Determining Amount of Water to Apply
Watering schedules should be managed to replenish the water lost to evaporation and used by the plant. This generally provides a healthier lawn and minimizes the potential problems associated with leaching and run-off concerns.
In most years, Minnesota's climate makes lawn watering optional. Water in the landscape is lost back to the atmosphere through evaporation of moisture from the soil and lawn surface as well as through the transpiration process described above. Together these two phenomena are known as "evapotranspiration", abbreviated ET (Fig. 9.2).
Figure 9.2. Evapotranspiration.
Potential evapotranspiration is the estimated amount of water a lawn would lose if there was sufficient water in the soil at all times. Figure 9.3 shows the average rainfall and the average potential ET in the Minneapolis/St. Paul area. On average, the amount of rainfall is greater than the amount of water needed by grass plants in each month of the year except June, July, August, and much of September. Consequently in most years, lawn irrigation during October, November, April and May should be minimal or unnecessary.
Figure 9.3. Average annual rainfall and potential evapotranspiration in Twin Cities area.
The graph in Figure 9.3, for an average year, indicates that turfgrasses use about 4 to 6 inches of water per month during June, July, and August. The precipitation during each of those months is usually 3 to 4 inches. Hence, there is potentially a deficit in that more water is being lost than is being supplied back to the lawn. This has given rise to the often quoted lawn watering rate of 1.0 to 1.5 inches of water per week (depending on weather conditions) including rainfall for the lawn to remain green and healthy throughout the growing season.
However, recent research has shown that Minnesota lawn grasses don’t necessarily need to have all of the water replaced in order to remain healthy and vigorous. This has resulted in adjusting downward the water required by lawns during the summer months. For example, Kentucky bluegrass, while being a relatively high user of water, can get by on weekly quantities of 3/4 to 1 inch of water rather than the traditional 1.0 to 1.5 inches of water. The more drought tolerant fine fescues have shown that they can tolerate weekly water quantities between 1/2 and 3/4 of an inch and still remain healthy and green.
The amount of water to apply also depends on the soil type and moisture content of the soil. How much to apply at one time can vary by the season as well. When grass roots are actively growing in the early spring and fall, watering deeply but infrequently is a good practice. However, at those times of the year, rainfall is often more frequent and additional watering may not be needed to keep that deeper rootzone moist.
As the roots of cool season grasses recede to shallower soil depths during the hot summer months, watering more frequently but to a shallower depth is a better practice than watering deeply and infrequently. This reduces the risk of too much water moving past the active rootzone and thereby being wasted. It should be noted that the surface soil should not be kept constantly saturated. Rather, allow some drying out of the rootzone before water is again applied. This slight drying out between watering sustains a much healthier grass root system and plant.
The rate at which water can enter the soil is sometimes less than the amount applied by the sprinkler. When too much water is applied at one time, much of it may be lost or wasted as it moves too deeply into the soil for the grass roots to be able to use. For sandy soil, it may take 1/2 inch of water to wet the soil to a depth of 5 or 6 inches, while 1 to 2 inches of water may be needed to wet a clay soil to that same depth.
However, 1 to 2 inches of water applied at one time on a heavier clay soil will likely result in ponding on the soil surface (Fig. 9.4) or even running off. Thus, the weekly allotment of water should be split into two or even three applications per week to avoid ponding or runoff issues while still providing sufficient moisture to the active rootzone. If puddles or runoff are noticed during the watering sessions and more water is needed to fully wet the soil, turn the water off for 30 to 60 minutes then resume watering until the desired amount has been added. Repeat that process until the needed moisture has been applied. Sandy soils may need two 1/2 inch applications per week rather than a single larger, 1 inch application.
Figure 9.4. Ponding of water on the soil surface as a result of too much water applied at one time.
To tell how deeply water has penetrated the soil, remove cores immediately following normal irrigation. Another method is to sink a shovel into the soil and spread the hole to view the wetted soil. Remove the shovel and press the soil back into place after examining.
Determining the amount and uniformity of water applied in a normal irrigation can be done by placing a row of equal-sized, straight-sided cans in a line at 1 or 2 foot intervals from the sprinkler and out to the farthest point of watering. After a specified length of time (such as an hour), measure the amount of water collected in each can. This will show how much water has been applied during that hour and how uniformly the water was distributed. From this, determine the appropriate length of time needed to apply the desired amount of water. Remember that watering during windy conditions even if early in the day can result in very distorted and uneven watering patterns often leaving some areas over-watered while others receive barely enough to wet the surface soil.
Proceed to Frequency of Watering