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Hydrozoning saves water

June 6, 2015
Dave Phelps

We all know the importance of saving water during a drought and that the greatest conservation can be accomplished by saving landscape water. Besides eliminating the obvious wasters of leaks, run-off and large nonrecreational lawns, hydrozoning is the next most effective way to save water. Investing in a “smart” controller won’t save much water if you haven’t taken this very important step.

Hydrozoning maximizes the efficiency of an irrigation system by putting plants with similar water needs on the same stations or valves. When a landscape irrigation valve waters plants with varying water needs, the plant with the highest water need sets the duration and timing for that valve — and the rest of the plants get more than they need, wasting water.

An easy way to visualize this is to envision a lawn — the area getting the least amount of water will turn yellow. If the irrigation timing is adjusted to make that yellow patch green then the rest of the lawn is overwatered and wastes our valuable resource. It would be better to maximize the distribution uniformity of the turf irrigation system by adjusting or changing the spray nozzles.

So how much water do plants need? While there is ample scientific data on most food crops, ornamental plants have not yet been scrutinized to such a degree. The answer is difficult and varies depending on what climate (and microclimate) the plant is growing in. It also depends on soil type, root depth, planting density and the amount of mulch applied. Different species and cultivars of the same genus may even need different amounts of water in different climates.

Thankfully, our tax dollars have been put to good use and we have Water Use Classification of Landscape Species (WUCOLS IV). Updated in 2013 to include many of the plants we love (3,546 entries), WUCOLS divides California into sixclimate zones and gives a water need value for each plant grown in each zone. This value has been debated and vetted by some of the best horticulturalists in each zone. The State Water Efficient Landscape Ordinance (WELO) has proclaimed WUCOLS the benchmark when calculating water budgets for landscapes.

This does not mean you have to move all your plants around. It is easiest and perhaps more logical to group together plants with similar water needs in the garden, but hydrozoning is about putting those similar plants on the same station or valve. WELO already mandates that trees be on their own station or valve — trees have much deeper roots and need less-frequent but deeper irrigation. Thus it is entirely possible to drip irrigate plants next to each other using different drip stations, if so desired. It just increases the complexity of the system.

WUCOLS groups plants into four water-use categories: very low, low, medium and high. When planners calculate a landscape water budget, these categories are used to assign plant coefficient values from .1 up to .9. If you’re wondering, these values are comparisons to a standard called ETo (EvapoTranspiration), which is the amount of water used by a fescue lawn grown under optimal conditions. Landscape planners multiply the water use coefficient by the number of plants, planting density and microclimate coefficients to obtain an estimate of landscape water use.

The current WUCOLS IV can be downloaded at ucanr.edu/sites/WUCOLS.

If you’re interested in having someone come out to your property to assess irrigation efficiency, hydrozoning, or water-wise plant use, consider calling UC Marin Master Gardeners for a free consultation at 415-473-4204.

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