Putting Green Root Zone Amendments and Irrigation Water Conservation
Ed McCoy and Kevin McCoy
School of Natural Resources
Ohio State University
Photo 1. The field study site showing the contrasting root zones of the experiment.
Peat and soil amendments increase the available water capacity (AWC) of sand based root zones, but to translate this into irrigation water savings, an irrigation scheduling protocol that uses AWC information must also be adopted by the superintendent. Thus, to explore the extent of water conservation that can be realized from using an amended root zone, we combined field based AWC data from a range of root zones with a deficit-based irrigation protocol to calculate the depth of irrigation required and number of irrigation events as influenced by a 10% peat or a 10% peat + 10% soil amendment to sand. Further, this estimation was conducted for a 20-year period using weather data generated for 6 diverse locations in the U.S. Using this calculation, savings in irrigation depth from using a peat amended root zone as compared with pure sand ranged from a modest 4% in Phoenix, AZ to a considerable 24% in Columbus, OH. Savings from amending pure sand with peat + soil ranged from 7% in Phoenix to almost 40% in Columbus. Irrigation event reductions, on the other hand, were considerable at all locations ranging from 30 to 60%.
Index Words: Irrigation Conservation, Putting Green Root Zones, Soil Amendments
Peat and soil are commonly used amendments in high sand root zone mixes for putting greens. Extensive research has shown measurable increases in water and nutrient retention from the addition to a specified sand of modest quantities of peat, soil, or both (Waddington et al., 1974; Brown and Duble, 1975; Taylor and Blake, 1979; Whitmyer and Blake, 1989; McCoy, 1992). For these high sand content mixes, the increased water retention delays the onset of injurious drought conditions between irrigations and the increased nutrient retention maintains a stable supply of nutrients to the turf between fertilizer applications. In a sense, these amendment materials provide a physical and chemical buffering capacity to sand to assist in the establishment and management of the turf. Consequently, increasing the available water capacity (AWC) of a sand based root zone through use of amendments would rationally provide a means of irrigation water conservation.
Yet, employing an amended root zone alone will not result in irrigation water savings. A golf course superintendent must also adjust irrigation practices, specifically using a protocol that employs available water information, and adjust irrigation accordingly. A widely recognized irrigation scheduling protocol that employs soil available water information is deficit-based irrigation (Water Management Committee of the Irrigation Association, 2004). Deficit-based irrigation employs rainfall and evapotranspiration (ET) information together with estimates of available water capacity within the root zone to schedule the frequency and amount of irrigation. The procedure can be used with regional, monthly mean values of daily rainfall and ET; or, when a local weather station is available, the procedure can be fine tuned to use actual daily rainfall and ET measurements. Thus, the potential for water conservation using a root zone amendment together with deficit-based irrigation practices clearly exists.
This study was conducted to quantify irrigation water savings that could be realized by employing peat alone, or both peat and soil as amendments to a high sand content putting green root zone; and by employing a deficit-based irrigation protocol. In addition, climatic conditions that generate rainfall and control ET vary greatly across the U.S., with time of the year, and reflect year-to-year variability. Thus, estimates of water savings due to amendment use in root zones must employ a wide range of locations, all seasons of the year and span a sufficient period of time to address year-to-year variability. For this reason, long-term weather data from diverse regions of the U.S. were employed in the water savings estimation.
Photo 2. The occurrence of foot-printing on the experimental greens. At this point it was presumed that the turf had depleted the AWC reservoir of the putting green root zone.
Root Zone Available Water Capacity
Central to a water budgeting using deficit-based irrigation is an estimation of available water capacity (AWC) within the rooting depth. Yet, the standard definition of available water published in textbooks and used in irrigation scheduling does not appear to be appropriate for a putting green system. Principally, the standard definition given as the volume of water retained in the soil from field capacity to the permanent wilting point, does not address the fact that a superintendent would apply irrigation long before the permanent wilting point is reached. Also, this definition is based on laboratory measurements of a soil sample and does not consider the layering of soil media characteristic of a modern putting green.
To improve our water budgeting, therefore, we redefined available water capacity as would be appropriate for a modern putting green. The basis for this redefinition was results from a 2-year field study wherein a complete water balance was performed on experimental greens supporting a bentgrass turf maintained under putting green conditions. The experimental greens consisted of a 300-mm deep root zone placed above a 100-mm thick gravel drainage blanket, all contained within a non-weighing lysimeter. The study employed six root zones: two containing pure sand, two containing sand +10% (vol.) sphagnum peat, and two containing sand + 10% peat + 10% (vol.) topsoil (Photo 1). Two different sands were used with one being slightly finer and one being slightly coarser but both containing about 74% medium and coarse particles.
This field research recorded all rainfall and irrigation inputs, all drainage losses, and from daily soil moisture measurements, calculated daily turf ET. For one instance each during years 2000 and 2001, irrigation was withheld to impose drought stress on the turf to the point where first wilt or "footprinting" became visually apparent (Photo 2). These dry-down periods were initiated by a heavy irrigation or rainfall. Thus, from tracking soil moisture changes and drainage losses during the dry-down period, a field-based estimation of water actually used by the turf from a well watered condition to first wilt was available. This was the basis for the AWC values used in this study (Table 1).
Following the procedure described above, AWC for a pure sand root zone, a sand + 10% peat root zone, and a sand + 10% peat + 10% soil root zone was 23, 31 and 39 mm of water, respectively. These values represent the depth of water available for turf uptake within a 300-mm root zone depth characteristic of a modern green.
The Weather Data
Due to climate diversity within the U.S., water savings estimates were conducted individually for 6 metropolitan locations across the country. Selection of the specific cities was further based on a map of soil moisture regimes of the U.S. (Soil Survey Staff, 1994) to ensure a wide span in possible climatic conditions. The six locations chosen were Phoenix, AZ; Sacramento, CA; Boulder, CO; Houston, TX; Miami, FL and Columbus, OH.
Table 1 Available Water
|Year 2000 (mm)||Year 2001 (mm)|
|Finer Sand + 10% Peat||32||33|
|Finer Sand + 10% Peat + 10% Soil||ND†||ND|
|Coarser Sand + 10% Peat||29||31|
|Coarser Sand + 10% Peat + 10% Soil||38||40|
|† Not determined because the actual root zone mix did not meet the soil amendment target.|
Table 1. Field estimates of available water contained within a 300 mm deep root zone overlying a gravel drainage blanket. Available water is defined as the depth of water removed by ET after a heavy rain or irrigation to the first indication of turf wilt (foot-printing).
For each location daily weather data including precipitation, maximum and minimum air temperature, solar radiation, dewpoint and wind speed were required to conduct the analysis. Further, a 20-year span of the daily weather data was chosen as suitably long to account for year-to-year variability. To access this weather data, we used a stochastic weather simulator called GEM6 (www.wcc.nrcs.usda.gov/climate/gem.html) obtained from Dr. Greg Johnson of the USDA-NRCS National Water and Climate Center in Portland, OR. This software delivers a time series (data stream) of daily weather data for as many years of simulated weather as desired, and for many locations in the continental U.S. The GEM6 generator used in this study is endorsed as the weather generation tool of choice by the USDA, NRCS and ARS.
The daily precipitation data for the 6 locations of this study were used directly in the analysis. The remaining weather data was used to calculate clipped grass reference ET (ETo) using the ASCE Penman-Monteith equation recommended in 2000 by the ASCE Task Committee on Standardized Evapotranspiration Calculations. ETo calculations were accomplished using the REF-ET software (www.kimberly.uidaho.edu/ref-et) from the University of Idaho. Finally, a factor was needed to convert ETo values corresponding to the 4-inch clipping height of the reference grass to comparable values for a closely mown putting green turf. The value of this conversion factor came from our 2-year water balance study wherein measured values of putting green turf ET were compared with an evaporation pan reference. Based on this comparison, a conversion factor value of 0.5 was chosen for this study.
Thus, the weather data used in this study consisted of a 20-year record of daily precipitation and putting green turf ET for the 6 metropolitan locations. As with the AWC values, these weather variables were expressed as a depth of water.
The analysis begins with the total available water capacity available for turf use. Each subsequent day, ET removes a depth of water from this reservoir. If rain occurs, then the specified depth of rainfall will partially refill the available water reservoir, completely refill the available water reservoir, or refill available water with excess lost to drainage.
Figure 1. Precipitation, irrigation and available water depths (mm) for 123 days starting May 1 in Phoenix, AZ. This is an example of the results for an arbitrarily selected year of the study. The upper graph is for a pure sand root zone with 23 mm of AWC and the lower graph is for a sand + 10% peat root zone with 31 mm of AWC.
Figure 2. Precipitation, irrigation and available water depths (mm) for 123 days starting May 1 in Columbus, OH. This is an example of the results for an arbitrarily selected year of the study. The upper graph is for a pure sand root zone with 23 mm of AWC and the lower graph is for a sand + 10% peat root zone with 31 mm of AWC.
|Table 2||Pure Sand||Sand + 10% peat||Sand + 10% peat + 10% soil|
|Irrigation depth||Irrigation events||Irrigation depth||Irrigation events||Irrigation depth||Irrigation events|
|† The percent depletion values correspond to management options whereby irrigation is withheld until the indicated proportion of available water is depleted by turf ET; 50% being the more conservative approach.|
Table 2. Estimated, 20-year irrigation depth and event count for a 300-mm deep root zone containing pure sand, sand amended with 10% (vol.) peat, and sand amended with 10% (vol.) peat + 10% (vol.) soil. The results correspond to deficit-based irrigation practices and are generated for 6 locations from distinct soil moisture regimes of the U.S. (Soil Survey Staff, 1994). The pure sand root zone contained 23 mm of available water, the sand amended with 10% peat contained 31 mm of available water, and the sand amended with 10% (vol.) peat + 10% (vol.) soil contained 39 mm of available water; where available water was defined as the depth of water retained in a 300-mm root zone following drainage to the first indication of turf wilt (foot-printing).
If available water is diminished to a specified threshold, then irrigation will be required to refill the reservoir. In this analysis, we chose two thresholds expressed as a percent of AWC. The more cautions threshold of 50% AWC means that if available water is diminished to 50% of its capacity, then an irrigation event would be required to refill it. A less cautions threshold of 70% AWC was also chosen, where irrigation would not occur until 70% of available water was depleted. The amount of irrigation applied is exactly the amount required to refill the available water capacity. Thus, the depth of irrigation applied for each irrigation event will depend on AWC and the specified threshold. Finally, irrigation was not applied if a 5-day moving average of the mean air temperature was below 42º F. This prevented an irrigation event from occurring when the turf was dormant due to seasonally cold weather.
Subsequently, the cumulative number of irrigation events and the total depth of irrigation applied were determined for the entire 20-year weather record of each location.
A deficit-based irrigation scenario was generated for approximately 7300 days for each of the 6 locations. This scenario indicated precisely when, given the local climate, an irrigation event was needed to refill the available water capacity and avoid drought stress. Further, this irrigation scenario was repeated for the various root zones of the study.
Examples of the analysis output are given in Figures 1 and 2. These figures show only a small portion of the data series; 123 days starting May 1 for just one of the 20 years. Also, the figures are paired, showing the results from a pure sand root zone (AWC = 23 mm) and a sand + 10% peat (AWC = 31 mm) root zone. A threshold of 70% AWC was used in both Figures 1 and 2. In these graphs, precipitation and irrigation amounts extend downward from the top, as shown on the left-hand axis, and the present state of available water extends upward from the bottom, as shown on the right-hand axis.
Figure 1 is for Phoenix, AZ, characterized by generally large ET rates and infrequent rainfall. Correspondingly, irrigation events were frequent (35 shown), particularly for the pure sand root zone. Whereas precipitation varied in amount as would be expected for natural rainfall, irrigation depths applied were always the same, such as would occur by setting the sprinkler run time and nozzle output. Available water peaked following an irrigation event and was then stepwise diminished by daily ET. Including 10% peat increased AWC such that the frequency of irrigation events could be reduced (26 shown), but with a greater depth of water applied during each event. The rainfall pattern remained the same for the pure sand and sand + 10% peat scenarios because the same Phoenix weather record was used for all root zone treatments.
|Table 3||Sand + 10% peat||Sand + 10% peat + 10% soil|
|Irrigation savings||Event reduction||Irrigation savings||Event reduction|
Table 3. Estimated, 20-year irrigation savings from the addition of 10% peat or 10% peat + 10% soil. Savings are based on the reduction of irrigation depth and the reduction of irrigation events as compared with a pure sand root zone.
As can be seen for just a few instances in Figure 1, an irrigation event could be delayed if rainfall occurred during the intervening period, refilling or partially refilling the AWC. By increasing AWC using the 10% peat amendment and extending the interval between irrigations, there is an increased probability that rainfall will perform the refilling delaying further an irrigation event and overall reducing irrigation requirements.
Figure 2 shows the results for Columbus, OH where, during the summer months, rainfall is more frequent, delivers greater depths of water and daily ET is less than in Arizona. As a result, few irrigation events are required, and these events are separated by relatively large time-intervals. For the period shown in Figure 2, there were 8 irrigation events for the pure sand root zone and 5 events for the sand + 10% peat root zone. Again, however, a greater depth of water was applied for the sand + 10% peat root zone than for the pure sand root zone.
A summary of the results of this study is given in Table 2, where estimated, 20-year irrigation depth and event counts are presented for the 6 locations and 3 root zones considered. Also shown are results for 70% and 50% AWC depletion scenarios. The locations are ordered in Table 2 from those requiring the greatest irrigation depth to those requiring the least irrigation depth when considering the pure sand root zone. In all cases, incorporating peat or peat + soil served to reduce both the irrigation depth and the number of irrigation events. This benefit is provided by the increased AWC of the amended root zones. Further, adopting a 70% depletion scenario as compared with a 50% depletion scenario also reduces irrigation depth and event count; although at a greater risk of turf drought stress.
The results also allow for calculation of percentage savings from using 10% peat or 10% peat + 10% soil amendment in a root zone. The savings in this case are based on the reduction in irrigation depth and number if irrigation events as compared with a pure sand root zone (Table 3). Using this calculation, savings in irrigation depth from using peat ranged from a modest 4% in Phoenix to a considerable 24% in Columbus, OH. Savings from amending pure sand with peat + soil ranged from 7% in Phoenix to almost 40% in Columbus. These savings reflect differences in irrigation amounts solely on the basis of replenishing AWC. Event reduction, on the other hand, was considerable at all locations ranging from 30 to 60%. Although not specifically determined in this study, reducing the number of irrigation events may also serve indirectly to conserve water by reducing irrigation system inefficiency losses. Finally, the amendment effect shown in Table 3 was not appreciably different between the 50% and 70% depletion scenarios.
The location effects of Table 3 can mostly be interpreted by considering rainfall frequency and ET differences that occur in the various locations. Because rainfall is more frequent in Columbus than Phoenix, by extending the irrigation interval using an amendment, there is a greater probability that a rainstorm will (partially or completely) replenish the AWC. Again, with natural precipitation replenishing AWC, the subsequent irrigation event can be delayed, overall reducing irrigation need. The smaller ET of Columbus than in Phoenix performs similarly in that the increased AWC of an amended root zone will take longer to deplete and also delay irrigation. Thus, both rainfall frequency and ET serve in extending the irrigation interval.
Rainfall amount for a given rainstorm, however, also contributes to the location effects of Table 3. Rainstorms occurring in Columbus generally deliver greater precipitation amounts and are more likely to fully replenish AWC than in Phoenix. For example, there were 330 days in Columbus (11.7% of all rain days) when rainfall equaled or exceeded 16.1 mm (70% of the AWC for sand); whereas there were 54 days in Phoenix (7.4% of all rain days) when rainfall equaled or exceeded this same amount. This implies that there was a 60% greater chance that a rainstorm in Columbus would completely replenish AWC than in Phoenix. In a sense, incomplete filling of AWC from a given rainstorm would only delay an irrigation event whereas completely filling of AWC would allow for skipping of an irrigation event. Of these three weather factors considered, however, the increased rainfall frequency of Columbus than in Phoenix is expected to serve the greatest role explaining location effects.
Thus, the results of Table 3 reinforce the role of natural rainfall in influencing the magnitude of irrigation savings when amendments are used to increase AWC. Greater proportionate irrigation savings occur when rainfall is sufficiently frequent, allowing natural precipitation the opportunity to replenish the AWC reservoir. Without frequent rainfall, even though increased AWC allows for less frequent irrigation, the differences are diminished by the system demand for greater irrigation amounts with each application.
Irrigation water conservation from the use of an amendment results from increasing the available water capacity of the putting green root zone such that less frequent irrigation is required. This provides a greater probability that a rainstorm, rather than irrigation, would replenish the AWC reservoir. The climate where the putting green is located, however, dictates the actual probability of a replenishing rain to occur. Thus, the location of the putting green within the U.S. will influence the absolute magnitude of irrigation water conservation.
- Root zone amendments can translate into irrigation water savings when accompanied with an appropriate irrigation scheduling protocol.
- The extent of irrigation savings is, however, climate dependent with lesser savings in generally arid climates and greater savings in humid climates.
- The increased rainfall frequency of a humid climate together with the less frequent irrigation requirement of a amended root zone yields a greater probability that rainfall rather than irrigation will replenish the root zone available water capacity.
This research was supported by funds received from the U.S. Golf Association, the Golf Course Superintendents Association of America and the Ohio Turfgrass Foundation.
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2005 USGA Turfgrass and Environmental Research summary.
Reproduced with kind permission of USGA Turfgrass and Environmental Research Program.
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