Water needs vary across course terrain

The level of sophistication in irrigation systems varies extensively from one golf course to another. Some superintendents get by with outdated systems that haphazardly deliver water to limited areas of the golf course. More and more superintendents, however, have state-of-the-art irrigation systems that deliver water uniformly and in just the amount desired. Technology has advanced to a point where you now can control individual irrigation heads and apply more or less water to different areas of the course as needed.

Installing an irrigation system today typically includes a single weather station. In addition to providing up-to-the-minute weather information, these units can provide an estimate of turfgrass evapotranspiration (ET), defined as water lost from soil and plants. Environmental conditions primarily influence ET. For example, an increase in ET occurs when day length, temperature or wind speed increases, and ET decreases due to higher relative humidity. Weather stations report ET as a daily estimate of the depth (in inches) of water lost from a turf area. Typically, weather stations communicate the data they collect to a personal computer, which inserts the information into a mathematical formula to generate an estimated ET value. (Computers can use various equations to estimate ET, but the most popular form is the Penman model.) You then can program your irrigation system-or zones within the system-to deliver all or some fraction of the estimated ET value.

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Variations in ET and irrigation need At Kansas State University's Department of Horticulture, Forestry and Recreation Resources, we hypothesized that ET and irrigation requirements could vary extensively across an individual golf course. We suspected this because most golf courses have many different microclimates within their boundaries. For example, a tee in an area that is shaded for much of the day may not require the same amount of water as one in full sun because temperatures on the shaded tee are cooler, relative humidity is likely higher, and wind speed is probably lower.

As you might expect, environmental conditions (and ET estimates) at the weather-station site may differ significantly from other areas of the course, again due to microclimate variability. In addition, superintendents often place weather stations in out-of-the-way locations on the course that may not be representative of the average conditions on the rest of the course.

For these reasons, we decided to measure the variability in evaporation at six diverse locations on one Kansas golf course. We also estimated ET at these six locations and compared it to ET estimates from the weather-station site.

Testing an ET hypothesis We performed this study at the Manhattan Country Club (Manhattan, Kan.) during the summers of 1995 and 1996. We measured evaporative demand using black Bellani plate atmometers (see photo, page G 13). These plates have a porous porcelain surface, and water evaporates through tiny holes in the plate's surface. We measured the amount of evaporation by weighing the water reservoir. During Summer 1995, we placed two of the plates at a 3-foot height on tees No. 3, 5, 7 and 18 and also near the weather station. In 1996, we placed the plates on tee No. 4, but we removed one from tee No. 5.

These tees varied in elevation, surrounding vegetation and microclimate. For example, tee No. 7 was closely bordered by trees and shrubs (about 10 feet high) to the east and west. The weather station was located in an open area, and turf was not irrigated at this site.

To determine the influence of slope on turf's water requirements, we installed atmometers on north- (about 28-percent) and south-facing (about 34-percent) slopes at a 20-inch height in 1996. We measured evaporation from each atmometer on a total of 62 rain-free days between June and August in 1995 and 1996.

We converted evaporation for each location on each measurement day to an ET estimate for perennial-ryegrass turf mowed at 0.5 inches. Using weather-station data, we compared ET estimates (provided by the Penman model) to ET estimates determined from the atmometers at various locations on the golf course.

The results: Differences do exist Evaporation from atmometers near the weather station was up to 22 percent higher than that at locations on tees (see table above). This may have been because turf under and surrounding the weather station was not irrigated. Past research has shown that air temperatures over non-irrigated areas can be up to 10F warmer than adjacent irrigated areas, with correspondingly lower relative humidities. Therefore, you should install your weather station in an area most representative of the entire golf course. Furthermore, you should situate your station on irrigated turf to improve the accuracy of ET estimates. Also, you must avoid false readings by installing an irrigation system that won't get the weather station's sensors wet.

The weather station did a reasonably good job of predicting turf water-use rates. However, we often observed that the weather station overestimated water need, particularly on hot, windy days when ET was greater than 0.16 inch per day. The Penman model that we used to estimate ET was originally developed to estimate ET for grassy vegetation at 6 inches tall-more than 4 inches higher than our mowing height. This difference likely contributed to the higher ET that the weather station in our test predicted compared to that measured at other areas on the course.

Evaporation varied by more than 20 percent among tees (see table at left), and evaporation on a north-facing slope was about 8 percent lower than it was on a level surface or south-facing slope. Tee No. 7 had the lowest evaporation rates in both years. If the superintendent at this course had irrigated this tee regularly based on 100 percent of the weather-station ET estimates, the tee would have received nearly 1 inch more water every week than it needed.

Our results show it would be beneficial for you to identify areas of your golf course with differing water requirements (that is, those with low, intermediate and high water-use areas). You then should adjust irrigation amounts and frequencies accordingly. Atmometers, such as the ones we used, would be useful for this purpose, as would soil-water measurements, visual turf appearance and your own general knowledge of your course.

Drs. Jack Fry and Steve Wiest are associate professors in the Department of Horticulture, Forestry and Recreation Resources at Kansas State University (Manhattan, Kan.). Hongfei Jiang is a former graduate assistant at Kansas State University and currently a research associate at the University of Illinois.

The authors wish to acknowledge the cooperation of Cliff Dipman, superintendent at the Manhattan Country Club, and the club's membership for allowing the use of their course for this research.

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