Principles of Turfgrass Irrigation
A practical watering program embodies three basic concepts. Each concept may be set forth as a question:
- How should water be applied?
- How much water should be applied?
- How often should water be applied?
While the basic concepts of a good watering program may appear simple, in actual practice there are many and varied problems associated with the successful application of each.
Manner of applying water
Water should never be applied at a rate faster than it can be absorbed by the soil. The ability of a soil to absorb moisture at a given rate depends upon a number of factors, most of which are directly or indirectly associated with certain physical soil problems. Soil properties that govern water infiltration (movement of water into the soil) are texture, structure, and the degree of compaction. Texture (size of soil particles) and structure (arrangement of soil particles) influences not only the infiltration of water, but also water-holding ability and soil drainage.
Likewise, soils that exhibit good aggregation (a measure of structure) permit more rapid infiltration of water than soils that display poor structural properties. Compaction refers to a condition in which aggregation is reduced or absent; hence, the soil is dense. The degree of compaction at or near the surface is of special importance insofar as infiltration of water is concerned. It has been shown experimentally that a very thin layer of compacted soil will substantially reduce the rate of infiltration.
Another very important factor that influences the ability of a soil to absorb moisture is the rate at which the water is applied. Sprinklers that do not adequately disperse moisture, as well as sprinklers that deliver a large volume of water within a concentrated area, tend to cause surface runoff. Whenever water is applied at a rate faster than it may be absorbed by a given soil, the water is being wasted.
Amount of water to apply
The amount of water to apply at any one time will depend upon the water-holding capacity of the soil, the amount of moisture present when irrigation is started, and drainage.
The water-holding capacity of the soil will, to a large extent, determine how much water will be needed at any one time. Loams and clay loams are generally considered to have desirable water-holding capacity, whereas sands display very little water-holding capacity. A sufficient amount of moisture should be applied to insure that the entire root zone will be wetted. Once the soil has been wet throughout the root zone or after contact with subsoil moisture has occurred, any additional water applied will merely fill the large pores and be “excess.”
Removal of excess water from soils is referred to as drainage. Unless soil are adequately drained, many problems arise because of the slow removal of excess water.
Frequency of irrigation
The frequency of irrigation depends on the type of grass, the soil’s physical properties, and the climatic condition — especially rainfall, humidity, temperature, and wind movement.
It is often said that many turfgrass problems may be attributed to improper watering. Perhaps one of the most important factors contributing to improper watering is frequent irrigation — watering too often. In general, it is an excellent idea to let the condition of the grass determine when to apply moisture. On most general turfgrass areas the time to apply moisture is just as the plants begin to wilt. As a matter of fact, with one possible exception, this could become a rule of thumb for watering turfgrass. The exception is on newly seeded areas which must be kept moist during the period the seed is germinating and seedlings are becoming established.
Frequent, shallow watering tends to keep the upper layers of soil near a point of saturation most of the time. This encourages shallow rooting and promotes weak turf which is susceptible to disease and insect attack as well as damage from traffic. The practice of watering deeply only when plants show signs of wilting is for most turfgrass areas a practical approach to a sound watering program and it is a big step forward in the development of healthy, vigorous turfgrass. Far too many of our turf areas are watered too frequently and for too short a time.
It is important that the sprinkler used delivers a uniform amount of water over the area covered by the sprinkler throw. Many commercially available home lawn sprinklers do not give uniform coverage. You can check your sprinkler output by placing a row of one pound coffee cans (or any cans of equal size) in a line at one to two foot intervals from the sprinkler to the point of furtherest throw. By allowing the sprinkler to run for a known time (½ to 1 hour) the amount of water in each container can be measured and these results plotted on graph paper to show the distribution pattern and application rate of that particular sprinkler. Normally this procedure should be used at a time other than during periods of peak community water use, as water pressure may be lower than normal during these periods.
Poor water distribution can also be due to human error. Often a stationary sprinkler is allowed to remain on one area of the lawn longer than on another. This can be overcome by using a traveling sprinkler that moves over the area at a uniform pace.
In summary, watering practices should provide for the proper distribution of water, permit good water infiltration, and assure sufficient water retention to support plant growth without irrigation for a reasonable time. Above all, good watering practices should provide for the removal of excess water. Finally, a sound watering program should utilize only as much water as is needed by the turfgrass plants to produce healthy, vigorous wear-resistant turf.