Drip Irrigation
Post date: Apr 4, 2016 2:08:18 AM
I decided to design a drip irrigation system, since I will have many new plantings that require water their first few years — until they are well established. After that they should manage on the local rainfall plus supplements when the rain doesn’t fall as expected.
To do this job ‘right’ is not simple. I have downloaded and read about a dozen articles — and that does not count the brand-specific instructions, which I am leaving ’til later.
First I need to determine the relative water needs of each of the 34 species I plan to have on my lot. These range from Spruce trees to short prairie grasses. OK. So I did that and it took a couple of hours. I found that two of the recommended trees are high water users. Either I hope they drive their tap roots to the water table (feasible here) or substitute another species.
I have determined that my soil type is clay, and that requires generally low-flow drippers. In clay soils (opposite of the sandy moraine we had in Seattle) the water can spread as much as six feet diameter from one dripper!
The micro-clime affects the evapotranspiration also.
- The north side of my house is largely shaded by the house — especially between the berm and the house. It is not much protected from the WSW winds, though.
- The east side is protected from the wind, gets afternoon shade.
- The west side is exposed to wind and has hot afternoon sun. Some of that area will be partially shaded by the spruce trees.
- South of the house the ground will be partly shaded by the existing deciduous trees and protected from wind.
- South of the south berm is exposed to sun and wind, but will be shaded by the neighbor’s house in Autumn and Winter.
I also found the spread of each plant. That will help tell how many drippers I need for each larger plant.
Most (if not all) of my plants need water at most weekly. The real trick is to determine how long to irrigate without wasting water or drying the plants. The official method computes this from the reference evapotranspiration rate (measured in inches of water per day), and environmental coefficient (the amount of sun and wind in each location), the evapotranspiration for the particular species, and the density of planting.
The reference evapotranspiration varies from month-to-month and can be achieved in various ways, including setting out pans of water and analyzing satellite images. Needless to say, not data are available for Taos. The closest is Santa Fe, not a bad substitute.
If I use drip irrigation, the drippers on top of the berms will have lower pressure than those below.
I’ve about given up on computing the precise number of gallons or inches of water per plant. I think I will use a moisture meter to calibrate the timing of the irrigation. As long as I properly group my plantings in each irrigation zone, measuring the soil moisture should tell me when and how much I need to water.
Albuquerque has a good irrigation manual complete with charts for irrigation rate and timing by month. I’m going with that for now.