Thinking ahead on irrigation
(Turf Business, March/April 2007)
With the welcome dry period at the beginning of April, thoughts now turn to irrigation. Within six weeks soils will go from field capacity to a condition of soil moisture deficit (ie. a condition where water must be added to retain field capability. Knowing the amount of water to apply and being able to apply accurately the programmed amount now becomes the main concern of the green keeper or groundsman. The latest irrigation control systems incorporate weather stations that produce evapotranspiration calculations. These estimates give a good idea of water losses but on 18 golf greens there can be variation due to the aspect, exposure and light – which all influence the degree of water loss.
Irrigation is only designed to supplement rainfall so it goes without saying that effective rainfall should be measured and tabulated. During dry periods many mangers opt to for apply a little water daily. This can be a hit and miss approach – particularly when one hot windy day all that has been applied is quickly lost in evapotranspiration.
The objective must be to maintain the top 100mm in a moist condition even if the surface is beginning to dry out. Daily watering keeping the surface wet favours the development of annual meadow grass (Poa annua). With the depth of roots on golf green not significantly greater than 50 to 75mm, the depth of the ‘reservoir’ is limited in the amount of available water that can be withheld. With not much more than 9mm available from this reservoir the supply can be depleted within three to four days.
Still one of the most practical means of assessing the water need in turf grass cover is to feel the soil taken from a probe inserted to the depth of the root zone. No tool is more invaluable during the hot summer months than a cut-off golf club with a 75mm side-cut eliptical notch cut out of it. On inserting the probe into the rootzone, the exposed core in the notch is easily removed and felt. When the contents are powder-dry there is already a problem and it is difficult to rewet these locations. Ideally the extracted core should be just moist (which is somewhere between field capability and wilting point). When the core is decidedly wet or very damp, there is adequate moisture and the decision not to irrigate become clear. Because of undulations, care should be taken from a number of points on the green to get a true picture of the water need.
The distribution of water
Nothing is more frustrating than to see water being applied unevenly to a turf grass area. Where the water programming has been carefully planned to meet the actual water needs, the accurate and even application becomes vital. Faulty nozzles, inadequate pressure and incorrect spacing between sprinklers are common weaknesses in irrigation systems. A periodic check of the uniformity of water distribution is a logical step to ensure water is applied effectively. The only way this can be done is by setting out a grid pattern of receptor cups to evaluate how evenly the water is applied. No system performs consistently to the originally designed level.
Nozzles become worn and are replaced, sprinkler heads settle and pressures at the nozzle can vary for a number of reasons. While the heart of the system is the pump station, periodic checking and adjusting becomes vital if the system is to apply water as it was designed to do.
Aerating and the use of wetting agents.
On golf greens daily light watering so often leads to dry patches developing. These dry patches can originate with the development of water-resisting fungal growth on sand particles. More generally they result from a condition where drying out of the surface creates such high tension between the sand particles that the water passages become too restrictive for downward percolation. Consequently it is common to see water droplets on the soil surface that are unable to enter the soil profile.
We all know the tedious procedure that now becomes necessary – forking holes into these dry areas and the application of wetting agents. These reduce the tensions with which water is held to the sand particles and downward flow can be restored. While most agents perform this task very well there are some that enable water entry but also increase the water holding capacity. This is effect means that water is able to enter the soil profile but is withheld by soil particles below the surface to create a better distribution of water within the root zone. This water is now not subject to evaporation loss and so provides a larger supply of available water.
Even approved root zone mixtures in greens and premier football pitches are prone to water run-off. This is particularly noticeable on undulated golf greens, bunkers and uneven pitches. Notwithstanding the high hydraulic conductivity rates measured in the laboratory, the root zone mixtures soon become subject to factors that steadily decrease this rate. Compaction at the surface and the ultimate build up of organic matter residue in the form of thatch soon restricts the infiltration rate and this will drop to between 5 and 50mm per hour. The formation of puddles on wetter areas in turf grass bears out this shedding of surface water.
The only solution is responding to this expected limitation is to reduce the irrigation period and increase the number of cycles in an effort to create greater infiltration. As many as three cycles in a right could be necessary. Where this approach limits the number of greens that can be irrigated at one time, staggering the watering programme to irrigate say six to nine holes in one evening could be the solution.