The effects of placement depths of driplines on seedless watermelon’s quality and yields
Watermelon [Citrullus lanatus (Thunb). Matsum. et Nakai] is an important cash crop for the southeast region of Colorado, especially for Arkansas Valley which stretches across the Eastern Plains outside Pueblo. Subsurface drip irrigation (SDI) is one type of drip irrigation that slowly emits water and nutrients under the soil surface directly to the crop root zone by buried drip tubes or drip tapes. The advantages of SDI system include improving crop yield and quality, accelerating crop maturity, suppressing weed, reducing plant disease, diminishing water losses from surface runoff and evaporation as well as curtailing applied fertilizer amounts.
The drip lines for a typical SDI system are placed 6-24 inches under soil surface. Watermelon under SDI system in the Arkansas Valley is grown on 60-inch center to center bed with a single dripline placed 7-8 inches deep in the center of the bed. However, less information regarding to the impacts of different burying depths of driplines on fruit quality and yield of watermelon are available for growers in Arkansas Valley.
1. Testing site
Field experiments will be conducted at Arkansas Valley Research Center (AVRC) located at Rocky Ford. There are approximately 100-acre field space at AVRC and the station has full access to the Rocky Ford ditch with sufficient water for field research.
2. Study design
The commercial seedless watermelon (Citrullus lanatus) variety Joy Ride (Seminis Vegetable Seeds, Bayer Crop Science) and the pollinator Wingman (Seminis Vegetable Seeds, Bayer Crop Science) will be planted in a 3:1 ratio in mid-May 2023 at AVRC. Trials will be planted by transplanting on 60-inch center-to-center beds, and crop management will follow commercial standard practices.
Three different placement depths for drip lines will be tested. The experimental design will be a split-plot design with the irrigation type being the whole-plot factor. The split-factors are subsurface drip irrigation (SDI) and none drip irrigation (ND). There are 3 treatments under drip irrigation: drip irrigation with drip line at 3-inch deep (D3), drip irrigation with drip line at 6-inch deep (D6), drip irrigation with drip line at 9-inch deep (D9). ND treatment will be irrigated by furrow irrigation. There are six 78-feet by 6-feet bed blocks to include 3 replicated SDI treatments with two plots per treatment. Each plot is 78 feet long and 5 feet wide with an in-row plant spacing of 3 feet. Treatment blocks will be separated by 22 feet buffer zones. ND treatment will be 300-feet × 2 beds along with SDI treatment blocks.
The subsurface drip system will be installed for D3, D6 and D9 treatments, and a single dripline will be installed at the center of each plot. The depths of drip lines 3-inch, 6-inch and 9-inch under soil surface will be established based on experimental design. The dripline is 13 Mil Rivulis T-Tape 5/8″ Drip Tape with 12 feet emitter spacing and a 1.85 L/h (0.45 Gal/h) nominal discharge per emitter. The black and non-degradable plastic mulch with a width of 4 feet and thickness of 0.0015 inch will be used for all SDI treatments. Both the SDI and ND treatments will receive same amounts of fertilizers. GPS technology will be deployed to precisely locate drip tape location.
Irrigation water will be from Rocky Ford Ditch for AVRC and water for SDI treatments will be sand filtered before applying to the trial.
3.1. Drip irrigation
The irrigation scheduling will be conducted based on the reference Evapotranspiration (ETref) data calculated using alfalfa reference equation (Kimberly Penman, 1982) provided by CoAgMET (https://coagmet.colostate.edu/), the network of agricultural weather stations run by Colorado State University.
The root depth for watermelon used in this project will be 2.0 feet and the maximum allowable depletion is 48%. Drip plots will be irrigated by replacing watermelon evapotranspiration (Ewm). Irrigation scheduling model has been successfully created based on the potential evapotranspiration (Etp) that depicted by using ETref data recorded by CoAgMET for 2022 watermelon growth. The Etp values for 2023 season will be calculated based on this model.
At 10 psi, it will take 6.4 hours to apply 1.0 inch of water for the drip irrigated blocks when the 13 Mil Rivulis T-Tape 5/8″ Drip with 12 feet emitter spacing will be used at flow rate of 1.85 L/h (0.45 Gal/h). The running time for drip irrigated plots will be calculated based on the watermelon evapotranspiration values (Ewm) during the project before applying to plots.
3.2. Furrow irrigation
At AVRC, water for non-drip beds will be delivered by gated pipes (diameter = 10 inches) and the flow rate (gpm, gallon per minute) of irrigated water will be measured by Powlus V Furrow Flumes (Honkers Supreme) following the method described by Trout (unpublished) and quantity of furrow irrigated water will be calculated using the equation: Q × t = d × A
[Q is the flow rate in cfs (cubic feet per second); t is the total time of irrigation (hours); d is the depth of applied water (inches), and A is the area irrigated (acres)]
4. Data collection and analysis
At maturity when the fruit begins to change color and detaches easily from its peduncle, one of the 78-feet plot for each replicated treatment will be harvested following grower practices of sequential harvesting by picking the larger fruits first. Multiple harvesting will be attempted for each testing site. The marketable fruits will be harvested, and the numbers will be recorded. Harvested fruits will be weighed to calculate total yield (kg/ha). The total quantity of water including rainfall and water applied by either drip or furrow irrigation will be calculated.
Four marketable watermelon fruits for each rep and a total of 12 fruits per treatment will be collected and sent to the Postharvest Lab at San Luis Valley Research Center, Colorado State University, for further fruit postharvest quality analysis. Four parameters pertaining to postharvest quality will be evaluated: the soluble solids (SS) from the extractable juice of fruit which is expressed in oBrix, carotenoid levels (lycopene) (mg per 100 gram of pulp), ascorbic acid (AA) or Vitamin C (mg per 100 gram of pulp) and flesh texture (hardness).
Water productivity (WP) was determined by dividing the marketable yield (Ywm, kg/ha) to the volume of water consumed by the crop: WP (Kg/ m3) = Ywm / ETwm (Geerts and Raes, 2009). Irrigation water productivity (IWP) was calculated as the ratio of marketable yield (Ywm, kg/ha) to the irrigation water applied (Howell, 2001).
Weed pressures for SDI treatments and none drip treatments will be recorded. Eight soil water sensors and eight soil temperature sensors will be installed for one replication for each treatment and daily data will be recorded by 900 CGM Watermark Data Logger (The Irrometer Company, Inc.). The watermark sensors will be placed at 6 inch and 18 inches below soil surface and 2 inches from drip tape for drip irrigation treatments.
Statistical analyses will be conducted using GLM procedure with SAS 9.3 software (SAS Institute, Inc., Cary NC) to evaluate the effects of irrigation methods and drip placement depths on water productivity, irrigation water productivity, watermelon yield and quality. Fisher’s protected LSD at P ≤ 0.05 will be implemented for separation of means.
Goals, Scope and Objectives:
With which stakeholder group(s) will the intern work?
What student learning outcomes do you anticipate and what are the opportunities for professional development?
From this internship, the student will have the clear vision of how to conduct an apply-based scientific research designed to solve grower’s concerns and challenges which may encourage the student to be an applied research scientist and a real-problem solver. By working with different personnel from extension and experiment station, the student may choose extension specialist as future career. The student may also develop passion and interests for agriculture production and may eventually set up future goal to become a grower, ag consultant, produce specialist or farm manager.