Adequate soil moisture is a fundamental concept when maximizing crop production and quality.
This is especially true when growing small fruits, vegetables and nursery crops in the Willamette Valley. Careful irrigation planning can result in great crops!
How do you know when you have enough water and what happens if you have too much or too little?
Over-irrigation, or too much water, can reduce root growth and nutrient uptake while encouraging root disease. Combined, these effects result in reduced crop production and crop quality. Over-irrigation can leach nutrients and pesticides from the plant rooting zone into groundwater, robbing them of applied fertilizer and pest control and adversely affecting water quality.Over-irrigation can also create surface runoff causing nutrients, pesticides, and sediment to leave the field.
Similar results can be expected from under-irrigation, or too little water. Plants are stressed so growth, crop yield and quality are reduced.
Either way, improper irrigation diminishes the return on your investment of time, effort and money.
Considerations for Irrigation
There are three primary considerations when determining your irrigation schedule:
In our area, sprinkler and micro-irrigation (drip) systems are both common. There are advantages and disadvantages to each system.
Sprinkler systems can be portable and offer flexibility. They provide cooling during heat events and frost protection when temperatures dip to freezing and below. However, uniform coverage can be adversely affected by poor management of overlap or drift due to wind, runoff, evaporation, inconsistent pressure and a lack of equipment maintenance. Higher labor and energy costs result from pumping and changing smaller settings. Wet foliage can also result in increased occurrence of disease.
Drip systems result in conservation of water by 15-30%, a uniform pattern, lower labor costs and dry foliage with fewer incidences of pathogens. They require good filtration and regular maintenance to prevent leaks or clogs, which are hard to see. They can be damaged by rodents and other animals and if the soil moisture is depleted it can be difficult to “catch up”.
Coarse soils with higher sand content (sandy loam) can take water in relatively quickly compared to fine textured soils (clay loam). They cannot store as much, losing it more quickly to surface evaporation and percolation below the crop’s rooting zone.
Your irrigation schedule must account for the specific crop you are growing and its stage of growth. Different crops have unique water demands which change as they develop from seed germination, or transplant, to maturity. Seeds need to be kept moist at the soil surface until they germinate requiring more frequent irrigation in smaller quantities. But, as the roots reach deeper and the leaves enlarge moisture demand increases requiring less frequent watering in larger quantities. This is, in part, due to stresses from weather events such as warm temperatures and wind.
Water demands from weeds can also have an impact. You will need to fiddle with your irrigation schedule if your weed control is inadequate.
Determining an Irrigation Schedule (when and how much?)
Irrigation scheduling is the process of determining when to irrigate, how much water to apply and recording the amount applied. It is similar to managing a checkbook. Through crop evapotranspiration water is removed from a field (withdrawals) reducing the available soil water (account balance). When the available soil water approaches the minimum balance (minimum account balance allowed) water should be applied through irrigation or rainfall (making a deposit).The penalty for overdraft below the minimum soil water is crop damage and reduced yield (overdraft charge). Repeated soil water depletion below the minimum may lead to severe crop losses or death. The upper limit of water that can be stored in the soil is the Available Water Capacity (maximum account value allowed).
Irrigation scheduling starts with estimating and monitoring the available water capacity in the crop’s root zone. You irrigate to have a full “checkbook” if the available water capacity isn’t already fully loaded. You manage your checkbook by following the crop and weather removal of moisture paired with additions either through rain or irrigation. It isn’t a difficult technique once you get used to it.
How do you set up your available soil water account?
- Identify your soil type. Typical for most vegetable crops, uptake of 70% of soil moisture comes from the upper 50% of the crop’s rooting depth, usually in the top 12-20 inches. Use the Soil Survey for your land, available through the Natural Resource Conservation Service (NRCS). There you will learn your particular soil’s infiltration rate in inches per hour at different depths and its available water capacity (AWC). In other words, how quickly it can take up water and how much it can store. Your local NRCS or Extension office can help you interpret the soil survey and make the necessary calculations. Local conditions like compaction, presence of a hardpan, organic matter content and amount of cultivation can make a significant difference. If you need to add to the AWC of your soil you can maintain crop residue, plant and incorporate cover crops into the soil, minimize tillage and minimize the use of equipment when soils are wet.
- Determine the output of your irrigation system in inches/hour. You will need to know the system’s flow rate, irrigated area and system efficiency. System efficiency is the amount of water your system can apply to the crop root zone. Sprinkler systems can vary from 50% for stationary guns to 90% for center pivot or linear move type systems. Micro or drip systems vary from 85-95%. Refer to the Irrigation Management Basics for in depth irrigation information and examples.
- Track the evapotranspiration (ET). ET is the sum of evaporation from the soil surface and transpiration by the plants for your specific crop, less any rainfall. ET information, often referred to as crop water use in inches per day, is available from weather station data collected all over the Pacific Northwest and posted on the Agrimet website.
- Know your crop’s maximum allowable depletion value (MAD), or the amount of AWC that can be depleted without hurting your crop. The MAD for most crops is 40-60% of AWC. Values below 40% risk damage to the crop. Irrigation is usually scheduled when the MAD is at 50%. Western Oregon Irrigation Guides provide crop specific information to aid growers with irrigation management.
Monitoring Soil Moisture
Monitor the soil moisture throughout the life of the crop. Some sophisticated, affordable and easy to install devices are available to track soil moisture, such as electrical resistance blocks, data loggers, and tensiometers. These sensors need to be placed in areas that are representative of the entire field (average soil and slope).
A soil probe is an easy way to check soil moisture anywhere in your field. It can be used to quickly collect samples from various rooting depths and observation by feel and appearance can approximate the percent AWC. Find out more about estimating soil moisture content by feel and appearance on the NRCS website: http://www.nrcs.usda.gov/wps/portal/nrcs/detail/wy/soils/?cid=nrcs142p2_026833
Irrigation System Rebates from Energy Trust of Oregon
Leaking gaskets on an irrigation system are a drain on operating costs and profits. Growers all over Oregon are replacing gaskets to save water, energy and money. They’re also qualifying for rebates from Energy Trust that in many cases cover the full cost of the new gaskets.
Replacing worn gaskets is a relatively quick and inexpensive project. Simply purchase new gaskets from your vendor, install them and apply for the Energy Trust rebate. To qualify, you will need to replace every gasket in your irrigation system and be a PGE or Pacific Power Customer. Don’t let this opportunity drip away.
An example gasket replacement project:
Average equipment cost
Energy Trust Rebate
Yearly Utility Bill savings
New Gaskets, average
A large farm that completed a gasket installation project reported more than $44,000 in rebates and energy savings over five years.
To learn how Energy Trust can help you save on irrigation system gaskets, visit www.energytrust.org/industrial-and-ag.
OSU Small Farms Irrigation Management: http://smallfarms.oregonstate.edu/sfn/sp10IrrigationManagement
OSU Extension Service, Western Oregon Irrigation Guides, EM8713: https://catalog.extension.oregonstate.edu/em8713
This publication gives an overview of the decisions and factors that go into irrigation system design, operation, and maintenance. It includes 16 crop-specific fact sheets that cover all of the major crops grown in the Willamette Valley. Each fact sheet covers the key water management issues and includes a simple worksheet for computing seasonal irrigation requirements. All essential calculations are described in appendices. A glossary of terms and a list of additional resources can also be found.
NRCS Soil Survey information: http://www.nrcs.usda.gov/wps/portal/nrcs/surveylist/soils/survey/state/?stateId=OR
NRCS irrigation webpage with commonly used irrigation handbooks and manuals available for download: http://www.nrcs.usda.gov/wps/portal/nrcs/detailfull/national/water/manage/irrigation/?cid=stelprdb1045075
Energy Trust of Oregon offers incentives for Scientific Irrigation Scheduling: http://energytrust.org/library/GetDocument/3183