay attention to what kind of debris comes out of the hose ends. Is it silt, clay, organic or chemical precipitates? If you are having emitter clogging this debris could be a good tool for understanding what is causing the clogging.
Clogged or partially clogged emitters are the most serious problem in drip systems. Drip emitter output can be measured by placing containers under emitters and measuring the amount of water collected over a known time period.
Grape growers spend lots of time calibrating sprayers, fertility sampling, measuring grape ripeness but many spend little time measuring the performance of their drip irrigation system. One irrigation performance characteristic that can be measured and tracked over the life of an irrigation system is Distribution Uniformity(DU). Distribution Uniformity is a measure of the uniformity of irrigation water over an area. Lower Distribution Uniformity in a drip irrigation system can be caused by pressure losses, pressure variations or by partial plugging of emitters by physical, biological or chemical buildup. Pressure testing of the drip line via a Schrader valve or pitot tube is a good practice to incorporate prior to or during a DU test to understand your irrigation system operating pressures.
A DU Low Quarter value is calculated by dividing the average output of the lowest one quarter of the emitters sampled by the average output measured of all the emitters sampled. A DU Low Quarter test can be done by selecting forty drippers across an irrigation block. Select emitters that best represent an irrigation block and choose emitter locations where you would expect to find high or low pressures. Collect emitter output for 30 seconds. While various containers can be used to collect emitter output a 100 ml graduated cylinder will be needed to measure the relatively small amount of output. 63 milliliters per 60 seconds = 1 gallon per hour. The formula is found below.
Convert the value found to a percentage by multiplying by 100.
An acceptable DU value is between 85% and 95%. A DU value between 75% and 85% should be improved and a DU value lower than 75% needs to be improved. A system with a lower DU value is applying extra water through the emitters that are in the top 75% of output in order to supply the vineyard with water for adequate vine growth for emitters that are in the lowest 25%. The investment in time of measuring DU over a period of years can alert you to problems of partial clogging of emitters or pressure issues with your irrigation system. This gives you time to implement management activities before the problem gets worse. Some management strategies are discussed below.
Many areas of the country have issues with poor water quality. Some ions dissolved in water can lead to chemical clogging of emitters. Calcium carbonate (Lime or Scale) is one of the most common of these compounds. Calcium carbonate formation is favored with water pH of greater than 7.5 along with bicarbonate levels of 100 ppm or higher. Chemical and microbial oxidation or iron and manganese can cause clogging. Other causes of emitter clogging can be bacterial or algal growth as well as suspended solids. Testing irrigation water quality every five years is a good practice in understanding what is contained in your irrigation water.
Acidification may be necessary for irrigation water that tends to form chemical precipitates such as calcium carbonates. Depending on water quality, acidification can be constant, or it can be done occasionally to prevent chemical clogging of the drip emitters. Acids that can be used include sulfuric, hydrochloric, phosphoric or urea sulfuric acid. An alternative to acids are synthetic scale inhibitors which reduce scale formation by preventing precipitation reactions from occurring. The amount of acid needed to reduce the pH of the water can be calculated by titration using a pH meter. Analytical labs can also recommend how much acid to use per volume of water. Target pH levels for different situations are found below.
Target Irrigation Water pH levels
If you are doing acid shock treatments check with emitter manufacturers to make sure that emitter parts are able to tolerate these low water pH levels.
Always add acid to water; do not add water to acid. Adding water to acid can cause a violent reaction, and may cause the acid to splash on the person pouring the water. Individuals working with acids should wear protective clothing and eyewear.
Once the amount of acid needed is determined are determined, you will need to know the volume of water that is being applied per treated irrigation block. This information can come from a flowmeter or by calculating drip emitters per block. Actual drip emitter output, which you get when you do a DU test, will be more accurate than what is provided by the drip emitter manufacturer when installed.
As a If you are doing infrequent or shock acid applications, inject the amount of acid in a known amount of irrigation water that will fill the drip lines at full operating pressure. After the acid has been injected and distributed to its furthest point in the irrigation block turn the system off and let the low pH water set for several hours(overnight) to dissolve chemical precipitates. Turn system back on and flush five to fifteen drip lines at a time.
For emitter clogging caused by biological problems such as algae, moss or bacterial slimes chlorination is the preferred treatment. Depending on how severe the problem is, chlorine can be injected continuously or occasionally.
Filter maintenance is critical to prevent physical clogging of drip emitters. If you do not have automatic back flushing filter systems you will need to monitor for pressure loss across the filter using pressure gauges. A pressure loss across the filters will alert you to debris clogging the filters.
Your irrigation system is a critical piece of vineyard infrastructure that needs to be maintained like all other parts of the vineyard. Take the time to measure your drip systems performance so it can deliver water efficiently and effectively. This is especially true if you use your system to deliver fertilizers and pesticides to your vineyard.
Here are some great videos to assist you:
Acidification to Clean Out a Drip Irrigation System: https://youtu.be/Kty6ykScfwQ
Calculating Acid Amount for Drip Irrigation System: https://youtu.be/eOJCDqC3n0E
Adding Acid to Irrigation System: https://youtu.be/5duPeWDAIns
Schwankl, Larry, Blaine Hanson, and Terry Pritchard. 2008. Maintaining Microirrigation Systems. University of California, Agriculture and Natural Resources: Oakland, California. Publication 21637.
Zellman, Paul. 2016. Drip Irrigation System Evaluations: How to Measure & Use Distribution Uniformity Tests. California Sustainable Winegrowing Alliance. https://www.sustainablewinegrowing.org/docs/DUArticle.pdf
Kirk Williams is a lecturer in Viticulture at Texas Tech University and teaches the Texas Tech Viticulture Certificate program. He is also a commercial grape grower on the Texas High Plains. He can be contacted at firstname.lastname@example.org