- Author: Gerry Spinelli
- Author: Michael Cahn
Determining how long to run your irrigation system can be challenging because typical recommendations on how much water to apply are expressed in units of inches (or feet) of water depth. Knowing the application rate of your irrigation system will allow you to convert these recommendations into the time (hours) to operate the irrigation system.
The application rate is the depth of water that the irrigation system applies during a period of time, and is typically expressed in units of inches per hour (in/hr). An application rate of 0.27 in/hr means that when the system is operated for one hour it will apply 0.27 inches of water to the field; if it is operated 45 minutes, 0.2 inches (0.27 in/hr × 45 min ÷ 60 min/hr) are applied and so on.
Note that the application rate is independent of the number of acres irrigated. In other words, an irrigation system would have the same application rate on a one-acre or a five-acre block. For raised bed crops such as vegetables and strawberries, the application rate includes the area of both the furrows and the bed tops.
Determining the application rate of drip systems
The variables determining the application rate of drip systems are: 1. bed width, 2. number of drip lines per bed, and 3. flowrate of the drip tape. The flow rate of the tape (also referred to as the discharge rate) corresponds to a specific pressure, usually 8 or 10 pounds per square inch (psi). The flow rate of the drip tape is usually specified on the label of the tape roll (Fig. 1) and is often expressed in units of gallons per minute per one hundred feet of tape (gpm/100 ft).
Figure 1. Examples of a drip tape label with flow (discharge) rate information.
Some tape manufacturers provide the discharge rate of the emitters and the emitter spacing rather than the flow (discharge) rate of the tape. The discharge rate of the emitters is usually expressed in units of gallons per hour (gph). To calculate the tape flow rate from the emitter discharge rate, use the following equation:
Tape discharge rate (gpm/100 ft) = Emitter discharge rate (gph) × 20 ÷ emitter spacing (inches) 
Example (from Fig. 1): emitter discharge rate = 0.16 gph; emitter spacing= 8 inches
Tape discharge rate (gpm/100 ft) = 0.16 gph × 20 ÷ 8 inches = 0.40 gpm/100 ft
The application rate of a drip system can be determined from the tape flow rate (gpm/100 ft), bed width (inches), and number of drip lines using the following equation:
Application rate (inches/hour) = tape flow rate (gpm/100 ft) × number of tapes × 11.55 ÷ bed width (inches) 
Example: tape flow rate = 0.45 gpm/100 ft; bed width = 48 inches; 2 drip lines per bed
Application rate = 0.45 gpm/100 ft × 2 drip lines × 11.55 ÷ 48 inches = 0.22 inches per hour
Table 1 also summarizes application rates of some common drip system configurations for strawberry. The application rate of the drip system can be estimated by finding the row with the flow rate that is closest to the tape that you use, and then reading the application rate under the bed and drip line configuration used at your farm.
Table 1. Application rate (in/hr) of drip systems for common drip tape flow rates and bed widths in strawberry.
Determining the application rate of solid-set and hand-move sprinkler systems
To estimate the application rate in sprinkler systems (aluminum hand-move pipes) one needs to know the spacing of the lateral lines, the sprinkler nozzle size and pressure at which the system is operated. For hand-move sprinklers, the distance that the sprinkler line is moved between irrigation sets is the lateral line spacing. Pressure can be measured at the sprinkler nozzle using a with a pitot tube fitted with a pressure gauge. Alternatively, a gauge or Schrader valve can be added to a riser so that the pressure can be measured on the lateral line. Discharge rate of sprinkler heads will increase with higher pressures and larger nozzle sizes. Most manufacturers provide data on discharge rate for each sprinkler head model. Table 2 shows the range of discharge rates for the Rainbird 20JH model.
Table 2. Discharge rates of Rainbird 20 JH sprinkler heads at varying nozzle sizes and pressures.
From the discharge rate, lateral pipe spacing, and discharge rate, the application rate for a sprinkler system can be estimated using the equation:
Sprinkler application rate (inches/hr) = discharge rate (gpm) × 96.3 ÷ [lateral spacing (ft) × head spacing (ft) 
Example: Sprinkler head = Rainbird 20JH with 7/64 inch nozzle operated at 45 psi; lateral spacing = 40 ft; head spacing = 30 ft.
From Table 2 the discharge rate of the sprinkler head = 2.63 gpm.
Sprinkler application rate (inches/hr) = 2.63 gpm × 96.3÷[30 ft ×40 ft] = 0.21 inches/hr
Alternatively, Table 3 can be used to estimate the application rate of an irrigation system using the Rainbird 20JH sprinkler heads.
Table 3. Application rates for sprinkler systems using Rainbird 20 JH heads at varying pressures, nozzle sizes, and lateral and head spacings.
Using the application rate to determine how long to irrigate
The application rate can be used to determine how long to irrigate a crop from estimates of crop water use. For an average year in the Pajaro Valley, a strawberry crop needs one inch of water during a typical week in August and about 28 inches for an entire crop cycle. A grower with an application rate of 0.22 inches/hour will need to irrigate four and a half hours (1 inch ÷ 0.22 in/hr = 4.5 hr) during a week in August and a total of 127 hours (28 in ÷ 0.22 in/hr = 127 hr) during the entire season.
Accurately estimating the application rate of an irrigation system requires precise knowledge of the discharge rate of the drip tape or sprinkler nozzle. Over time, nozzles become worn and drip emitters clog, or the pressure of the system may not be operated at the specifications of the manufacturer. The best way to accurately know the application rate is to directly measure it in the field. For direct measurements of the application rate, schedule an on-site irrigation evaluation with the RCD of Santa Cruz County at: 831-464-2950, email@example.com.
- Author: Michael Cahn, Irrigation and Water Management Advisor
- Contributor: Thomas Lockhart, Staff Research Associate
- Contributor: Laura Murphy, Staff Research Associate
An important benefit of drip irrigation is the ability to apply fertilizer through the irrigation water, permitting growers to spoon-feed nutrients, such as nitrogen (N), to their crops. By avoiding applications of large amounts of N fertilizer when the crop is small and uptake rates are low, losses of nitrogen by leaching can be minimized. Also, unlike furrow and overhead sprinklers, drip can deliver fertilizer in the zone where roots are most concentrated.
While drip fertigation offers several advantages for managing nitrogen fertilizer during the season, success depends on the management of the drip system and using best practices for fertigation. Drip systems with poor distribution uniformity may likely cause fertilizer to be unevenly distributed within a field. Also, the strategy of injecting fertilizer into a drip system can affect the distribution of fertilizer to the crop. Proper fertigation requires injecting at a steady rate and at a location that provides sufficient mixing of fertilizer with irrigation water. To assure that the fertilizer uniformly distributes within the field after an injection, sufficient irrigation time with clean water is needed so that all of the fertilizer is flushed out of the drip tape before the irrigation ends.
For drip to be economical for vegetable growers on the central coast, most farming operations retrieve drip tape after each crop is harvested and repair and reuse the tape for 8 to 12 crops. Breaks and leaks in the tape are repaired using a splicing machine (Figure 1). Growers have expressed concern that fertigating through their drip systems is not resulting in even applications of N fertilizer after the tape has been reused for multiple crops. Splicing machines often do not fully repair leaks in tape, and emitters tend to plug over time unless the tape was adequately maintained by flushing and chemical treatment.
In response to grower concerns, we evaluated the uniformity of applied water and nitrogen fertilizer for surface placed drip in 11 commercial lettuce fields during the fall of 2012 and during the spring of 2013.
Fig. 1. Splicing machines are used to repair leaks and breaks in drip tape
All fields were planted with romaine or iceberg lettuce varieties on 40-inch or 80-inch wide beds. At each site irrigation, pressure, and fertilizer uniformity were evaluated during a single irrigation event. Field sizes ranged from 8 to 20 acres, and the maximum row lengths ranged from 600 to 1340 ft. Drip tape at all field sites was 7/8 inch diameter, medium flow tape (0.34 gpm/100 ft), but varied by manufacturer and age. The location where fertilizer was injected into the irrigation system, and start and end time of the fertigation, as well as the duration of the irrigation, were recorded. Before irrigating, couplers fitted with ¼ gallon per hour pressure compensating emitters that were spliced in to the drip tape at 24 locations within the field, representing the head, tail and middle areas. Water from these emitters was collected into 5 gallon containers during the entire irrigation (Figure 2) and analyzed for NO3-N and NH4-N. The discharge rate of 4 emitters and pressure of the tape was measured near each of the 24 fertilizer sampling locations (total of 96 emitters). Mass (lbs) of N applied at each of the 24 collection locations within a field was estimated by multiplying the measured discharge rate of the drip tape by the irrigation time and by the concentration of N in the collected water. Uniformity of applied water, tape pressure, and fertilizer was calculated by comparing the lowest 25% of measurements to the average of all 24 measurements. In addition to evaluating fertilizer distribution uniformity, we evaluated the time for fertilizer to travel to the furthest distance from the injection point by injecting food dye for a 5 minute period into the irrigation system and monitoring the water for color at the furthest point from the injection location.
Fig. 2. Low flow (1/4 gph) pressure compensating drip emitters were used to collect samples of irrigation water during the entire irrigation cycle.
Distribution uniformity of applied water for the 11 fields averaged 73% and ranged from 38% to 88% (Table 1). The industry standard for irrigation uniformity of surface drip is 85%. Fertilizer application uniformity averaged 67% and ranged from 46% to 82%. The distribution uniformity of the drip systems of 7 fields evaluated was greater than 74% (avg = 82%) and fertilizer uniformity was greater than 72% (avg = 77%) (Figure 3).
One of the causes for poor distribution uniformity of some drip systems may have been related to pressure. Pressure uniformity averaged 80% and ranged from 43% to 99% (Table 1). Average pressures in the drip tape ranged from 3.5 to 13.8 psi (Table 2). Where the system pressure averaged 4.3 psi, the tape discharge rate was 30% less than the manufacturer's rating. Irrigation distribution uniformity decreased substantially when the average field pressure was less than 5 psi (Figure 4). Additionally, a substantial percentage of emitters of some drip systems were plugged (Table 2) which would reduce irrigation system uniformity. Leaks were evaluated in 5 fields and ranged from 1 to 5 leaks per 1000 ft of tape (Table 2). Significant leaks can potentially reduce drip uniformity by lowering the downstream pressure. Other limitations to good drip uniformity included mixing different types of tape in the same field, fluctuating pressure during the irrigation, and row lengths longer than 800 ft.
Field 8 had a high uniformity of pressure and irrigation distribution but a low fertilizer uniformity. We speculate that the fertilizer which was injected at a “T” connecting the valve in the field with the submain did not have sufficient time to mix with the irrigation water before the flow split into opposite directions. Hence, the average concentration of N on one side of the field was approximately half the concentration measured on the other side of the field. The distribution uniformity of fertilizer on individual sides of the fields was greater than 87%.
With the exception of field 8, fertilizer distribution uniformity was closely related with irrigation system uniformity (Figure 4). Fields with the lowest fertilizer uniformity were operated at the lowest average pressure and/or had the highest level of plugged emitters (Table 2).
Fertilizer was injected at the well in 4 of the fields and at the submain valve in the other 7 fields (Table 3). Injections were made simultaneously using 2 valves at 3 of the fields. Fertilizer was injected during an average of less than 30 minute period often at the beginning of the irrigation (Table 3). The time required for the fertilizer to travel to the furthest point of the irrigation system averaged 42 minutes but ranged from as short as 22 minutes to as long as 1 hour. Field size, row length, and injection location appeared to affect the travel time of the fertilizer. The average time for flushing the fertilizer was 3.75 hours, which was ample time to allow the fertilizer to completely flush from the system. The irrigation industry recommends that for long irrigations (> 4 hours), fertilizer should be applied in the middle of the irrigation cycle. Only at field 10 was the fertilizer applied during the middle of the irrigation. The long flush time after injecting could potential leach nitrate forms of fertilizer below the root zone of the crop. On average, half of the applied fertilizer N measured in the collection buckets was in the nitrate form.
This survey of commercial lettuce fields demonstrated that N fertilizer applied by drip has an average distribution uniformity of 77% when the injection is properly made and the drip system is operated and maintained to achieve an average distribution uniformity of 82%. The results also showed that N fertilizer applied by drip is frequently distributed to fields unevenly due to poor uniformity of the drip systems, or because proper injection procedures were not followed. Operation procedures observed at these sites would suggest that irrigators may need training to better understand the principles of fertigation so that fertilizer is applied at the highest uniformity possible, and in a manner that will prevent leaching losses of nitrate.
We thank the California Leafy Green Research Board for funding this project and the many growers that cooperated with the field trials.
Table 1. Summary of irrigation, fertilizer, and pressure uniformity of drip irrigated lettuce fields.
Table 2. Drip tape characteristics at commercial lettuce sites.
Table 3. Irrigation summary for drip irrigated lettuce fields.
Fig. 3. Relationship between distribution uniformity of retrievable drip systems and fertigation uniformity. Each symbol denotes a commercial lettuce field evaluated during the study.
Fig. 4. Effect of tape pressure on the distribution uniformity of retrievable drip systems. Each symbol denotes a commercial lettuce field.