Final Report for GS07-063
Keeping water and nutrients within the rootzone of vegetable crops is the main goal of nutrient Best Management Practices. Ideally, a no-leach situation may be created if the flow rate of irrigation matches exactly the rate of crop evapotranspiration. A reduced irrigation operating pressure (OP) could decrease flow rate such that near-continuous watering of plants is achieved without increasing the risks of water and nutrient leaching. The study was conducted to determine the effect of using a reduced irrigation OP (6 psi) on the flow rate and uniformity of water application as well as the size and shape of the wetted zone. With the prospect of improved water and nutrient efficiencies with low OP, the effects of reduced fertilizer and irrigation rates on plant nutrient status and fresh market tomato marketable yields were also compared with the standard OP (12 psi).
With reduced OP, flow rate was decreased by 23-38% for drip tapes from three major manufacturers. Uniformity of water application was not significantly decreased by the reduced OP. Tape length had a significant effect on the uniformity of two drip tapes with greater variation and less uniformity at 300 ft compared to 100 ft. Reduced OP seems to be more appropriate for short runs as they allow for greater uniformity. This makes reduced OP a practice better suited for small fields, or for large fields with multiple zones and short rows.
For the same volume of irrigated water, depth of wetted front was the same but width was increased for reduced OP compared to standard OP. Depth response to volume of irrigated water was quadratic with D = 5.34 + 0.16V – 0.0007V2 (p&lt;0.01, R2 = 0.80) at 12 psi and D = 4.42 + 0.21V – 0.001V2 (p&lt;0.01, R2 = 0.72) at 6 psi. Due to reduced emitter flow rate at low OP, it took 3 hours of irrigation to go below the typical maximum crop rootzone of 12 inches whereas it took 1.5 hours at 12 psi, thus allowing extended irrigation that better meets hourly crop evapotranspiration without the risks of deep percolation. The width of the wetted front also showed a quadratic response to volume of irrigated water with W = 7.58 + 0.17V – 0.0006V2 (p&lt;0.01, R2 = 0.77) at 12 psi and W = 6.97 + 0.25V – 0.002V2 (p&lt;0.01, R2 = 0.70) at 6 psi. Maximum W was reached at a lower V (V = 62.5 gal/100ft) for the 6 psi treatment at 14.8 inches, which was 53% of the 28-inch wide bed, which indicated that at reduced OP, if a single drip tape were used, only a single row of plants should be planted as watering would not be sufficient for two rows of plants.
Fresh market tomato yields were significantly higher at the reduced OP for one crop season where the crop cycle was short (10 weeks). In the early part of the season when plants were small, the reduced OP in combination with reduced irrigation (75% recommended irrigation) and fertilizer rate (60% N recommended rate) would be a good strategy. . A reduction in irrigation volume of 7% and in fertilizer usage of 15% could be achieved if the reduced OP strategy were adopted for the first four weeks of the season. As the season progressed, the standard OP would be necessary to meet higher crop demands. An adjustable in-line pressure regulator could be installed in the drip system for the grower to easily change the irrigation operating pressure to adjust the flow rates at various growth stages.
These results suggest that reducing OP can be a practical tool to reduce water application and fertilizer rates without reducing yields. Growers can easily reduce OP by inserting a pressure regulator in the drip system, but will need to determine the actual flow rate at the reduced OP. Reduced OP seems to be more appropriate for short runs as they allow for greater uniformity. This makes reduced OP a practice better suited for small fields, or for large fields with multiple zones and short rows. It would be helpful if manufacturers would provide uniformity and flow rates at reduced OP on the drip tape label so that growers do not have to determine them on their farms. Growers also need to understand the connection between pressure and flow rate, and need to be informed of when a flow rate (established by reduced OP) becomes too low to meet crop water needs. It is unlikely that growers will switch the use of pressure regulators during the season. However, if pressure change becomes a part of the automation of irrigation and is linked to automated soil moisture measurement (not requiring grower intervention), then this practice is more likely to be adopted. Growers’ acceptance would also be improved if the additional investment for the pressure regulators could be cost-shared by the BMP program. Overall, these results suggest that reduced OP can be considered a BMP. However, these results also show that reduced OP alone is not enough to keep the water within the root zone, thereby reducing the risk of leaching – not completely eliminating it.
Florida is a major producer of fresh market tomatoes with approximately 35,000 acres planted and having an annual value of nearly $500 million. Drip irrigation is commonly used for growing tomatoes in North Florida. Drip irrigation allows better control of water applications as well as more precise delivery of fertilizers through the dripline. Current IFAS irrigation recommendation consist of (1) a target irrigation rate of 1000 gallons/acre/string/day, (2) fine tuning based on a measurement of soil moisture, (3) a rule for splitting irrigation, (4) a method for accounting for rainfall, and (5) keeping irrigation records (Simonne and Dukes, 2009). Detailed recommendations also exist for crop fertilization when drip irrigation is available (Olson et al., 2009). Because of the low water holding capacity of Florida’s sandy soils, the leaching of nutrients below the root zone of vegetables (typically 12 inches) requires an adequate management of irrigation. In sandy soils, the number of daily irrigation cycles is as important as the total daily irrigation rates. Using currently available drip tapes (with flow rates ranging from 0.15-0.24 gal/h), a typical irrigation schedule may consist of 2 to 3 daily irrigation events of 1.5 to 2 hours.
The Federal Clean Water Act (FCWA) enacted in 1972 (U.S. Congress, 1997) required states to monitor the impact of nonpoint sources of pollution on surface and ground waters and to establish Total Maximum Daily Loads (TMDLs) entering impaired water bodies (Section 303(d) of FCWA). In 1987, Florida legislated the Surface Water Improvement and Management (SWIM) Act (Section 373.451 F.S., Florida Senate, 2009) to protect, restore and maintain Florida’s highly threatened surface water bodies. The 2001 Florida Legislature authorized FDACS to develop interim measures, BMPs, cost-share incentives and other technical assistance programs to help agriculture to reduce pollutant loads in target watersheds (Section 570.085 F.S., Florida Senate, 2009). Best Management Practices (BMPs) are specific, scientifically-based cultural practices, which are determined to be practical and effective in reducing pollutants from agricultural operations. In Florida, statewide agricultural BMPs had been adopted by rule (5M-8 Florida Administrative Code) for vegetables and agronomic crops (FDACS, 2006). By law, growers who voluntarily implement the BMPs will receive a “presumption of compliance” with state water quality standards and will be given a waiver of liability for costs and damages associated with reparation of contaminated surface and ground water resources. As a result, growers are encouraged to adopt and implement BMPs in their crop production.
The visualization of water movement in the soil can be achieved using a dye that moves with the water (German-Heins and Flury, 2000). The dye is injected at the beginning of the irrigation cycle, the soil is exacavated and the shape of the wetted zone (depth: the longest distance from the drip tape to the bottom of the blue dye width: the horizontal length perpendicular to the bed axis at the widest point of the wetted zone and length: the horizontal length parallel to the bed axis at the widest point of the wetted zone) can be measured (Simonne et al., 2006). Dye tests conducted throughout Florida have shown that the rate of vertical movement of water in drip-irrigated fields ranged from 0.6 to 0.9 inch/10 gal/100ft (Simonne et al., 2003, 2006, ). These dye tests have also shown that the rate of vertical water movement was generally greater than the rate of horizontal/lateral movement of water (Simonne et al., 2006). Hence, the potential for water moving below the root zone of vegetable crops grown in sandy soils with drip irrigation is high.
Despite efforts in implementing BMPs, it is possible that soluble nutrients (especially nitrate) move below the root zone when current UF/IFAS recommendations are followed. Ideally, a no-leach situation may be created if the flow rate of irrigation matches exactly the rate of crop evapotranspiration. This would require having drip tapes with emitters that could allow for very low flow (during the part of the day when crop evapotranspiration, Etc, is low) and that could change as ETc changes. Conceptually, ETc should be approached on an hourly basis rather than a daily basis, and irrigation rates should match hourly ETc. Currently, such technology is not available, and non-pressure compensating emitters are not available, and the vast majority of drip irrigation systems are operated at constant pressure. Most drip tape manufacturers guarantee flow rate and uniformity at a set pressure, for a maximum length of drip tape.
The emitter flow rate is empirically related to water pressure by q=kP^x ,where q is the discharge rate (volume/time), P is the water pressure (force/area), k is the emitter discharge coefficient and x is emitter discharge exponent (Thompson, 2003, Smajstrla et al., 2008). Lowering the operating pressure could be an option to achieve lower flow rates. Recent work by the industry on the use of low pressure drip irrigation (Dowgert et al., 2007) reported higher water use efficiencies with systems that are gravity-based and operate on pressures as low as 4-5 psi. Such systems are claimed to have lower flow rates that allow longer irrigation durations without generating runoff or deep percolation. The amount of water and fertilizer to be applied per crop could potentially be reduced using the low pressure system. This type of approach needs to be tested in Florida with vegetable crops. On the other hand, using low pressure has been known to result in poor uniformity of water application. However, documented effects of low pressure drip irrigation system on water movement patterns in the crop root zone and on crop growth and yield responses is lacking. This study was conducted to determine the effects of low irrigation pressure on the flow rate and uniformity of water application, water movement in the soil/crop root zone and on fresh market tomato growth and yield. Additionally, with the prospect of better water use efficiency, the use of reduced fertilizer rates and irrigation volumes was investigated.
The goal of the project is to assess the feasibility of minimizing nutrient leaching through irrigation management by reducing the irrigation operating pressure (OP) (and thereby flow rate) without reducing crop yields. The objectives are:
1. To establish flow rate and uniformity of water application under reduced pressure for three commonly used drip tapes.
2. To determine if reduced OP reduces the depth:width ratio of the wetted zone for three commonly used drip tapes.
3. To measure the effects of reduced N fertilizer schedules and irrigation water management on tomato nutrient status and marketable yields.
Objective 1. Effects on flow rate and uniformity of water application
The objective is to measure the changes in flow rate and uniformity when reduced pressure is used and to determine the longest possible drip tape run for the reduced pressure system. Two tests were conducted in the experimental field located at the North Florida Research and Education Center – Suwannee Valley near Live Oak, FL, in Fall 2009. Three drip tapes with 12-inch emitter spacing but different flow rates were tested (Tape A: EuroDrip, 0.16 gph, Madera, CA; Tape B: Netafim, 0.24 gph, Fresno, CA; and Tape C: John Deere Ro-Drip, 0.24 gph, San Marcos, CA) at the two pressures (12 and 6 psi), and at 100 and 300 feet lengths. Typical drip tape lengths were 300-500 ft long. The low-end 300 ft was chosen as the standard in comparison to a shorter length of 100 ft as reducing OP was expected to perform better on short lateral runs. The irrigation system consisted of a well, a pump, a back-flow prevention device, a 150-mesh screen filter, in-line pressure regulators of 6 and 12 psi (Senninger Irrigation Inc., Orlando, FLA) and drip tape. Drip tape was laid flat on top of the ground, without bed and without mulch. Pins were used approximately every 50 ft to keep the drip tapes in place. The 12 treatments (3 drip tapes x 2 OP x 2 lateral length factorial) were replicated four times in a split-plot (OP as main effect) randomized complete block design. The drip irrigation system was allowed to charge and to reach stable pressure according to treatments. The volume of water discharged in 10 mins from two emitters (Figure 1) at every 50 feet along the drip tape was measured and was used to calculate flow rate per emitter. Uniformity was assessed using parameters of emitter flow variation, qvar, coefficient of variation, CV and uniformity coefficient, UC, which were calculated according to Camp et al. (1997). Flow rates and uniformity parameters were analysed by SAS PROC Mixed (SAS, 2008).
Objective 2. Effects on water movement in the soil (size and shape of wetted zone)
This objective is to determine if there is greater lateral movement of water in the soil when the pressure is reduced, and how this is affected by duration of irrigation for three different drip tapes. Treatments included two OP (12 and 6 psi), and three drip tapes (A, B and C) in a split-plot (OP as main effect) randomized complete block design replicated four times with four irrigation durations (45, 90, 180 and 240 mins) per OPxtape treatment. The experiment was carried out in 2008 and 2009 using new tapes from a different roll each year.
Dye tests were conducted in raised plasticulture beds without plants. Before the day of the test, 28-inch wide plots were made and laid with drip tape and plastic mulch. The plots with the shortest irrigation times were located farthest away from the water source. Drip tapes between plots were connected by on/off valves. After pressurizing the system, about one gallon of blue dye (Terramark SPI High Concentrate, Prosource One, Memphis, Tenn.) was injected at 1:49 dye:water dilution rate for about 30 mins followed by irrigation according to treatment of 45, 90, 180 or 240 mins. At the end of each irrigation time, water flow was shut off for the plot. To balance the decreased length of the remaining tape, one additional drip tape of equal length was opened every time a section of drip tape was turned off. Transverse sections of the beds below selected emitters were dug by hand after each irrigation time to expose the dye pattern which was indicative of the wetted zone (Figure 2). Measurements of the maximum depth (D, vertical length from the top of the bed to the bottom of the blue dye) and width (W, maximum width of the blue dye) was taken. Depth and width responses to volume of irrigated water were analysed by regression using SAS PROC GLM (SAS, 2008).
Objective 3. Effects of reduced N fertilizer schedules and irrigation water management on tomato nutrient status and marketable yields.
The objective is to determine if low pressure drip irrigation is sufficient to meet crop water losses without affecting crop yields, and at the same time if reduced fertilizer and irrigation rates are feasible when combined in such a system.
Only drip tape C with flow rate of 0.24 gph was used at the two pressures (OP, 12 and 6 psi) to provide nitrogen (N) at three rates (100%, 85% and 60% of 200 lb/A) and to irrigate at two rates (IRR, 100% and 75% of 1000 gallons/acre/string/day). The 12 treatments were replicated four times in a completely randomized block design. The experiment was repeated in the Spring of 2008 and 2009, respectively. Fertilization was based on the results of Mehlich-1 soil test and followed current recommendations (Olson et al., 2009). Pre-plant fertilizers (N-P2O5-K2O 13-4-13, Mayo Fertilizer Inc, Mayo, FLA) at 50 lb/acre of nitrogen and potassium, respectively, were applied during bed preparation three weeks before transplanting. Two drip tapes were laid together in the middle of each bed for the independent application of irrigation water and fertilizer. Six -week-old ‘Florida 47’ tomato transplants were established on raised beds (Alpine-Foxworth-Blanton sand) with plastic mulch spaced 5-ft apart and 18-inch within row spacing for open-field tomato production (Figure 3)on April 30, 2008 and April 8, 2009 (Days after Transplanting, DAT = 0). The remaining N and K fertilizers were injected weekly through the dripline (Olson et al, 2009). Operating pressure was regulated by installing pressure regulators (Senninger Irrigation Inc., Orlando, FLA) for 12 and 6 psi. Current IFAS recommendation for nitrogen for tomato (seasonal application of 200 lb/acre) was used as the 100% N fertilizer rate, which was reduced to 80% (160 lb/acre) and 60% (125 lb/acre) for other N treatments. Ammonium nitrate (34-0-0, Mayo Fertilizer Inc, Mayo, FLA) was used to provide the required N. Potassium chloride (Dyna Flo 0-0-15, Chemical Dynamics Inc, Plant City, FLA) was injected weekly through the dripline as well to provide K. An irrigation rate of 1000 gal/acre/day/string commonly used in the industry was designated as 100% irrigation rate (IRR) and was reduced to 750 gal/acre/day/string for the 75% IRR treatment. The volume of irrigation water applied was controlled through timers (Orbit Irrigation Products Inc., Bountiful, UT). Water meters installed at the sub-mains of each OP treatment were read weekly and were used to monitor actual amounts of water applied. The duration of irrigation was progressively increased as greater irrigation volumes were needed during later growth stages and based on monitoring of soil moisture. Other cultural practices (staking, pest control) followed current IFAS recommendations for tomato production (Olson et al., 2009).
Plant nutritional status was monitored using petiole sap testing on 5, 6, 7, 8 WAT in 2008 and 5, 7, 9, 11 WAT in 2009. Ten petioles from most recently fully expanded leaves were sampled per plot, cut into half-inch long pieces and placed into a garlic press to extract sap to determine nitrate (NO3–N) and potassium (K+) concentrations using ion-specific electrodes (Cardy meter, Spectrum Technologies, Plainfield, IL) (Studstill et al., 2009). Tomatoes were harvested when 50% of fruits were at breaker stage on 69 DAT in 2008, and on 75 and 84 DAT in 2009. Fruits were graded as extra-large, large, medium and culls (USDA, 1991) and weighed and counted. Marketable fruit yields were calculated by adding the extra-large, large and medium grades. Petiole sap NO3–N and K+ concentrations, and total and marketable yield responses to the treatments were analyzed using SAS PROC GLM and treatment means were compared using Duncan’s multiple range test (SAS, 2008).
Objective 1. Effects on flow rate and uniformity of water application
For pressure-compensating emitters, x ranges from 0 to 0.1 (Thompson, 2003) and the flow rate is expected to change by less than 10% when the pressure drops from 12 to 6 psi. In this study, the interaction between OP and drip tape type was significant (p &lt;0.01 and the flow rates were significantly reduced by 32%, 23% and 38% for Drip Tapes A, B and C, respectively (Table 1). Flow rate was also significantly affected by drip tape length with reductions of 13%, 10% and 21% at longer lengths for Drip Tapes A, B and C, respectively. These results showed that the emitters of the drip tapes were not fully pressure-compensating and flow rates vary substantially with pressure and tape length.
Uniformity was not significantly affected by the reduced OP in all three drip tapes. Tape length had a significant effect on the uniformity of Tape B and C with greater variation (qvar and CV) and less uniformity at the longer length. All the drip tapes showed acceptable uniformity with UC &gt;90%. Drip tape type, probably due to differences in emitter design, is important in determining responses to OP and tape length. Hence, growers contemplating using low OP can do so simply by inserting a pressure regulator, but will need to determine the actual flow rate at the reduced OP. Reduced OP seems to be more appropriate for short runs as they allow for greater uniformity. This makes reduced OP a practice better suited for small fields, or for large fields with multiple zones and short rows. It would be helpful if manufacturers would provide uniformity and flow rates at reduced OP on the drip tape label so that growers do not have to determine them on their farms.
Objective 2. Effects on water movement in the soil (size and shape of wetted zone)
Depth response to volume of irrigated water was quadratic with D = 5.34 + 0.16V – 0.0007V2 (p&lt;0.01, R2 = 0.80) at 12 psi and D = 4.42 + 0.21V – 0.001V2 (p&lt;0.01, R2 = 0.72) at 6 psi (Figure 4A). The intercepts and regression coefficients for both lines were not significantly different which indicated that depth response to volume was not affected by OP. The depth of wetted zone went beyond the active crop rootzone of 12 inches when V was about 45 gal/100ft, which represented about 3 hours of irrigation at 6 psi and 1.5 hours of irrigation at 12 psi. These results show that reduced OP allowed extended irrigation without increasing the wetted depth.
The width of the wetted front also showed a quadratic response to volume of irrigated water with W = 7.58 + 0.17V – 0.0006V2 (p&lt;0.01, R2 = 0.77) at 12 psi and W = 6.97 + 0.25V – 0.002V2 (p&lt;0.01, R2 = 0.70) at 6 psi (Figure 4B). The regression coefficients of V and V2for both lines were significantly different from each other at p=0.05 and p=0.02, respectively. This means that for a fixed V within the tested V range of 9-108 gal/100ft, the increase in width at 6 psi was greater than at 12 psi. However, maximum W was reached at a lower V (V = 62.5 gal/100ft) for the 6 psi treatment at 14.8 inches, which was 53% of the 28-inch wide bed. At 12 psi, maximum W of 19.6 inches (70% of bed width) was predicted at V = 142 gal/100ft. For irrigation scheduling purposes, using a reduced OP of 6 psi can wet up to an area of about 7.5 inches on either side of the drip tape, and with complete emitter-to-emitter coverage at V = 25 gal/100ft (about 1.5 hours of irrigation). The width response was in agreement with Bar-Yosef and Sheikhoslami (1976) who reported greater lateral movement at a lower emitter discharge rate. Since the depth response for both OP treatments was the same, it implied that, at reduced OP, the wetted volume was greater and the soil moisture content per unit volume was lower. Due to the reduction in flow rate, reduced OP is a good irrigation management tool to extend irrigation duration without moving water and nutrients deeper into the soil. Growers need to know the flow rate achieved from the reduced OP so as to estimate the duration of irrigation. Also, as the maximum wetted bed width at reduced OP was narrower (about 53% of bed width in this study), reduced OP is more suited for a single row of plants if a single drip tape were used.
Objective 3. Effects on plant nutrient status and fresh market tomato marketable yields.
Because different amounts of N and irrigation were applied in 2008 and 2009, the responses of tomato petiole sap nutrient concentrations and yields were analyzed separately by years. Nitrogen rate did not affect the petiole sap NO3–N concentration although in general the concentration was higher with higher nitrogen rate. Similarly, operating pressure and irrigation rate treatments had no significant effect on sap NO3–N concentration. For all treatments, petiole sap NO3–N and K concentrations in all the growth stages were sufficient or exceeding for plant growth, which showed that crop nutritional status could be maintained at reduced OP and with reduced fertilizer and irrigation inputs. In 2008, total and marketable yields of the respective N and irrigation rate treatments were not significantly different (Table 2). OP, however, had highly significant effect on the total (p&lt;0.01) and marketable (p&lt;0.01) yields as well as the Extra-large and Medium tomato classes. With the 6 psi treatment, a significant 21% increase in total yield was obtained over the 12 psi treatment. In 2009, total (p=0.04) and marketable (p=0.04) yields were significantly higher at 100% N treatment. The effect of OP was also significant for the marketable yields (p=0.05) and for Extra-large and Large tomato classes, with lower yields obtained at 6psi. Irrigation rate had no significant effect on the yields in both years.
The yields obtained in 2008 were half of that in 2009 as the crop cycle was shorter with a single harvest in 2008. With the reduced yields and probably reduced water demands, the low pressure treatment was found to give significantly higher yields. In the early part of the season when plants were small, the low operating pressure (6 psi) in combination with reduced irrigation (75% recommended irrigation) and fertilizer rate (60% N recommended rate) would be a good strategy. Using reduced OP would extend the irrigation duration to better meet the uptake capacity of small plants and at the same time minimize the downwards movement of water into the soil. In this study, by four weeks after transplanting, the tomato plants at reduced OP were irrigated for four hours daily from 9-11am and 3-5pm so that plants were provided with near-continuous watering throughout the day. If the low OP strategy were adopted for the first four weeks of the season, a reduction in irrigation volume of 7% and in fertilizer usage of 15% could be achieved. As the season progressed, the reduced OP would not work as well possibly because the water output per unit of time was not sufficient to meet the higher crop water demand. During the spring in FL, daily ETc values of 0.16 to 0.24 inch/day are common (FAWN, 2009), it would be necessary to revert to the standard OP. An adjustable in-line pressure regulator could be installed in the drip system for the grower to easily change the irrigation operating pressure to adjust the flow rates at various growth stages.
These results suggest that reducing OP can be a practical tool to reduce water application and fertilizer rates without reducing yields. However, growers need to understand the connection between pressure and flow rate, and need to be informed of when a flow rate (established by reduced OP) becomes too low to meet crop water needs. Futhermore, uniformity of water application could be adversely affected when using an OP below the manufacturer’s recommendation so preferably short lateral runs of the drip tapes should be used with small irrigation zones controlled by each pressure regulator, which should be installed as close to the drip tapes as possible. Additionally, the risk of emitter clogging would be high for the low OP treatment so the water source should be of high quality and well-filtered and weekly chlorination of the drip system should be carried out.
Educational & Outreach Activities
- A drip irrigation workshop was conducted on November 5, 2009, using the dye test to demonstrate the movement of the water front in the soil. Some 20 growers attended the workshop.
Findings from the study had been shared with participants of the BMP Workshop in October 2009.
Results on movement of the waterfront in the soil had been presented during SR-ASHS in February 2010 with an abstract published and FSHS in June 2010 with a paper under review. Other results on fresh market tomato yields would be presented during ASHS in July 2010.
Use of the low 6 psi OP to create an ultra-low emitter flow rate is an innovative way for irrigation management. The results show that reduced OP did not severely affect the uniformity of the irrigation system at the tested lengths of 100 or 300 ft and it could be incorporated into the irrigation scheduling strategy, which could potentially reduce deep percolation and water and nutrient losses. It is unlikely that growers will switch the use of pressure regulators during the season. However, if pressure change becomes a part of the automation of irrigation and is linked to automated soil moisture measurement (not requiring grower intervention), then this practice is more likely to be adopted. Overall, these results suggest that reduced OP can be considered a BMP. However, these results also show that reduced OP alone is not enough to keep the water within the root zone, thereby reducing the risk of leaching – not completely eliminating it.
The initial capital investment for a 100-acre tomato drip irrigation system was estimated to be $67,400 (Pitts et al., 2002). An in-line adjustable pressure regulator could cost as much as $36 each. As reduced OP is more suited for small irrigation zones with short tape runs, if each acre were divided into four zones, 400 pressure regulators would be needed in 100 acres leading to an additional capital cost of $14,400 (an increase of 21% in initial investment). Current cost of inline adjustable pressure regulator is too prohibitive for ready adoption by commercial growers. Another possibility is to use two regulators with preset pressures of 6 and 12 psi at the sub-mains of each zone controlled by a valve that will divert water flow through the 6 or 12 psi pressure regulator. Such a pressure regulator costs about $10 each, and 800 pieces for 100 acres cost $8000, an additional 12% in initial investment, which could be more acceptable for growers, especially in light of potential savings in pumping and fertilizer costs (Table 3). Furthermore, growers would be more likely to adopt this technology if the additional investment could be cost-shared by the BMP program.
UF is considering including OP manipulation in irrigation recommendations. However, it is unlikely that growers will switch the use of pressure regulators during the season. However, if pressure change becomes a part of the automation of irrigation and is linked to automated soil moisture measurement (not requiring grower intervention), then this practice is more likely to be adopted. Growers’ acceptance would also be improved if the additional investment for the pressure regulators could be cost-shared by the BMP program. Overall, these results suggest that reduced OP can be considered a BMP. However, these results also show that reduced OP alone is not enough to keep the water within the root zone, therby reducing the risk of leaching – not completely eliminating it.
Areas needing additional study
- Testing effect on growth and yield and irrigation system functionality of having low OP at early plant growth stage followed by standard OP at later stages.
Determining the size of field that the low OP could be operated on without severely affecting flow rates and uniformity.
Measuring water movement in the soil at longer durations of irrigations at the low OP.
Field testing in a commercial farm of such an irrigation strategy incorporating low OP.
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