Evapotranspiration Rates (ET Rates)
ET Rates, what are they and how do I use them?
What is an ET Rate? (No, it is not the rate of invasion from extraterrestrials)
ET Rates stand for Evapotranspiration rates. What this means for the normal person is the amount of water that is evaporated from the soil plus the amount of water that is transpired (used) by plants. In other words, an ET rate is the amount of water that is lost in a bed, lawn or any area.
As most people know, Evaporation rate is how fast water is converted from liquid to gas. This process slows in humid conditions, low temperature conditions, and low wind conditions. Evaporation is also effected by the soil type from which the water is evaporating. In Houston, we obviously have months that are low humidity and other months that are very high humidity.
Transpiration is the technical term for the use of water by plants, we respire and plant transpire. Plants use their roots to pull water from the soil convert water into gas which is then evaporated off of the leaves and stems. The next paragraph explains this process in technical jargon, if you are not interested skip to the following paragraph.
The technical explanation for this process is Transpiration, which is the evaporation of water from the aerial parts of plants, especially leaves but also stems, flowers and roots. Leaf transpiration occurs through stomata, and can be thought of as a necessary "cost" associated with the opening of stomata to allow the diffusion of carbon dioxide gas from the air for photosynthesis. Transpiration also cools plants and enables flow of mineral nutrients from roots to shoots. This mass flow is caused by the decrease in hydrostatic pressure in the upper parts of the plants due to the diffusion of water out of stomata into the surrounding atmosphere. Water is absorbed by the roots through osmosis, and any dissolved mineral nutrients travel with it through the xylem. The rate of transpiration is directly related to the degree of stomatal opening, and to the evaporative demand of the atmosphere surrounding the leaf. The amount of water lost by a plant depends on its size, along with the surrounding light intensity, temperature, humidity, and wind speed (all of which influence evaporative demand in soil as well). Soil water supply and soil temperature can influence stomatal opening, and thus transpiration rate. Transpiration rate of plants can be measured by a number of techniques, including potometers, lysimeters, porometers, and heat balance sap flow gauges. The transpiration ratio is the ratio of the mass of water transpired to the mass of dry matter produced; the transpiration ratio of crops tends to fall between 200 and 1000 (i.e., crop plants transpire 200 to 1000 kg of water for every kg of dry matter produced) (Martin, Leonard & Stamp 1976, p. 81). About 90% of the water that enters a plant's roots is used for this process. On the other hand, desert plants and conifers have specially adapted structures, such as thick cuticles, reduced leaf areas, sunken stomata and hairs to reduce transpiration and conserve water. Many cacti conduct photosynthesis in succulent stems, rather than leaves, so the surface area of the shoot is very low. Many desert plants have a special type of photosynthesis, termed Crassulacean acid metabolism or CAM photosynthesis in which the stomata are closed during the day and open at night when transpiration will be lower.
The bottom line is that a fully grown tree may lose several hundred gallons of water through its leaves on a hot, dry day. Some cacti and conifers will pull very little water from the surrounding soil.
So, what do all this have to do with your sprinkler system?
If we know the historical ET rates for an area and we know the historical precipitation of that area, we can calculate an estimate of how much water we need to make up for ET rates. This is what the purpose of an irrigation system, to make up for losses due to evaporation and plant transpiration.
Almost everyone in our area that has a sprinkler system over-waters their yards and shrubs. So what is the problem with that? you might ask. Well there are several issues that are caused by over-watering. The first is that water is a precious resource that should not be wasted. Then there is a cost associated with the excess water that your are basically, and sometimes literally, throwing down the drain. And finally there is a cost to the vegetation. Plants are very specific about how much water they do well in. Beyond that point, we start stressing the plants and invite disease like fungi to infect the vegetation.