Reducing nutrient loss below the root zone of drip-irrigated vegetables using low-pressure, increased irrigation time

Project Overview

Project Type: Graduate Student
Funds awarded in 2007: $9,966.00
Projected End Date: 12/31/2010
Grant Recipient: University of Florida
Region: Southern
State: Florida
Graduate Student:
Major Professor:
Bee Ling Poh
University of Florida

Annual Reports


  • Vegetables: tomatoes


  • Crop Production: crop rotation, nutrient cycling, tissue analysis
  • Education and Training: demonstration
  • Soil Management: soil analysis


    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<0.01, R2 = 0.80) at 12 psi and D = 4.42 + 0.21V – 0.001V2 (p<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<0.01, R2 = 0.77) at 12 psi and W = 6.97 + 0.25V – 0.002V2 (p<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.

    Project objectives:

    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.

    Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.