Gravity-Fed Drip Irrigation Systems

Once we finished building the elevated water barrels and connecting the pipes and fittings needed to push water from the cisterns to the barrels and from the barrels to the garden beds, we were ready for the next step. Our irrigation goal was to conserve water but also supply enough water to grow healthy and productive plants.

According to Washington State University, 80% of water withdrawals in Washington State are used for agricultural purposes. Out of the 1.8 million acres that are irrigated in this state, 80% use sprinklers (overhead watering); 15% use surface (where water is distributed by gravity, such as in a furrow system) and 5% use drip systems (with watering on top or below the soil or mulch). Sprinkler and drip systems require pressure to deliver water to the crop. If public utilities are not available or are not being used, gravity can produce the pressure necessary to move the water.

Overhead watering is an inefficient watering technique because much of the water is lost to evaporation or runoff.  It puts extra strain on farmers and landscape managers, particularly during times of prolonged drought. Overhead watering contributes to soil erosion and surface water or ground water contamination. Researchers at WSU Research and Extension Center conducted an unreplicated study in 2001-2003 and found that “the drip system used 50% less water, produced 50% fewer weeds with 75% less biomass than the overhead system (Efficiency of Drip and Overhead Irrigation Systems, Carol Miles, Martin Nicholson and Madhu Sonde, WSU Vancouver Research and Extension Center). They concluded that drip is preferable to overhead watering despite the increased time and cost required for installation and labor. Drip systems, however, must be monitored for damage to the lines from animals or clogging. Note that other studies dispute that less water is used in drip irrigation systems (Food and Agriculture Organization of the U.N. Drip Irrigation) although they do see value in using drip systems to reduce loss due to surface runoff and evaporation.

Drip irrigation is a viable option for gardeners and small farmers. It is more efficient than overhead watering because water is delivered closer to plant roots, reducing loss to evaporation. It is easier to overwater or underwater when watering by hand with a hose or watering can. Overwatering can contribute to root rot. Water flows so slowly through a drip irrigation system that overwatering is less likely unless there is a break in the line that allows water to gush out. Drip irrigation also aids in disease prevention by keeping the foliage dry and creating a less hospitable environment for weeds. Some systems can be customized to water specific plants or containers.

Soil characteristics affect how well an irrigation system functions. The type of soil plays a role in what type of irrigation system to use and how often to turn it on. Soils have varying structures, depending on what they are composed of and what outside forces they have been exposed to. The types of mineral particles (sand, silt and clay), how closely the particles fit together and amount of organic matter affect how quickly water percolates through the soil, where it flows, and how long it takes for the soil to dry out. Soil compaction influences how efficiently plant roots can get the moisture they need. Using heavy equipment, tilling wet soil and leaving soil uncovered through a rainy season all contribute to soil compaction. Slope is another factor to consider. For more information on irrigation and soil properties, see Univ. of CA Coop. Extension’s PowerPoint, “ Soil Physical Characteristics: Impacts on Irrigation, Management Choices” and United Nations Food and Agriculture Organization website’s document on “Irrigation Water Management: Irrigation Methods“.

The soil in our beds appears to be adequate for using with drip irrigation. We plan to continue to investigate the way water travels through the soil and how deeply it percolates downward. We have begun digging up plants as they finish producing to see how big the roots grew as one way to assess this. 

There are a number of manufacturers that sell drip irrigation equipment and a few that advertise products that they claim will work when hooked up to an elevated rain barrel or cistern. There is little information available from farmers or gardeners who have tried these products.

The soil in our beds is level and holds water well so we decided that drip irrigation is a good fit for or beds. We decided to test three different systems that are advertised for use in gravity-fed irrigation in order to assess how well each one works for the food crops we grow to donate to the University Food Bank.

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