Evapotranspiration
Basics
Irrigation
Scheduling
Water
- Fertilizer Issues
Turfgrass
"Factoids"
Evapotranspiration is a culmination of water loss from the plant through natural process and the evaporation of water from the soil. The amount of water loss depends upon temperature, wind, light intensity, and humidity. Plants transpire more at higher temperatures because water evaporates more rapidly as temperature rises. Plants also transpire water in order to cool themselves, so additional water is used by plants on a hot day. Evapotranspiration will increase as wind speeds pick up, because moisture is more readily pulled from the plant. Grass will also transpire more rapidly in the light than in the dark. This is largely because light stimulates the opening of small pores (stomata) on the leaf blade, causing water to be easily pulled from the plant. In addition, water escape from the leaf blade increases when it is surrounded by dry air or in other words low relative humidity. Irrigation scheduling using evapotranspiration data is a useful tool to apply only enough water to your lawn to keep it green by replacing only what is used or escapes from the soil and plant. Our published Evapotranspiration data is based upon the Penman Monteith equation which more information can be found at this site.
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Soil type is one of the fundamental requirements in order to estimate an accurate irrigation scheduling techniques. The most prominent soil type in Moscow is the Palouse Silt Loam series. One soil characteristic is infiltration rate, which is how fast the soil will absorb the water placed on it. Palouse Silt Loam has an infiltration rate of 0.6 – 2.0 inches of water per hour depending upon soil physical characteristics. It will be on the low end of the scale if the soil is compact or hard with little organic matter. As organic matter increases and compaction decreases, higher values can be obtained.
Probably the most important variable in soil to determine water application times is the water holding capacity of the soil. This is the amount of water the soil can hold per depth of soil. The Palouse Silt Loam soil can hold 0.19 – 0.21 inches of water per inch of soil.
The rooting depth of your grass also must be known so you can determine the maximum duration you can go without watering your lawn. Lawns typically have root zone depths of about 6 inches. This is important because you will only apply as much water as is needed to fill this first six inches of the soil. Any water that goes past this 6 inch root zone is unavailable to the plant and therefore wasted. This information is used along with the idea of Maximum allowable deficiency to come up with an irrigation interval, or time between irrigations.
Maximum allowable deficiency can be related to how hard you can suck a thick milkshake through a straw before you have to take a breath. The grass in your lawn can only pull so much of the water out of the soil. It is incapable of pulling all the water out of the soil. As the soil becomes dryer and dryer, the soil particles hold onto the water even more tightly. Maximum Allowable Deficiency is how much of the soil moisture a plant can suck out of the soil before it turns red in the face. Grass can typically use only 50% of the water that is in the soil. Therefore, even though the soil can hold about 0.2 inches of water per inch of soil and the root zone is about six inches deep, yielding 1.2 inches of water, only 0.6 inches of this water can be used by the plant. Therefore your lawn will need to be watered at no greater intervals than in which 0.6 inches of evaportranspiration has occurred. To use this however, it assumes that the soil is fully saturated, or the full amount of water that the soil can hold is filled.
Light, frequent irrigations produce shallow, weak root systems. The shallow root system prevents efficient use of plant nutrients and water in the soil. Roots generally grow where the soil is moist, and some turfgrass roots do seek out water deeper in the soil as the surface moisture is depleted. Therfore, it is better for you lawn to apply more water less frequently. Ideally, you would only water your lawn when 0.6 inches of evapotranspiration is approached.
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When your lawn is healthy is has enough water and nutrients. If its’ basic needs are meet then it will not experience stress. Stress can lead to weakness, disease, and higher insect damage. The amount that you water your lawn is also very important when it comes to nutrients. Over-watering will lead to leached nutrients and will cause your lawn to become deficient in Nitrogen. Nitrogen is easily leached past the root zone if too much water is applied. Every time you put more water on than just fills the 6-inch root zone, you are pushing nitrogen past the root zone and making it unavailable to the plant. If the soil is flooded then it will more readily leach nitrogen than if water is applied slowly with a proper rate from a sprinkler. Never allow runoff or puddling to occur when watering your lawn.
Other nutrients like phosphourus, potassium and sulfur are not susceptible to leaching because the soil particles and these nutrients are attracted to one another and are readily held by the soil.
Above Ground…
Grass plants are 70 to 80% waterBelow Ground…
Grass clippings are 90% water
Grass clippings contain 4% nitrogen, 2% potassium and 0.5% phosphorus
A 10,000 square foot lawn will contain:
6 grass plants per square inch
850 plants per square foot
8.5 million plants total
90% of the weight of grass is in its rootsTurfgrass sod is a superior form of erosion control, with tests documenting:
A single grass plant has 387 miles of root
There are 329,000 miles of root per square foot
3 billion miles of roots in a 10,000 square foot lawn
A dense lawn is 6 times more effective than a wheat field and 4 times better than a hayfield at absorbing rainfall.A 50 by 50 foot lawn (2,500 square feet) releases enough oxygen for a family of four, while absorbing carbon dioxide, hydrogen fluoride and perosyacetyle nitrate.
Sediment losses from sodded areas will be 8 to 15 times less than for tested man-made erosion control materials and 10 times less than for a straw covered area.
Runoff from a sodded area will take 28 to 46 times longer than for five popular erosion control materials.
Source: http://www.turfgrasssod.org/trc/faqs.html
Now, onto Calibrating Your Sprinkler