As with most things in life, finding the correct balance is everything. This is true of drip irrigation, too. Although the majority of emitters sold through most retailers are rated at 1 gallon per hour (gph) that does not mean that 1 gph is the correct rate for every application. Since the purpose of drip and micro-irrigation systems is to apply the optimal amount of water, at the most efficient rate for any given plant, it is important to know how to figure the gph rate that works best for your particular plants, and that is all about finding the right balance.
To address this, agriculture and drip irrigation design experts consider the “soil, plant, water” relationship when creating a new system. They must work to balance the sometimes conflicting needs of soils vs. plants to arrive at the most efficient watering rates. As you can imagine, in huge agricultural applications, growers cannot afford to waste water, or invest in unneeded irrigation capacity—and neither can you.
The first factor in determining the correct drip rate is soil type. Depending on whom you ask, that can be 12 or more types. The U.S. Department of Agriculture lists seven types, ranging from very coarse sand to clay. For non-commercial applications, most people condense this down to three key types: sand, loam, and clay. Knowing the soil type is important because water moves, or percolates, at different rates through these different soils, with sands having the fastest percolation rates and clays the slowest.
A simple visual means of identifying the soil type is to note its wetting pattern. When an emitter dispenses drops of water, a distinctive wetting pattern is created on and below the surface of the ground depending on the soil type. In sandy soil, the water percolates downward very quickly, creating a small area of moisture at the surface, while clay soils tend to spread the water horizontally, creating a larger wetting pattern. You want to create a small, compact wetting pattern, so by watching the way the water spreads as it drips you will be able to see if you are applying too much or just enough. For the most part, sandy soils require drip rates in the 1 to 3 gph range, while clay soils require a drip rate of just 0.5 to 2 gph.
Plant anatomy is the second key factor. The root zone of the plant is the area at the base of the plant that contains all of its roots and root hairs. The entire root zone is normally about the same size as the plant’s canopy. The larger the root zone, the more water the plant needs for consistent growth. The root hairs are the thin filaments extending out from the main roots, and they are the part of the root system where water is absorbed. The root hairs of most landscaping plants are relatively shallow in the soil, residing in the top few inches. This may create a dilemma: A plant may have a large root zone, which requires a lot of water, but on the other hand, the root hairs may be shallow.
So, if you are using traditional irrigation methods, watering at high flows, the water percolates right past the shallow root hairs and much of it is not used by the plant at all. A good rule of thumb to remember is that the top 25% of the root zone absorbs 40% of the water, and the bottom 25% absorbs only about 10%. One of the principle advantages of drip irrigation is that drip allows you to apply small amounts of water, at precisely timed intervals. This limits percolation through the soil and creates a state of consistent moisture for the plant. The shallow root hairs absorb as much water as they need, when they need it. The result is greatly improved plant health and reduced water waste.
For the average homeowner, the best way to figure out what drip rates are right for which plants is to understand the basic root system characteristics of the plants and know the type of soil they are growing in. For instance, a large plant with a large root zone in clay soil could use an emitter with a medium flow rate, whereas the same plant in sand might require a high flow rate.
The final key factor is time. In drip irrigation, longer run times (the number of minutes the system waters), at slow rates appropriate to the soil type, create an area of uniform moisture throughout the plants’ root zones without ever saturating the soil with water. This is the perfect growing environment for plants, and minimizes water waste and evaporative loss.
Getting to this state of perfection requires a little experimentation. It is as much an art as a science, but once these few factors have been taken into account it is easy to achieve the outcome you need using standard 1 gph emitters and adding emitters to increase the drip rate for specific plants. Or, if you prefer, you can buy higher drip rate emitters–it all comes down to creating the right balance.