DuPont, S.T., Peters, T., February 14, 2022. Draft publication under review.
Old inefficient irrigation systems can contribute to low fruit quality and cost grower profits. Evaluate the irrigation system in older blocks to identify common challenges and potential solutions.
Common Challenges
Cork, small fruit, insufficient water availability for the block, plugging filters, uneven pressure on hills and slopes, over and under-watering, irregular distribution across the block, and sandy soils with low water holding capacity are all common challenges in pear irrigation systems in North Central Washington pear orchards (Table 1).
Table 1. Common pear irrigation system challenges and potential solutions
| Challenges | Potential Solutions and Tools |
| Cork, small fruit from insufficient irrigation | Increase irrigation system capacity (larger flow rate to the field). Increase the irrigation frequency (shorter sets, return to zone sooner). Increase water holding capacity of soil with organic matter additions or mulch. Irrigation system upgrades. |
| Cork, excess spring suckering, and growth when roots sit wet in spring | Match initial irrigation in the spring to orchard water use using soil moisture monitoring and increased knowledge of tree needs. Improve water infiltration and soil structure with deep tillage at planting, organic matter additions, or bio-drilling cover crops. |
| Insufficient water availability | Calculate water needs to identify potential ways to re-distribute water. Consider changes to the mainlines to enable watering every other row. Consider automatic valve systems to facilitate irrigation in irregular sets for the hours needed with less labor. Increase water holding capacity with organic matter additions. |
| Filters plugging at canal intake Pressure loss overnight | Check for sticks blocking the intake. Consider self-cleaning filters. Use larger filters. Try to keep filters in the middle of the water intake, away from the top of the water surface and away from the bottom. Install flow meters to trigger cleaning. |
| Uneven pressure on hills | Install flow control nozzles (pressure compensating) and flow control valves. |
| Match irrigation schedules with tree water needs | Use weather-based irrigation scheduling models (such as Irrigation Scheduler Mobile). Use soil moisture sensors. |
| Cork and soil-borne disease from over-watering | Calculate irrigation needs. Use soil moisture sensors. Install automatic valves or variable speed pumps. |
| Secondary filters plugging | Check for worn nozzles and replace them. Replace nozzles with uniform size nozzles. Use flow control (pressure compensating) nozzles. Check flow rates. Switch to nozzle types that do not clog as readily. Improve filtration if nozzles are plugging. Periodically inject chlorine if algae are plugging nozzles. |
| Too long of an interval between irrigation sets because of the number of days required to water a block using the existing system | Reduce nozzle size so a larger section of the orchard can be irrigated in a set. Reduce set times to apply less water, but more frequently. Change mainlines to allow irrigating every other row. |
| Sandy soils with low water holding capacity | Increase water holding capacity of soil with organic matter additions or mulch. Install supplementary drip systems. Consider systems that allow for frequent pulses of irrigation with automatic valves, variable speed pumps, etc. |
| System inefficiencies | Upgrade intake line from the canal for the correct number of shares. Fix intake delivery line if clogged. Make sure the canal filter is working. Check for leaky main lines. Check whether shared mainlines need better valves. Fix leaky valves. Replace worn or inconsistent sprinklers. Ise flow control nozzles on hillsides. |
Steps to evaluating your irrigation system
To identify potential areas for improvement in an irrigation system orchardists can answer the following questions using the tools outlined in the sections below.
- Is water application per set no more than the maximum amount the soil can hold?
- Does water application meet but not exceed tree water needs?
- Is the water application rate, flow, and pressure uniform throughout the block?
- Is water distribution under the tree uniform?
Irrigation system measurements
To begin gather information on your irrigation system and take a few simple measurements.
1. Gather irrigation system description information
Table 2. Irrigation system information
| Number of days to irrigate the block | |
| Pounds per square inch (PSI) of pressure at the pump | |
| Mainline size | |
| Filter type | |
| PSI at the lines | |
| Spacing between irrigation lines (ft) | |
| Spacing between irrigation heads in the row (ft) | |
| Sprinkler head type | |
| Nozzle type and size (often different sizes are different colors) | |
| Soil type‡ | |
| Water holding capacity to 36 in‡ | |
| Elevation at the highest point | |
| Elevation at the lowest point |
‡To determine your soil type and predicted water holding capacity use the Web Soil Survey at https://websoilsurvey.sc.egov.usda.gov/App/HomePage.htm
2. Measure sprinkler head flow rates
Use a measuring device like a 5-gallon bucket. Direct the flow from your irrigation sprinkler into the bucket using a piece of hose (Figure 1). Allow the water to enter the bucket for exactly one minute.
Figure 1. Graduate students divert the flow from an irrigation sprinkler into a container for one minute in order to measure the flow rate.
Pour the water from your bucket into a measuring container. Measure each sprinkler head twice and use the average. Measure the flow from at least six sprinkler heads distributed across your block to calculate an average flow rate. If your block is on a hill measure six heads at the top and six at the bottom of the hill.
Table 3. Sprinkler flow rate (GPM)
| Sprinkler 1 | Sprinkler 2 | Sprinkler 3 | Sprinkler 4 | Sprinkler 5 | Sprinkler 6 | Average | |
| Section 1 (top of the hill) | |||||||
| Section 2 (middle) | |||||||
| Section 3 (bottom of the hill) |
3. Measure pressure
Remove the sprinkler heads from six or more sprinklers and screw a pressure gauge into the riser. The gauge shows your water pressure in pounds per square inch (PSI).
Figure 2. Installing a pressure gauge on a sprinkler riser in order to measure pressure at the sprinkler head.
Table 4. Sprinkler pressure (PSI)
| Sprinkler 1 | Sprinkler 2 | Sprinkler 3 | Sprinkler 4 | Sprinkler 5 | Sprinkler 6 | Average | |
| Section 1 (top of the hill) | |||||||
| Section 2 (middle) | |||||||
| Section 3 (bottom of the hill) |
4. Measurements for irrigation efficiency and distribution
An under-tree sprinkler system averages approximately 75% efficiency where efficiency is the percentage of the flow that reaches the soil. Drip irrigation generally provides almost 100% efficiency. To measure irrigation the efficiency of your irrigation system place a series of measuring containers (pint paint buckets work well) in a grid pattern where some buckets are in line with the tree row and some are in the middle of the grass strip and others are halfway in between (Figure 3). Run the irrigation system for one full set (at least 1 hr). Measure the amount of water in each container.
Figure 3. Place catch cans in a grid pattern with a set of cans in each tree row one row is in the grass strip and the other two rows are halfway between the middle of the grass strip and the tree row.
Table 5. Catch can measurements. Measure the volume in each container (oz per hr).
| Lateral 1 | Lateral 2 | Average | |||
| Sprinkler 1 | Sprinkler 3 | ||||
| Sprinkler 2 | Sprinkler 4 | ||||
| Overall average | |||||
5. Calculate irrigation efficiency
Use the average amount of water caught in the catch cans (overall average Table 5), the spacing between the sprinklers, the test run time, and the average sprinkler flow rate (Table 3) to calculate irrigation efficiency (Equation 2).
Equation 1. Find the opening area of your catch cans (inch2)
Equation 2. Use the spacing between the sprinklers to find the area that each sprinkler irrigates. We assume that any water leaving the area is equal to the amount of water coming into the area from neighboring sprinklers.
Equation 3. Use the average catch in each can (in fluid ounces) to estimate the actual amount of irrigation water that reached the ground from the system.
Equation 4. Use the average sprinkler flow rate (in gallons per minute from Table 3), the area each sprinkler irrigates (from Equation 2 above), and the run time of the catch can test to estimate the amount of water that should have reached the ground if it was 100% efficient.
Equation 5. Irrigation efficiency is the actual amount of water that reached the ground (Equation 3) divided by the theoretical maximum amount of water that would have reached the ground (Equation 4).
Is water application per set no more than the maximum amount the soil can hold?
Soil available water capacity (AWC) is the maximum amount of water that soil can store to be extracted by the plants. It is the water held between field capacity and the permanent wilting point. The total available water in the soil root zone for a specific crop is equal to the crop’s rooting depth multiplied by the available water-holding capacity per unit depth of the soil. Generally, irrigation managers choose a specific allowable depletion of water from the root zone between irrigations (management allowable depletion = MAD). For trees, the MAD is most often chosen to be 50% of AWC. As an irrigation manager, you can adjust the allowable depletion to be sure to avoid water stress. For example, you may determine that 30-40% depletion is more appropriate for your management system.
We usually assume two feet of root depth in young or dwarf rootstock orchards, and about 3 feet of root depth in older, vigorous rooted orchards. In very high-quality soil or older trees, you may assume up to 3.5 or 4 feet. Root studies have determined that 2/3 of the root volume of large, old trees is in the top two feet of soil. About 80 percent of the roots are in the top three feet. Thus, even for large trees, it is generally recommended to assume no more than 3 feet of rooting depth.
To determine your soil type the web soil survey is a useful tool. You can also send a soil sample to your local lab for textural analysis.
Table 6. Plant available water (AW) per foot of soil (in) for common soil types.
| Soil Texture | Inches AW |
| Very coarse sands | 0.4 – 0.75 |
| Coarse sands, fine sands, loamy sands | 0.75 – 1.25 |
| Sandy loams, fine sandy loams | 1.25 – 1.75 |
| Very fine sandy loams, loams, silt loams | 1.50 – 2.30 |
| Clay loams, silty clay loams, sandy clay loams | 1.75 – 2.50 |
| Sandy clays, silty clays, clays | 1.60 – 2.50 |
Equation 6. Maximum irrigation per set
Does water application meet tree water needs?
Calculate the application rate (Equation 1) for your block and compare it to estimated tree water needs. To determine your application rate multiply the average flow rate in GPM from your measurements in step 2 by the sprinkler heads per acre and convert from gallons to acre inches. Below is a sample calculation where the grower measured the output from their sprinkler heads at 1.87 GPM per head and the irrigation efficiency at 0.58.
Equation 7. Application rate example
Compare the application of a set for the block to estimated tree water needs. The amount of water that trees need in a given week is calculated based on the evapotranspiration rate (ET). You can use data from AgWeatherNet to find apple ET in your area. Log in and click on water use. Select your crop and a station near you. The water use model will predict tree water use over the prior week (Table 5). For general estimates see Table 6.
Table 7. Estimated weekly water use from AgWeatherNet data for N Cashmere pears
| 2017 | 2018 | ||
| Date | Inches | Date | Inches |
| 7-May | 0.6 | 7-May | 0.6 |
| 14-May | 0.5 | 14-May | 0.8 |
| 21-May | 0.7 | 21-May | 0.8 |
| 28-May | 1.0 | 27-May | 1.0 |
| 4-Jun | 1.0 | 3-Jun | 0.9 |
| 11-Jun | 1.0 | 10-Jun | 1.0 |
| 18-Jun | 1.1 | 17-Jun | 1.2 |
| 25-Jun | 1.7 | 24-Jun | 1.7 |
| 2-Jul | 2.0 | 2-Jul | 1.6 |
| 9-Jul | 2.2 | 9-Jul | 1.7 |
| 16-Jul | 2.1 | 16-Jul | 1.9 |
| 23-Jul | 1.8 | 23-Jul | 2.0 |
| 30-Jul | 2.0 | 30-Jul | 1.8 |
| 6-Aug | 1.7 | 6-Aug | 1.8 |
| 13-Aug | 1.7 | 13-Aug | 1.6 |
Table 8. Average tree water use
| Irrigation season | …cooler | …average | …warmer |
| inches/week | inches/week | inches/week | |
| early April | 0.28 | 0.35 | 0.42 |
| late April | 0.56 | 0.63 | 0.7 |
| early May | 0.8 | 0.9 | 1.1 |
| late May | 1.2 | 1.4 | 1.8 |
| early June | 1.4 | 1.6 | 1.8 |
| late June | 1.8 | 2.0 | 2.2 |
| July | 1.9 | 2.2 | 2.7 |
| early August | 1.9 | 2.2 | 2.5 |
| late August | 1.5 | 1.7 | 2.1 |
| early September | 1.1 | 1.3 | 1.5 |
Calculated for standard rootstock trees.
Another tool for estimating simple irrigation schedules during an average year for different areas of Washington state is available online.
Is flow and pressure even throughout the block?
If pressure and flow are not even in your block some trees or sections of the block will be receiving more water than others. When the irrigation rate is uneven you will be constantly over or under-watering some trees.
Consider your pressure and flow measurements in Tables 3 and 4. A certain amount of pressure variation is normal (5-10 psi). Flow variation over 10% is a signal that your irrigation system may need some upgrades.
There are multiple solutions to uneven flow and pressure depending on what is causing the problem. Consider flow control nozzles. Check for worn nozzles and replace them. Replace nozzles with uniform size nozzles. Switch to nozzle types that do not clog as readily. Water more frequently so algae does not clog nozzles.
Acknowledgments
This article is part of a project supported in part by funds from the Fresh and Processed Pear Committee and the Bonneville Environmental Foundation. Thank you to project partners Cascadia Conservation District, Larry and Renee Caudle, Erica Bland, Brandon Long, Aaron Hargrove, Phil Guthrie, and Bob Gix. Thank you for in-kind support from Wilbur Ellis, and Sentek Technologies.
Additional Information
Improving Irrigation Efficiencies in Pears Case Studies. DuPont, S.T., Kalcsits, L., Peters, T.2021. https://treefruit.wsu.edu/orchard-management/irrigation-management/improving-irrigation-efficiencies-in-pears-case-studies/
DuPont, S.T., Peters, T. Using Soil Moisture Sensors in Pears. Fruit Matters. October 2021. https://treefruit.wsu.edu/article/moisture-sensors-pears/
