Water Cycle
The key to understanding irrigation is understanding the cycle of how water travels in and around plants. Most people know that water is only absorbed by the root system, but most people don't understand that supplying the root system is the sole purpose of irrigation. Different types of soil hold different amounts of water. In Houston, most of the soil is clay which has the capacity to hold a lot of water compared to other more sandy soils. The root system is like a gas tank, we must refill the tank as it gets low. How long it takes for the tank to get empty depends on how much the soil can hold. This is a function of the soil type and the depth of the root zone. Below is a diagram of water cycle in and around the root zone.
So lets look at each one of these factors to see how we can developed an watering plan that works for our landscape.
Precipitation:
In Houston, we get a lot of rain. In fact we average over 40 inches of rain annually. Last year (2007) we received 35.7 inches and yet our plants only needed (at most) 23.73 inches for the entire year. So why do we even need to use a sprinkler system? Well, if we could control the rain so that it only rained when our root zones were empty, the answer is we would not need to irrigate. Obviously we can't control when it rains, but we can control when we irrigate.
Evaporation and Transpiration:
Notice in the diagram above that the two ways that water leaves our soil through the air is from Evaporation from the soil and transpiration from the plant's leaves. Together these two processes are referred to as Evapotranspiration or ET. This is a very important concept in determining the overall watering requirements of our Houston area vegetation. In fact, we can calculate this requirement based on Solar Radiation, relative humidity, wind speed and temperature. Fortunately, there are weather stations around Texas that keeps track of this data on a daily basis. The one the we use is located in Kingwood at the golf course. Essentially, ET lets us know how fast we are using water in the root zone which lets us determine how often we need to refill the root zone with water. We have included a very detailed discussion of ET Rates if you would like to learn more about it.
Infiltration and Run Off:
The infiltration rate is a rate (how fast) water enters into the surface of the soil. The most important thing to consider, for our purposes, is that if the sprinkler system can deliver water faster than some soils can accept it. When that occurs, we end up with run off. Run off is a major waste of water and money. Run off is to be avoided at all cost. Run off is another major reason that we need sprinkler systems in Houston in the first place. A large percentage of our rain is from very fast downpours that exceed our soil's ability to absorb it leading to a lot of run off during rain showers. An interesting note; if your sprinkler system can deliver 1 in/hour and you have clay soil, you will start to see run off in about 8 minutes. Yet, most sprinkler systems in Houston run for 10, 15, 20 or more minutes at a time. A good controller will allow you to set a "soak/cycle" period that will allow you to only run that zone for maybe 5 minutes, go to another zone and come back and continue watering the first zone. This is particularly important for water sloped areas although, it is import for all areas. Just remember that our clay soil in Houston can hold a lot of water and that is good, but it takes a while for it to absorb that water.
Deep Percolation:
This is not something that is particularly important in our clay soils, but it is worth mentioning for those of your who live in areas with sanding soils. Deep percolation is another way for us to waste irrigation water. Once the water moves below the root zone, it is no longer available for our landscape vegetation. I will just say that sandy soils require more frequent irrigation with lower runtimes. Again, this is not a major consideration in Houston with our clay soil.