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IRRIGATION

Cyclic Irrigation of Turfgrass Using a Shallow Saline Aquifer

^a Dep. of Biol. Sci., Univ. of Nevada–Las Vegas, Las Vegas, NV 89154
^b Dep. of Environ. and Resour. Sci., Univ. of Nevada–Reno, Reno, NV 89557
^c Coop. Ext., Univ. of Nevada–Reno, Reno, NV 89557

^* Corresponding author (dev50{at}clark.nscee.edu)

Received for publication May 13, 2002.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
A 2-yr cyclic irrigation study using shallow saline groundwater was conducted on a sports field in southern Nevada [bermudagrass (Cynodon dactylon L. ‘Tifway’) overseeded with perennial ryegrass (Lolium perenne L. ‘Champion’)]. Shallow groundwater with a salinity of 3.3 dS m^-1 was substituted for municipal water (0.9 dS m^-1) at a rate of one, two, three, or four times per seven irrigation events during the peak water demand periods of 15 May to 15 October. Salinity sensors and tensiometers were installed at depths of 10, 25, and 40 cm and recorded weekly. Midday leaf xylem water potential, canopy temperature, and turfgrass color and cover ratings were taken on a bimonthly basis. Soil salinity cycled up (as high as 24 dS m^-1) and down (baseline values of 4.0–10.0 dS m^-1) in response to substitution periods. However, the duration in which soil salinity exceeded salt tolerance threshold values for bermudagrass were short in all treatments (<21 d during the 2-yr period at the 10-cm depth). These short durations of threshold values being exceeded combined with the successful return to baseline soil salinity values during the cyclic off periods (freshwater only) led to little change in turfgrass color, cover, and plant water status. Freshwater savings as high as 50 cm yr^-1 and a reduction in as many as 62 municipal irrigation days (days irrigations took place) during the peak water demand periods occurred. Results of this experiment indicate that the cyclic irrigation strategy is feasible in the urban setting with large turfgrass areas.

Abbreviations: EC_e, saturation paste electrical conductivity • ET, evapotranspiration • ET_a, actual evapotranspiration • ET_o, potential evapotranspiration • SAR, sodium adsorption ratio

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
LAS VEGAS, NV, is one of the fastest growing communities in the United States with as many as 6000 new residents each month. Based on the current rate of growth and with no new sources of freshwater available, conservation and the use of nonpotable water will be relied on to a greater extent to meet the increasing water demands of the community. It is estimated that as much as 60% of the municipal water is currently used for residential use and as much as 70% of that water is used outdoors, primarily in the irrigation of landscapes (K. Sovocool, Southern Nevada Water Authority, personal communication, 2002). One source of nonpotable water available in the valley is a shallow perched saline aquifer. This water (urban agricultural drainage water) exists because of the overwatering of urban landscapes by homeowners and because of highly stratified sediments of low permeability present in the vadose zone. Although the exact size of this shallow system is unknown, it has been conservatively estimated to be at least 12 300 ha-m (100 000 acre-ft) (Zikmund, 1996). Using this water as a supplemental irrigation source on landscapes would free up potable water for higher priority uses, lower water tables in contact with structural foundations, reduce the potential of contaminating the primary aquifer, and reduce the flow of shallow groundwater to the Las Vegas Wash and the Colorado River system. However, such water does contain a significant salt load. Salinity in the shallow system varies from 1.8 dS m^-1 in the central part of the valley to >10.0 dS m^-1 in the southeastern part of the valley (Dean 1996; Schaan 2001).

Many studies have demonstrated the feasibility of using saline water for irrigating agricultural and horticultural crops (Ayers and Wescot, 1985; Rhoades et al., 1989; Grattan, 1994; Dean et al., 1996; Leskys et al., 1999). Results from previous investigations into the potential use of the shallow saline aquifer in the Las Vegas Valley indicated that bermudagrass is well suited for irrigation with such water if threshold irrigation/potential evapotranspiration (ET_o) ratios are not exceeded (where ET_o is generated with an empirical-based equation such as the Penman Combination equation and threshold values relate to a well-defined point where turfgrass color and cover ratings decrease with declining irrigation/ET_o ratios) (Dean et al., 1996, 1998) and that irrigations are based on an evapotranspiration (ET) feedback system with adequate leaching and high uniformity of application (Leskys et al., 1999). However, Dean et al. (1996)(1998) concluded that this water should not be looked on as a sole source for irrigation purposes but should be considered as a supplemental source, especially at the higher salinity levels.

When at least two qualities of water exist for irrigating, blending or cyclic irrigation can be employed. However, Grattan and Rhoades (1990) point out the many advantages to selecting the cyclic irrigation option, which include: no blending facility would be required (soil dilution), more salt-sensitive plants could be included in the rotation, and soil salinity could be reduced at critical times of physiological growth. Turfgrass (bermudagrass overseeded with ryegrass in particular) is well suited for such an irrigation strategy when using saline water. Bermudagrass has been documented to be a highly salt tolerant species (Dudeck et al., 1983; Devitt, 1989; Devitt et al., 1990, 1993; Marcum and Murdoch, 1990), whereas the overseed and establishment period of the ryegrass is a time in which lower soil salinities are desired. This irrigation strategy in which poorer quality waters would substitute for potable water also coincides nicely with reducing peak summertime demand problems associated with the current water distribution system in the Las Vegas Valley. Final decisions must also be based on economic analysis, such as those conducted by Dinar et al. (1986), who studied the blending of saline and nonsaline waters for irrigation. If the saline water has a salinity level that is not excessive (<6.0 dS m^-1), if a decline in yield (turfgrass) is acceptable as long as no loss in color and cover occurs, and if the saline water is inexpensive relative to the potable water, conditions should be ideal for the partial substitution of saline water for potable water for irrigation purposes. The shallow saline groundwater in the Las Vegas Valley is classified as nuisance water, and its current price at the time of use is dictated only by pumping and distribution costs.

Although several field-scale studies have been conducted to test the feasibility of using the cyclic irrigation strategy with saline water on agricultural crops (Ayars et al., 1986; Grattan et al., 1987; Rhoades, 1989; Sharma and Rao, 1998), no research on the implementation of this strategy on turfgrass in the urban landscape has been published. The objective of this study was to determine the long-term effects of applying shallow saline groundwater using a cyclic irrigation strategy to a turfgrass sports field. In particular, the research was conducted to determine the optimum substitution rate, the resultant increase in soil salinity, the cyclic nature of the soil salinity, the physiological response of the turfgrass, yearly savings in freshwater, and the reduction in the number of freshwater irrigation days during the peak water demand months (where irrigation days are defined as days in which an irrigation occurs).

	MATERIALS AND METHODS

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A 2-yr cyclic irrigation study was conducted in the Las Vegas Valley on the practice football field at the University of Nevada–Las Vegas. The site had been planted with Tifway hybrid bermudagrass, which was overseeded in the fall of each year with Champion perennial ryegrass. The soil was classified as a McCarran fine sandy loam (coarse loamy, mixed, thermic typic Haplogypsid).

Before starting the experiment, a shallow well was drilled to tap the shallow saline aquifer. The well was drilled to a depth of 30.5 m, yielding approximately 130 L min^-1. Depth to the freestanding water table was 4.5 to 5.5 m. Water was pumped to an aboveground storage tank (38 000 L) to help offset the low yield and to provide a distribution system that would help minimize the possibility of cross contamination. A 11 190 J s^-1 (15 horsepower) booster pump was installed on the discharge line of the tank to deliver saline water to the field. An irrigation delivery line was laid parallel to the existing main irrigation line to facilitate delivery of shallow aquifer water from the storage tank. The new line from the tank was connected just beyond the existing valves (downstream) on all fresh lines and fitted with additional valves (Toro 252 Series 2, Toro Corp., Minneapolis, MN). The dual-valve system allowed freshwater and saline water to be cycled to the field using the existing irrigation system. All fresh and saline lines were fitted with 5-cm water meters to monitor irrigation volumes. Backflow prevention devices were also installed on both the fresh and saline irrigation lines to ensure that no cross contamination occurred. All irrigation valves in the field were connected to irrigation clocks so that accurate irrigation times could be scheduled.

Experimental plots (replicated twice) were 12.2 by 12.2 m. All plots were established within designated irrigation zones (separate irrigation stations) with enough buffer to ensure no irrigation overlap from adjacent zones (limiting the number of replicates possible). All plots were instrumented with salinity sensors (Soil Moisture Corp., Santa Barbara, CA) and tensiometers (Soil Measurement Syst., Tucson, AZ) at 10, 25, and 40 cm below the soil surface. All salinity sensors and tensiometers were inserted into undisturbed soil, placing the measurement point at a distance >30 cm from the wall of a meter box that allowed easy access. Tensiomters were read with a pressure transducer and filled on a twice-weekly basis. All measurements occurred on Thursdays; thus, the irrigation sequence before measurement varied by treatment. Irrigation distribution uniformities were assessed and improvements made such that all irrigation cells had Hart and Reynolds (1965) Christiansen uniformity coefficients (CUCs) >0.80.

An automated weather station (Weather Watch 2000, Campbell Sci., Logan, UT) was positioned at the site to monitor climatic variables. Rainfall, solar radiation, wind direction and speed, temperature, and relative humidity were recorded on an hourly basis. Hourly and daily ET_o estimates were made based on a modified Penman equation (Campbell Sci., Logan, UT).

All irrigations were based on imposing a 0.15 leaching fraction by setting irrigation volumes based on I = ET_a/(1 - LF), where I is the irrigation volume based on pressure volume time curves, ET_a is the actual ET estimated by multiplying ET_o with a crop coefficient (Kc; Devitt et al., 1992), and LF is the leaching fraction. Total irrigation volumes scheduled for each advancing time period used this ET feedback approach based on the previous seven-irrigation-event period (which ranged from 9–10 d during the substitution period). The plots were irrigated with saline water one, two, three, or four times per seven irrigation events (occurring during this 9–10 d period). Days designated for saline water received 100% saline water, whereas days designated for municipal water received 100% municipal water. Where possible, an equal spacing was made between the scheduled saline and municipal water (for example, four saline irrigations per seven irrigation events followed a saline–fresh–saline–fresh–saline–fresh–saline sequence). Personnel at the site had final approval on all irrigation scheduling, as sporting events required irrigations to be cancelled. Rainfall was included in the irrigation estimates; however, the amount occurring during the substitution periods was <5 cm yr^-1. The same irrigation approach was followed during the nonpeak demand period of 15 October to 15 May, except that no saline substitution occurred as all irrigations were completed with municipal water.

Turfgrass was mowed with a reel mower twice per week at a height of 2.5 cm. Clippings were allowed to remain on the field at all times. Nitrogen was applied as NH₄NO₃ at a rate of 2.44 g N m^-2 mo^-1.

Before the first (May 1997) and second salinization periods (May 1998) and at the end of the experiment, soil samples were taken using a 4.5-cm-diam. soil auger. Samples taken in 1997 were based on a 4 x 4 grid (2.44-m spacing between grid locations, 16 samples) that was set up within the plot at equidistant locations from the edges. Sampling in 1998 and 1999 was based on a 5 x 5 grid (2.03-m spacing between grid locations, 25 samples). Samples were collected at depths of 0 to 15, 15 to 45, and 45 to 75 cm, with an additional depth of 75 to 105 cm added in 1999. Additional soil samples were taken at the end of the experiment at depths of 105 to 135, 135 to 165, and 165 to 195 cm at the 2-2, 3-3, and 4-4 grid locations. Soil samples were dried and extracted using the procedures outlined by the United States Salinity Laboratory staff (1954). Soil solution extracts were analyzed for Na, K, Ca, Mg, CO_3, HCO₃, Cl, SO₄, and electrical conductivity (flame photometer, atomic absorption, titration, chloridometer, spectrophotometer, and conductivity bridge) (Black et al., 1983). However, only electrical conductivity and sodium adsorption ratios (SARs) of these soil extracts are reported in this manuscript. Water samples taken from the shallow well and the municipal irrigation line were analyzed on a monthly basis for the previously mentioned parameters and are reported as average values in Table 1.

Soil parameters measured bimonthly included surface soil moisture content (theta probe, Delta T Devices, Vernon Hills, IL) and bulk soil conductivity (EM38, Geonics, Mississauiga, ON, Canada), whereas soil salinity (salinity sensors, Soil Moisture Corp., Santa Barbara, CA) and soil matric potentials (tensiometer and tensimeter, Soil Measurement Syst., Tucson, AZ) were measured weekly.

The data collected were analyzed using descriptive statistics, analysis of variance (ANOVA), or linear and multiple linear regression analysis (SigmaStat, Chicago, IL). Multiple regressions were performed in a backward stepwise manner, with deletion of terms occurring when probability levels for the t test exceeded 0.05. Average treatment values were compared based on a least significant difference (LSD) generated from a mean square of the error term from the corresponding ANOVA.

	RESULTS

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Irrigation
The water in the shallow aquifer was a Ca–Mg–SO₄ dominated water (Table 1) that had a salinity level (3.28 dS m^-1) 3.8 times higher than that of the municipal water (0.86 dS m^-1). Because of the higher Ca–Mg levels relative to Na in the shallow aquifer, the adjusted SAR of the saline water was only 24% higher than that of the municipal water (4.7 vs. 3.8) and still within an acceptable range for irrigation purposes (Ayers and Wescot, 1985).

Irrigations based on the ET feedback approach paralleled the ET_o curve in a sinusoidal fashion as shown for the four-out-of-seven saline substitution treatment in Fig. 1 . Actual substitution rates during the peak demand periods were very close to the set saline substitution rates (actual percentage within ±3% of the set percentage; slight differences were due to site personnel overrides). Total irrigation amounts for the 2-yr period were 2.7 ± 0.2 m for all five irrigation treatments (slight differences were due to site personnel overrides). Because the total irrigation volumes were scheduled based on the previous seven-irrigation-event time period, an obvious response lag was observed during the ET_o rise period of the bell-shaped ET_o curve. Irrigation projections based on this previous time period did not adequately compensate for the continual rise in the ET_o (March, April, and May) and would suggest that a shorter time period during this ET_o rise period would be justified. However, during the ET_o declining period of the bell-shaped ET_o curve (August, September, and October), irrigations did adequately exceed ET_o such that total irrigation amounts exceeded total ET_a estimates for all treatments for the 2-yr period (2.7, 2.7, 2.8, 2.7, and 2.9 m of irrigation vs. 2.1 m of estimated ET_a). It should also be noted that the ET_o estimate of 1.6 m is significantly lower than previously published ET_o values for Las Vegas of 1.9 m. However, the weather station was situated in a large, irrigated mixed-landscape area near the research site surrounded by buildings and large trees that reduced wind speed significantly (3.0 vs. 1.9 m s^-1 for the 1.9- vs. 1.6-m ET_o) and prevented localized advection from occurring.

View larger version (24K): [in this window] [in a new window]   Fig. 1. A comparison of actual evapotranspiration (ETa), potential evapotranspiration (ETo), and irrigation for the four-out-of-seven saline irrigation treatment for the experimental period of 15 May 1997 to 15 May 1999.

View larger version (24K): [in this window] [in a new window]   Fig. 2. Change in soil salinity by depth increment for each saline substitution rate (May 1999 vs. May 1997). Saline substitution of one, two, three, and four out of seven irrigation events (0.00, 0.16, 0.30, 0.46, and 0.57).

Soil salinity was also assessed over time using salinity sensors at the 10-, 25-, and 40-cm depths. Average sensor values in dS m^-1 plus standard deviations are reported in Table 3 based on the on–off periods of saline substitution (where on is defined as a saline substitution period beginning and off as a saline substitution period ending; during off periods, only freshwater was used). Statistical analysis (ANOVA) indicated significant differences (P < 0.05) based on time, treatment, and treatment x time interactions for all depths (with the exception of time for the 40-cm sensors). These differences were also somewhat complicated by the differences in initial soil salinity (Table 3, 1 and 3 substitution plots had higher initial salinity than the 2 and 4 substitution plots, especially at the 25- and 40-cm depths). However, because sensors are only a point measurement and each plot had two sensors per depth, we relied on soil sampling to assess salt buildup and sensor measurements to assess the cyclic nature of soil salinity based on the on–off periods and different substitution rates.

View larger version (40K): [in this window] [in a new window]   Fig. 3. Average salinity sensor values for the one-out-of-seven saline substitution treatment for the experimental period of 15 May 1997 to 15 May 1999, with a soil solution salinity threshold of approximately 13.8 dS m-1 (salinity at which yield would be expected to decline).

View larger version (31K): [in this window] [in a new window]   Fig. 4. Average matric potentials vs. average soil salinity (salinity sensors) for the one-out-of-seven saline substitution treatment at the 40-cm depth for the experimental period 15 May 1997 to 15 May 1999.

Sodium adsorption ratio values in the saturation extracts ranged from 0.4 to 14.1, with average values for all plots below 5.0 (Table 4). The number of samples required to estimate the mean depth-weighted SAR for the 0- to 15- and 0- to 75-cm depth within 10% at the 95% confidence level was calculated (0–15 cm: 108, 21, 42, 11, and 35; 0–75 cm: 253, 137, 145, 57, and 37 for the one-, two-, three-, four-, and zero-out-of-seven saline substitutions, respectively). Based on the 50 soil samples taken per depth in each treatment, sample requirements were met in all treatments except the one-out-of-seven treatment in the 0- to 15-cm depth. However, the sample-number requirement in the 0- to 75-cm depth was only met in the control treatment.

View larger version (28K): [in this window] [in a new window]   Fig. 5. Average midday leaf xylem water potential for all treatments for the experimental period 15 May 1997 to 15 May 1999.

Turfgrass color ratings for all plots at the end of the experiment were >9.5, with statistical separation based on treatment (P = 0.05); however, this separation was based on a 0.1 rating difference. Turfgrass cover percentages were unaffected by the treatments, with all plots having cover percentages of 99%.

Freshwater Savings
The average numbers of freshwater irrigation days saved during the peak demand months of 15 May to 15 October were 16.5, 32.0, 49.0, and 61.5 d for the one-, two-, three-, and four-out-of-seven saline substitution treatments, respectively. As the number of saved freshwater irrigation days increased, the quantity of freshwater savings increased from 11.1 to 49.8 cm (Fig. 6) . Large increases in water savings were associated with small increases in soil salinity (1.5 dS m^-1). At the same time, color ratings and leaf xylem water potentials remained virtually unchanged.

View larger version (29K): [in this window] [in a new window]   Fig. 6. Number of irrigation days and quantity of freshwater saved per saline treatment per peak demand period (15 May to 15 October).

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

Although soil salinity assessed with sensors represents single-point locations in the field and only two sensors per depth per treatment were included in this study, changes over time were still valuable in assessing the impact that cyclic substitution during designated on–off periods had on baseline soil salinity. Switching from a cyclic strategy with saline water to a noncyclic strategy with freshwater during fall and winter months successfully returned soil salinity to initial baseline values in all treatments, critical for the successful overseed and establishment of the ryegrass. The sensors were also valuable in assessing the impact that soil drying had on fueling greater oscillations in soil salinity (Fig. 4). A similar response was reported by Rhoades (1972), suggesting that the timing and amount of irrigation water relative to the plant water requirements is critical in controlling rapid rises in soil salinity and osmotic stress. Even though soil salinity exceeded biomass threshold values for bermudagrass during the cyclic on periods, the durations were short in all treatments (<21 d during the 2-yr period at the 10-cm depth). These short durations of threshold values being exceeded combined with the successful return to baseline values during the cyclic off periods led to little change in color, cover, and plant water status. These results would suggest that the sensors could be used as a valuable feedback tool before soil sampling to make changes in substitution rates, duration of on–off periods, and leaching fractions.

If the cyclic irrigation strategy is to be used by turfgrass irrigators, they must rely not only on accurate ET_o estimates, but also on the availability of suitable crop coefficients. However, crop coefficients by definition are derived under nonstressed conditions. Results derived from previous work by Devitt et al. (1990) would suggest that bermudagrass ET would remain unchanged (high frequency, 1.5, 3.0, and 6.0 dS m^-1 irrigation water in three contrasting soil types) even though cumulative yield clippings would decline over a depth-weighted soil salinity (EC_e) range up to 15.0 dS m^-1. More research is needed to better understand the range in soil salinity that crop coefficients would still be suitable. However, with salt-tolerant species such as bermudagrass, one would expect the range to exceed the threshold value for biomass. Because sports turfgrass fields are cut multiple times per week, a difference in cumulative clipping biomass on ET would not be as significant as a loss in percentage cover and/or stand density, neither of which were altered in this experiment or in the work by Devitt et al. (1990), suggesting that crop coefficients generated under nonsaline conditions could still be applicable as soil salinization occurred. Obviously, there must be an upper limit of soil salinization for crop coefficients to still be suitable, at which point reclamation would be required. All of this underscores the need for turfgrass managers utilizing poor quality water for irrigation purposes to maintain a scientific approach to management.

Finally, water savings as high as 50 cm yr^-1 is significant when one considers the thousands of hectares of potential turfgrass sites on schools, parks, golf courses, and roadways in southern Nevada. Based on the results of this experiment, higher substitution rates may be feasible, leading to even greater water savings. Utilization of this water also minimizes the negative effect this water has on foundations and other structures. The shallow groundwater below the swimming pool on the campus of the University of Nevada–Las Vegas must be pumped continually to prevent the pool from shifting due to the rising water table. However, during the period in which water was pumped from the shallow groundwater system for this experiment, no pumping was required by the maintenance crews. The cyclic strategy also freed up as many as 62 irrigation days during the peak demand months of May through October (days that municipal water was not required for irrigation). If significant turfgrass hectarage was converted to this irrigation strategy, this could provide greater flexibility in meeting peak water demands during summer months without the costly expansion of the water delivery system.

	ACKNOWLEDGMENTS

 
We thank the Las Vegas Valley Water District for financial support. We also thank Mr. Jeff Andersen and Ms. Lesa Cox for their assistance in both the field and laboratory. Research was supported in part by Nevada Agricultural Experiment Station (Publ. no. 5202343).

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 

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