Turfgrass Tolerance to Salinity
2.4. TURFGRASS TOLERANCE TO SALINITY
The future for irrigating turf may rely on the use of moderate- to high-salinity water and, in order to ensure that the turf system is sustainable, will rely on the use of salt-tolerant grasses and an improved knowledge of the effects of salinity on turfgrasses.
High levels of soluble salts in the turf rootzone are detrimental to most turfgrasses. Excess soluble salts can affect growth by osmotic inhibition of water uptake (physiological drought) by the specific ions (Harivandi et al. 1992). Salinity affects different species in different ways and the effects can vary according to the age of the plant: salinity effects are generally greater at germination and planting (when vegetative material is used) than in the mature plant. Salinity tolerance in turfgrasses is related to the plants' ability to reduce NaCl uptake.
A number of studies to investigate salt tolerance in turfgrasses and the mechanisms affecting salt tolerance have been undertaken. Younger et al. (1967) observed significant variation in the salt tolerance of creeping bentgrass (Agrostis spp.) varieties. The main effect of high salinity was the reduction in top growth; the old variety 'Seaside' had the highest salt tolerance and 'Penncross' the lowest. It was noted that 'Seaside' had high variation between individual plants and Engelke (pers. comm.) has selected new varieties (e.g. 'Mariner') with improved salt tolerance and turf quality based on this variation. McCarty and Dudeck (1993) reported that when germinating bentgrasses in high-salt solutions, 'Streaker' red top and 'Seaside' creeping bentgrass were the most salt-tolerant. 'Kingston' velvet, 'Exeter' colonial and 'Highland' colonial had intermediate tolerance while 'Pennlinks', 'Penncross' and 'Penneagle' creeping bentgrass were the most salt-sensitive. Marcum (2000) has studied the salt tolerance in the modern bentgrass varieties. He tested 35 bentgrass cultivars, with increasing salinity concentrations from 1 decisiemans/metre/day up to 8 decisiemans/meter/day at which time data was collected. The most salt tolerant cultivars were Mariner, Seaside II, Grand Prix, Seaside, 18th Green and Century. The least tolerant cultivars suffered complete death after ten weeks exposure and they included Avalon (velvet bent) Ambrosia (colonial bent) as well as Regent, Putter, Penncross and Penn G-6.
Dudeck and Peacock (1993) carried out a study on warm season grasses and demonstrated that 'Emerald' zoysiagrass (Zoysia spp.), FSP-3 Seashore paspalum (Paspalum distichum) and 'Tifway' couchgrass (Cynodon dactylon × C. transvaalensis) were the most salt-tolerant. 'Floralawn' St Augustinegrass (Stenotaphrum secundatum), 'Tifway II' couchgrass (Cynodon dactylon × C. transvaalensis) and 'FSP-1' Seashore paspalum had intermediate salt tolerance while Centipedegrass (Eremochloa ophiuroides) and Bahiagrass (Paspalum notatum) were very salt-sensitive. Dudeck and Peacock (1993) also demonstrated that as salinity increased, plant K levels decrease and to a lesser degree there is a decrease in Ca, Mg and P.
Duncan and Carrow (2000) have demonstrated that some selections of Seashore paspalum can tolerate undiluted seawater under the correct management regimes. Seawater has an EC of 54 dS/m (34 560 mg/L) and these new salt-tolerant varieties provide an opportunity to use very brackish sources of water though a high level of management is required.
Salinity effects on turfgrass growth have been summarised by Harivandi et. al. (1992) as:
- reduced water uptake due to osmotic stress;
- reduced nutrient uptake-for example, K may be depressed by absorption of Na;
- root biomass may increase to improve water-absorbing ability; and
- Na and Cl reduce growth by interfering with photosynthesis.
Harivandi et al. (1992) have also listed the common turfgrasses and their estimated salt tolerance (Table 2.9).
2.5 MANAGING HIGH SALINITY WATER
The future management of golf courses may be dependent on the use of lower-quality and higher-salinity water. While the use of salt-tolerant plant species and varieties may increase the viability of using high-salinity water, it is also essential that the golf course manager has a good understanding of complete soil/turf/drainage system to ensure long-term sustainability.
If water of high salinity is the only available water supply, several management techniques can be used to minimise salt damage. These are described below:
- Establish salt-tolerant species and varieties of turfgrasses. Establishment will most likely have to be done using a freshwater source
- Construct the greens and tees using high drainage rate sands that meet the USGA specification (1993) for greens construction and include a subsoil drainage system to ensure that leaching of salts occurs.
- Ensure that irrigations are sufficient to leach salts out of the rootzone and prevent accumulation but do not leach pollutants into groundwater The amount of water required for leaching, when rainfall is not sufficient for leaching, can be calculated:
Leaching requirement = ECiw
where ECiw is the electrical conductivity of the irrigation water and ECdw is the electrical conductivity of the drainage water (VIRASC 1980). One method of calculating the leaching requirement is to assume that the concentration of the drainage water is the same as that of the saturation extract (ECe) at the bottom of the rootzone. An appropriate value can be chosen from table 2.4 and then the calculation made.
For example, if Creeping bentgrass (Agrostis stolonifera) has an ECe of 3 - 6 dS/m and the irrigation water is 2 dS/m (say, 1400 mg/L) the leaching requirement is 33 - 66%. That is, the amount of irrigation required is 33 - 66% greater than if low salinity water is used.
The DNRQ (1997) has produced a water facts sheet (DNRQ97089) on salinity that includes the salt tolerance of a wide range of ornamentals that are useful for non-turf and landscaped areas.
- To avoid short-term high salt concentrations, do not allow excessive drying out of the soil.
- Maintain adequate soil permeability through subsoil aeration and thatch control.
- Irrigate at night to avoid salt burn.
- Irrigate with freshwater whenever possible to aid leaching.
- Conduct soil analysis to monitor soil soluble salt and cation levels. Adjust as required, e.g. apply gypsum to counteract Na accumulation.
- Maintain adequate nutrient levels including K, Ca, Mg and P.
- Construct on sandy soils whenever possible.
- Install subsoil drainage into low-lying and poorly drained areas that are likely to accumulate salts.
2.6 IMPROVING IRRIGATION EFFICIENCY
Water is a valuable resource that is shared with the entire community and therefore must be managed responsibly and used efficiently. Irrigation is a considerable cost on all golf courses, whether it is the direct cost of the water supply or the cost of pumping it. Irrigation systems can be inefficient for several reasons.
| Factors affecting irrigation efficiency include:|
These all contribute to the ineffective application of water and uneven watering. As a result, the system will be operated to pick up dry areas, which in turn will result in the over-watering of other areas. This not only wastes water, it also results in a turf of uneven health and quality as a playing surface.
The performance and management of the irrigation system must be evaluated on a regular basis (Connellan 2000). This includes both the performance of the irrigation system (i.e. the mechanics of the system and how uniformly water is applied) and how well the system was managed over the irrigation season (i.e. the amount of water applied compared to the amount that should have been used).
2.6.1 Irrigation management indicator
The quantitative measure of how much water is applied versus the demand can be used both as a post-mortem of the water use for the previous season and as a prediction of the expected water use for the season ahead. The irrigation index is a seasonal performance indicator that can be used to compare the amount of water actually used versus the estimated quantity required (Connellan 2000). It is expressed as follows:
Irrigation Index (I) = Water applied to site
Estimated water required
The amount of water applied can be easily determined from total water consumption at the site and the size of the area being irrigated. To assist in this process, regular meter readings should be taken. Modern pumping and control equipment will also provide this information.
The estimation of water required or plant water use is the estimation of the amount of water that should have been used by the site over a particular period or season and is somewhat more involved; however, the basic information is readily available.
Plant water use or evapotranspiration (ET) can be calculated using local climatic data and, in particular, evaporation from an A-pan evaporimeter.
Plant water use (ET) in mm = Epan × Crop Factor (CF)
The value of CF will vary depending on the turf type, available soil water, management practices and, most importantly, the quality of turf required. In the following examples, crop factors of 50%, 60% and 70% have been used for couchgrass (Cynodon spp.). A higher CF is used for putting greens than for fairways, where the CF could be as low as 30%.
Rainfall needs to be factored into the water requirement equation and, most importantly, the amount of rainfall actually used or available to the plant, i.e the effective rainfall. The effective rainfall is that proportion of the rainfall that is used by the plants after all the rainfall losses have been taken into account. The main factors to consider are:
- Rainfall in excess of soil storage capacity is lost through drainage beyond the rootzone.
- Rainfall intensities greater than the infiltration rate of the soil will result in some runoff.
- Very small amounts of rainfall will add very little water to the rootzone due to losses by evaporation from the turf surface. Rainfall of less than 2 mm can be ignored.
The irrigation requirement to satisfy plant requirements can then be expressed by:
Net Irrigation Requirement (IR) in mm = ET - Effective Rainfall
Due to inefficiencies, the sprinkler system needs to apply more water than the estimated irrigation requirement (IR). Some water is lost due to wind drift and evaporation, some may drain below the rootzone and there is always unevenness in application. The system efficiency, which accounts for the losses, can range from 50% to 90%, with a minimum acceptable efficiency of 80%. The water required can then be calculated as follows:
Water required in mm = Net Irrigation Requirement (IR)
System Efficiency (use 75% as a minimum)
The Bureau of Meteorology has an excellent website (www.bom.gov.au) where climatic averages are available for various locations. This information can provide a very good pre-season predictive model. Appendix 1 provides example tables of rainfall and evaporation for various New South Wales locations.
As an end-of-season assessment of irrigation efficiency or on a month-by-month assessment, the irrigation index should be calculated and ideally will be about 1.0.
2.6.2 Sprinkler uniformity performance
It is not possible to achieve efficient irrigation if the water is not applied uniformly. The 'catch can' test is used to determine sprinkler uniformity. Cans are placed at regular sprinkler intervals within the sprinkler pattern and the system is then run for sufficient time to ensure that a measurable amount of water is collected (figure 2.2). The preferred measure of uniformity for turf is DU (Connellan 1997).
The DU places emphasis on areas of turf that receive low amounts of water and is calculated by comparing the average of the lowest 25% of can readings to the overall average. The equation is as follows:
DU (%) = Lowest 25% of readings × 100
Average of all readings
A DU greater than 85% is considered acceptable for turf sprinkler systems. The can test not only provides information on system uniformity, it also gives a precipitation rate in millimetres/hour. Too often, irrigation is scheduled on run times in minutes, rather than the amount of water applied.
Irrigation is a relatively complex task, with numerous factors affecting the efficiency of water use. The following is an example of a checklist that could be developed to provide a quick guide to some of the tasks that need consideration.
2.6.3 Irrigation control
An efficient irrigation system is one that has been competently designed, incorporates quality hardware, is installed correctly and is managed according to plant water needs and soil conditions. The crucial link between the irrigation hardware and the achievement of efficient water use is effective control (Connellan 1995).
The requirements of an automatic irrigation controller have been described by Peasley (1992) and are summarised below.
(i) Basic requirements
- To operate reliably when required and to complete the planned schedule completely without fault.
- To apply the correct depth of water to the irrigation area during every scheduled irrigation and throughout every irrigation season.
- To apply the correct depth of water at the correct rate to suit the soil infiltration rate and the crop requirement.
- To be easy to operate.
(ii) More sophisticated requirements
- To apply the correct depth of water to the irrigation area as efficiently as possible by optimising:
the plant/moisture synergy that causes healthy growth;
the design of practical watering schedules;
uniformity of application;
labour costs; and
- To provide specialised functions for specific management tasks such as:
scheduling based on moisture monitoring;
scheduling based on weather station monitoring;
multiple repeat cycle or pulse irrigation;
historical record keeping and statistical analyses;
heat stress suppression;
soil electrical conductivity (salinity) monitoring;
frost control by temperature monitoring;
fertigation metering; and
burst pipe isolation.
On golf courses, the irrigation control system is typically a Master/Satellite local network control system that features a central computer, controlling a multitude of satellites by various methods of communication-two-wire cable, radio, microwave etc. These systems can be interfaced with soil moisture sensors, weather stations, pumping plants etc.
In terms of the controller satisfying the requirements of the golf course it must have the following characteristics (in order of importance):
- ease of programming
- sensor inputs
- program performance
- recording capability
- alarm facility
- remote communication.
Quality irrigation systems that aim to achieve efficient water use should incorporate some form of feedback into the control process (Connellan 1995). Two basic approaches can be taken: (i) use of soil moisture sensors, and (ii) use of predictive plant water use models.
2.6.4 Soil moisture sensors
Turfgrasses obtain their water needs from the soil and it makes sense to monitor the changing soil moisture status as a method of irrigation scheduling. Soil moisture sensors that are accurate and reliable can provide real-time information on the soil moisture status. They are particularly useful in providing feedback on the effectiveness of an irrigation or rainfall event and at what point the dry-down cycle is as a means of determining when the next irrigation is likely to be necessary. Soil moisture sensors also have the capacity to provide additional information such as soil nutrient status (by EC) and soil temperature. Trials carried out by Krieg (1994) demonstrated that using soil moisture sensors could reduce water use by up to 50% compared to irrigating by observation/experience.
Different types of soil moisture monitoring devices use different methods of determining soil moisture content. These include gypsum or porous ceramic blocks and tensiometers, neutron probes, capacitance probes and heat pulse soil moisture sensors.
Independent trials undertaken by the Australian Irrigation Technology Centre (AITC) tested the performance of twelve different environmental devices at various sites around Australia (AITC 1996). They included rain switches, evaporimeters and soil moisture sensors. The trial demonstrated a wide range of performance and the importance of the correct installation of this equipment. Two products, a small plastic evaporimeter ('Aquamiser') and a differential dehydration tension type soil sensor ('Watermatic'), demonstrated good performance.
In Western Australia, nine types of sensors were evaluated for use on irrigated horticultural crops on sandy soils (Luke et al. 1994). Of the sensors evaluated, the 'Watermatic', 'Hydroprobe', 'Enviroscan' and 'Loktronic' performed reliably, in accordance with the manufacturers' instructions. Moller et al. (1996) demonstrated the effectiveness of using the 'Enviroscan' for scheduling irrigation with up to a 60% reduction in the requirement for irrigation on warm season grass sportsfields. In the trials undertaken, they also detailed the potential cost-effectiveness of adopting a managed irrigation regime (table 2.10)
The use of 'Watermatic' sensors has also demonstrated considerable savings in fertiliser by minimising leaching losses on sandy soils. Neylan and Robinson (1995) reported that with irrigation control by observation an application of soluble fertiliser was leached out of the rootzone within 5 days of application, whereas under sensor control the fertiliser lasted up to 25 days.
The key requirements for soil moisture sensors (Connellan 1995) are that they:
- operate effectively in a wide range of soil types;
- respond accurately and rapidly to changing soil moisture conditions;
- have simple calibration;
- operate in confined rootzones;
- are easily installed and provide a good soil/sensor interface;
- have output signals that are compatible with irrigation controllers and/or computers;
- are electrically and electronically sound and reliable;
- have long-term wetting and drying reliability;
- are of robust construction; and
- have minimum ongoing maintenance requirements.
The efficiency of irrigation management on golf courses in New South Wales is unknown, though many golf courses have the equipment to provide a high level of water use efficiency.
2.6.5 Control prediction models using weather stations
The use of predictive models to calculate plant water use has been described in section 2.6.1; however, with the introduction of on-site weather stations, localised predictive models can be used to schedule irrigation. The main advantage of the on-site weather station is the climate data will be more site-specific than Bureau of Meteorology data, which is likely to be from a more remote location.
The accuracy of this technique is dependent on the quality of the mathematical expression used to calculate the evapotranspiration rate (ET) and the quality of the climatic data used in the calculations. Generally, the accuracy will increase with the number of parameters measured, the frequency of the readings and the accuracy of the readings.
The most significant advance in irrigation pumping technology for golf courses has been the introduction of the variable frequency drive (VFD) (Brockway 1997). In the USA, VFD systems represent 80% of the pump stations sold to golf courses. VFD stations vary pump speed to meet flow demands, whereas with the fixed speed system the pump operates at a fixed speed and a pressure-reducing valve constantly opens and closes to maintain a constant irrigation pressure as the flow changes. It is common for fixed speed booster pumps to operate at 20-50 PSI more pressure than is required.
The VFD station uses a pressure transducer to relay pressure information back to the VFD. As the flow demand increases, output pressure decreases. The VFD senses this and increases motor speed to increase pressure. Brockway (1997) provides a cost analysis of the VFD system, described below:
- Initial cost: VFD booster systems typically cost 20-35% more than a comparable fixed speed system.
- Operating cost: VFD systems generally reduce electricity costs by 20-50%.
- Maintenance costs: Over the life of the pump system, the maintenance cost of a VFD station should be less than that of a fixed speed station. However, a poorly maintained system can be very expensive to repair.
- Smoothness of operation: A VFD system will be much softer on the piping system. The gradual 'ramping up' to speed and continual speed modulation minimise water hammer and pressure surges.
- Complexity: Basic VFD controls are simpler than those of fixed speed stations. VFD stations are simpler to calibrate, although the electronics can be quite sophisticated.
- Susceptibility to lightning and power surges: High-quality surge protection devices have been developed that protect VFDs in the golf industry.
- Headache factor: VFD systems overall absorb less maintenance and repair time than a comparable fixed speed system.
Overall, the operational goal of a pump station manufacturer is to 'build a pump station that reliably and efficiently sequences pumps to provide variable flow rates at a constant discharge pressure. Pumping systems should eliminate air and offer sufficient alarms and shutdowns to protect the integrity of both the pump station and irrigation system'.
| CASE STUDY: PUMP REPLACEMENT BENEFITS
In the eco-efficiency survey at Horton Park Golf Course it was recommended that a review and upgrade of the pumping system be undertaken. It was proposed to install four 5 kw pumps to replace the existing 21 kw pump so there was greater flexibility in operation, depending on the water demand. For this particular site it was estimated that there would be a power saving of 29 200 kwh.
Cost of multistage pump $22,000
Other than the power savings, there is less wear and tear on the pumps, irrigation pipework and sprinklers.
2.6.7 Water audits
An irrigation audit is a critical first step in improving irrigation efficiency. The irrigation audit determines the overall condition and effectiveness of the irrigation equipment and its operation.
Some of the problems that may be identified in undertaking an audit include:
- poor sprinkler uniformity;
- leaks (e.g. valves, pipework, sprinklers);
- inadequate operating pressure;
- malfunctioning valves;
- sunken sprinkler heads;
- incorrect nozzles;
- incorrect rotation of sprinkler heads,
- inadequate control system;
- broken casings and missing parts;
- distorted spray distribution;
- broken seals; and
- tilted irrigation heads.
The Irrigation Association of Australia (IAA) offer a Water Audit training course designed for hands-on operators that provides them with the skills to evaluate turf irrigation systems.
| IRRIGATION ASSOCIATION OF AUSTRALIA (IAA) - WATER AUDIT TRAINING|
Undertaking an irrigation system audit and documenting the findings benchmarks the current condition of the system and allows decisions to be made on what needs to be done to improve the operational efficiency. This then allows appropriate budget allocations to be made.
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Appendix 1: Examples of long term monthly evaporation and rainfall data for various locations in NSW