There are five things every homeowner should know when it comes to watering the lawn. How does the grass use water? What kind of soil type is it and what is its moisture holding capacity? How much is the allowed water depletion from the root zone, and when & how much should you water.

But first some plant watering myths. Deep root watering does not work. The active root zone in turf grass is 3 to 4-inches deep at most. True a few roots may go deeper, but roots to not grow through dry areas to search out moisture, and most of the water uptake by roots occurs in the top 2-inches of the turf root zone. The active root zone for trees and shrubs is in the top 12 to 18-inches of the soil, so watering below that level with a root feeder or surface watering is a waste of water.

Soil water extraction by plants will vary, but a safe assumption for calculating soil water extraction is the 40-30-20-10 rule. This rule states that 40 percent of the water will be withdrawn from the top 25 percent of the root zone, 30 percent will be withdrawn from the second 25 percent of the root zone, 20 percent from the third 25 percent, and 10 percent from the lowest 25 percent of the root zone.

Every third day watering throughout the growing season does not save water. The turf needs to be watered when 50 percent of the moisture is gone from the active root zone. This time span varies depending on the time of year, how hot it is, and other climatic factors.

How Grass Grows. Transpiration occurs when the plant moves water through the roots and stomates into the air as a result of a difference in tension or atmospheric suction. The plant uses this water to draw up nutrients, maintain structure and to cool the leaf surface. The description of this process and how much water it requires is called evapotranspiration (ET). ET is the combination of transpiration and water that is evaporated from the soil surface. It is usually defined as the quantity of water (in inches) that needs to be replaced in the soil in order to maintain ideal plant growth and appearance. Evaporation from the soil surface usually affects only the top few inches of soil. Daily irrigation of a clay soil with turf grass can result in a higher soil evaporation rate than every other day or every third day watering. This is not much of a problem in sandy soils because the water drains faster. Crop cover and mulches also reduce the amount of soil evaporation.

The climatic factors that effects ET are air temperatures, wind speed, relative humidity, and solar radiation. Solar radiation is one of the primary climatic factors that effect ET. This is the source of energy necessary to convert water to vapor in both plants (transpiration) and soil (evaporation). Solar radiation peaks in late June in the Northern Hemisphere at latitudes above 22 degrees. Temperatures usually peak in June or July, and after these months, the ET rate begins to decline leading to reduced irrigation through the remainder of the season.

Soil Types. Soil is the water storage reservoir or "fuel tank" for most plant material. Additionally, water movement into the soil is important in order to prevent runoff and conserve water. How plants interact with soil and their available water supply is critical to understanding how water conservation practices can be achieved.

SOIL types and texture are determined by the particle size of minerals that make up the soil. Soil types range from very course sand with a particle size of up to 20 mm to clay soil with particle sizes below 0.002 mm. For our purposes we will group the soil categories into three types:

Soil pore space is the area between particles that is available to hold water and air. Sandy soil has the least amount of pore space at approximately 35 percent of total volume, while clay soils have up to 60 percent pore space. A high percentage of pore space results in a greater soil water holding capacity. When compacted, clay soils have reduced pore space that can be as low as 25 percent of the total volume. Tilling the soil prior to planting is important to create space for needed air and water.

The Soil Moisture Holding Capacity of a soil type is defined as the amount of water that remains in the soil after the gravitational water has drained away and after the rate of downward movement has decreased to a significant extent.

Water holding capacities of soil will vary by soil type, and are generally measured in inches of water per inch of soil thickness as shown in the table below:

If the plant root zone is 4-inches deep in a loamy soil the water holding capacity would be 0.50-inches (4 x 0.125 = 0.50), a clay soil 4-inches deep would have a water holding capacity of 0.668-inches (4 x 0.167 = 0.668).

Allowed Soil Water Depletion. Wilt results when plants can't retrieve enough moisture out of the soil. The difference between a full saturation and the wilting point for the plant is known as available water holding capacity and is the percentage of moisture available to the plant from the soil. This is usually 50 percent for turf grass. This is the amount of water that can be removed from the soil by the plant before irrigation should occur.

How Much & When to Water. The turf cover as well as compaction, slope and thatch will decrease the water infiltration rate. During the first few minutes of irrigation the infiltration rate is very high at 5 to 10 inches per hour and then it rapidly decreases to very slow rates. Small fixed arc pop-up sprinkler heads with a 15 foot radius or less should run no more then 7 minutes at a time, and then the cycle repeated after 30 minutes if additional water is needed. These heads have an average application rate of 1.5 to 2-inches per hour. During the heat of the summer the maximum runtime should be about and hour per week. If you’re watering 3 days per week, apply a maximum 21 minutes per day or three 7-minute cycles. Larger rotor heads with a single stream (20 foot plus radius) will need to water about 3 times as long as sprays, or 20 minutes or more per cycle. Longer water cycles will cause runoff.

Small fixed arc pop-up sprinkler heads with a 15-foot radius or less will apply approximately .20-inches in 7 minutes or .60-inches in 21 minutes. Rotor heads will apply about .20-inches in 20 minutes or .60-inches in one hour. You can put out cans and catch the water for 7 or 20 minutes (pop-ups or rotors) to determine the actual application rate for your system.

The best time to water is between the hours of 10 PM and 6 AM because of less evaporation loss, less wind, and its better for the turf. The average historical ET rate for various months is as follows:

Be sure to adjust your irrigation clock for the differing water requirements each month

Plant roots need oxygen to function and grow. A soil that is saturated most of the time is not conducive to healthy plant growth because oxygen movement in the soil is limited. In Clay soils this can be a problem and can be avoided by infrequent or "just in time" irrigation, and in sandy soils this is not much of a problem. Frequent aeration allows oxygen to readily enter the soil and also improve the intake rate of the soil.

Overwatering causes runoff & pollution, water drains below the root zone, leaches nutrients from the soil, and causes disease in the turf.