Irrigation Frequency, Drip Tape Flow Rate, and Onion Performance

Clint C. Shock, Erik Feibert, and Lamont Saunders

Malheur Experiment Station

Oregon State University

Ontario, OR, 2002

Introduction

Onion production with subsurface drip irrigation has been tested at the Malheur Experiment Station since 1992. In 1997 and 1998 onions were submitted to five soil water potential treatments using an automated, high frequency irrigation system. The soil water potential was maintained relatively constant by applying 0.06 inch of water up to eight times a day depending on soil water potential readings. The soil water potential at 8-inch depth that resulted in maximum onion yield, grade, and quality after storage was determined to be -20 kPa. An irrigation frequency of up to eight times a day in small increments is not very feasible on a commercial scale. Would reducing the irrigation frequency result in lower water use efficiencies and lower onion yield and quality?

The tape that has been used at the Malheur Experiment Station has a flow rate of 0.22 gal/min/100 ft of tape. A reduced flow rate could theoretically result in an improved soil wetting pattern and less water lost to deep percolation. An improved soil wetting pattern could result in the onions on the outside row of a double row receiving more uniform soil moisture. New "ultra low flow" drip irrigation tapes with reduced emitter flow rates are being introduced by drip tape manufacturers. This trial tested four irrigation frequencies and two drip tape flow rates for their effect on onion yield and quality.

Materials and Methods

The onions were grown at the Malheur Experiment Station, Ontario, Oregon on an Owyhee silt loam previously planted to wheat. Onion (cv. 'Vaquero', Sunseeds, Morgan Hill, CA) was planted in two double rows, spaced 22 inches apart (center of double row to center of double row) on 44-inch beds on April 4, 2002. The single onion rows in the double row were spaced 3 inches apart. Onion was planted at 150,000 seeds/acre. Drip tape (T-tape, T-systems international, San Diego, CA) was laid simultaneously with planting at 6-inch depth between the two double onion rows. The distance between the tape and the double row was 11 inches. The drip tape had emitters spaced 12 inches apart and either of two flow rates: low flow (0.22 gal/min/100 ft) and ultra low flow (0.11 gal/min/100 ft).

Immediately after planting the onion rows received 3.7 oz of Lorsban 15G per 1,000 ft of row (0.82 lb ai/acre), and the soil surface was rolled. The trial was irrigated on April 8, April 20, April 24, April 27, and April 30 with a minisprinkler system (R10 Turbo Rotator, Nelson Irrigation Corp., Walla Walla, WA) for even stand establishment. Risers were spaced 25 ft apart along the flexible polyethylene hose laterals that were spaced 30 ft apart. Onions started emerging on April 18.

The experimental design was a randomized complete block with four replicates. The onions were submitted to eight treatments consisting of a combination of two drip tape flow rates and four daily irrigation frequencies (Table 1). The onions in each plot (4 double rows by 50 ft) were submitted to one irrigation frequency and one tape flow rate. The irrigation frequencies were the daily time interval by which the datalogger (CR10, Campbell Scientific, Logan, UT) checked the sensors and made irrigation decisions. Each plot was irrigated independently when the average soil water potential at 8-inch depth in the plot reached -20 kPa. The irrigation duration for each treatment was adjusted so that when irrigated the maximum number of times all treatments had the capacity to deliver a maximum of 0.48 inch of water per day.

Soil water potential was measured in each plot with four granular matrix sensors (GMS, Watermark Soil Moisture Sensors Model 200SS, Irrometer Co., Riverside, CA) installed at 8-inch depth in the center of the double row. Sensors were calibrated to soil water potential (swp) (Shock et al. 1998). The GMS were connected to the datalogger via five multiplexers (AM 410 multiplexer, Campbell Scientific, Logan, UT). The datalogger read the sensors and recorded the soil water potential every 3 hours. The irrigations were controlled by the datalogger using a controller (SDM CD16AC controller, Campbell Scientific, Logan, UT) connected to solenoid valves in each plot. The pressure in the drip lines was maintained at 10 psi by pressure regulators in each plot. The amount of water applied to each plot was recorded daily at 8:00 a.m. from a water meter installed between the solenoid valve and the drip tape. The automated drip irrigation system was started on May 22. Irrigations were terminated on August 30.

Onion evapotranspiration (Et_c) was calculated with a modified Penman equation (Wright 1982) using data collected at the Malheur Experiment Station by an AgriMet weather station. Onion Et_c was estimated and recorded from crop emergence on April 18 until the final irrigation.

Fertilizer was applied through the drip tape as ammonium phosphate at 50 lb N/acre and 96 lb P/acre on June 12. On June 19 and June 27, ammonium phosphate at 25 lb N/acre and 48 lb P/acre were applied through the drip tape. On June 13, zinc chelate at 0.25 lb Zn/acre, copper chelate at 0.2 lb Cu/acre, and boric acid at 0.1 lb B/acre were injected through the drip tape. On June 20, magnesium sulfate at 5 lb Mg/acre was injected through the drip tape. On July 3, magnesium sulfate at 5 lb Mg/acre and copper chelate at 0.2 lb Cu/acre were injected through the drip tape.

Postemergence weeds were controlled by an application of Prowl (1.7 lb ai/acre) and Poast (0.2 lb ai/acre) on May 2 and an application of Goal (0.12 lb ai/acre), Buctril (0.12 lb ai/acre), and Poast (0.28 lb ai/acre) on May 23. Approximately 0.3 inch of water was applied through the minisprinkler system on May 2 to incorporate the Prowl. Thrips were controlled with four aerial applications of Warrior and Lannate (June 19, July 2, July 18, and August 6) and one aerial application of Warrior on June 10. Warrior was applied at 0.03 lb ai/acre and Lannate was applied at 0.26 lb ai/acre.

On September 10 the onions were lifted to field cure. On September 19, onions in the central 40 ft of the middle two double rows in each subplot were topped and bagged. The bags were placed into storage on September 23. The storage shed was managed to maintain an air temperature of approximately 34°F. The onions were graded out of storage on December 10. Bulbs were separated according to quality: bulbs without blemishes (No. 1s), split bulbs (No. 2s), and diseased bulbs. The No. 1 bulbs were graded according to diameter: small (<2¼ inches), medium (2¼ -3 inches), jumbo (3-4 inches), colossal, (4-4¼ inches), and supercolossal (>4¼ inches). Bulb counts of supercolossal onions were made during grading. Marketable onions were considered perfect bulbs in the medium, jumbo, colossal, and supercolossal size classes.

Results and Discussion

Soil water potential at 8-inch depth over time remained fairly close to -20 kPa during the season for all treatments (Figs. 1 and 2). The amplitude of the oscillations was slightly higher for the once-a-day irrigation frequency with either tape flow rate. The average soil water potential for the season was the same for all treatments (Table 1). The standard deviation of the soil water potential increased as the irrigation frequency was decreased, but the increases were small for either tape flow rate. The soil water potential at 20-inch depth over time remained close to soil water potential at 8-inch depth and with no differences between treatments.

Water applications to all treatments closely followed Et_c during the season (Figs. 3 and 4). Evapotranspiration totaled 30.2 inches from emergence to the last irrigation. The total amount of water applied was 32 inches and was the same for all treatments. The total amount of water applied includes 2 inches of water applied with the minisprinkler system after emergence and 0.84 inches of precipitation. There was no significant difference in water use efficiency between treatments (Table 1).

There was no significant difference in onion yield, grade, or quality between treatments (Table 2).

Reducing the irrigation frequency to once per day did not reduce irrigation efficiency, water use efficiency, onion yield, or quality. We thought that longer irrigation sets would result in the possibility of greater leaching of water and lower irrigation efficiencies. The reduced tape flow rate did not result in any improvement in irrigation efficiency, water use efficiency, onion yield, or quality.

References

Shock, C.C., J.M. Barnum, and M. Seddigh. 1998. Calibration of Watermark Soil Moisture Sensors for Irrigation Management. Pages 139-146 inProceedings of the International Irrigation Show, Irrigation Association, San Diego, CA.

Wright, J.L. 1982. New evapotranspiration crop coefficients. J. Irrig. Drain. Div., ASCE 108:57-74.

Table 1. Drip tape flow rate and irrigation frequency treatments applied to onions and total amount of water applied, water use efficiency, and average soil water potential. Malheur Experiment Station, Oregon State University, Ontario, OR, 2002.

			Water applied			Average soil water potential
Tape flow rate	Irrigation frequency*	Irrigation duration	Per irrigation	Total †	Water use efficiency‡	8-inch depth	20-inch depth
gal/min/100 ft	-------- hours --------		------- inches --------		cwt/inch	--------------- kPa -------------
0.22	3	1	0.06	32.7	31.6	-20.3 (3.1)§	-23.6
0.22	6	2	0.12	32.2	29.8	-21.7 (3.4)	-23.3
0.22	12	4	0.24	32.5	32.0	-20.3 (3.5)	-18.6
0.22	24	8	0.48	31.5	32.4	-19.5 (4.0)	-31.4
0.11	3	2	0.06	32.6	29.6	-19.8 (2.9)	-21.4
0.11	6	4	0.12	31.2	31.5	-20.7 (3.9)	-23.3
0.11	12	8	0.24	32.9	29.6	-20.3 (4.2)	-26.4
0.11	24	16	0.48	31.5	31.9	-20.6 (5.4)	-15.3
LSD (0.05)				NS	NS	NS	NS

*Time interval used by datalogger for checking sensors and making irrigation decisions.
^†Includes 2 inches of water applied with minisprinklers after emergence and 0.84 inches of precipitation.
^‡cwt marketable yield per inch of water applied.
^§Standard deviation in parentheses.

Table 2. Onion yield and grade response to subsurface drip tape flow rate and irrigation frequency, Malheur Experiment Station, Oregon State University, Ontario, OR, 2002.

Figure 1. Soil water potential over time for onion drip irrigated using a tape flow rate of 0.22 gal/min/100 ft and four irrigation frequencies (time interval used by datalogger for checking sensors and making irrigation decisions). Soil water potential is the average of 16 sensors. Malheur Experiment Station, Oregon State University, Ontario, OR, 2002.

Figure 2. Soil water potential over time for onion drip irrigated using a tape flow rate of 0.11 gal/min/100 ft and four irrigation frequencies (time interval used by datalogger for checking sensors and making irrigation decisions). Soil water potential is the average of 16 sensors. Malheur Experiment Station, Oregon State University, Ontario, OR, 2002.

Figure 3. Cumulative water applied and Et_c over time for onion drip irrigated using a tape flow rate of 0.22 gal/min/100 ft and four irrigation frequencies (time interval used by datalogger for checking sensors and making irrigation decisions). Water applied is the average of four plots. Malheur Experiment Station, Oregon State University, Ontario, OR, 2002.

Figure 4. Cumulative water applied and Et_c over time for onion drip irrigated using a tape flow rate of 0.11 gal/min/100 ft and four irrigation frequencies (time interval used by datalogger for checking sensors and making irrigation decisions). Water applied is the average of four plots. Malheur Experiment Station, Oregon State University, Ontario, OR, 2002.

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