25. Landscape Sustainability: Irrigation Water Use

As you probably know, the long term trends show that water will be increasingly scarce in California as the years go by.  Here at the end of 2013 the rainfall totals have been disappointingly low, it may be the driest calendar year ever recorded.  At the Oakland Airport, the National Weather Service has recorded only 2.07 inches of rain from July 1 to December 28, 2013.  The average rainfall for this time period is more than 3 times that amount at 7.24” and this time last year we had almost 11 inches.  Our rainy season is only halfway through, so it is too early to start using that ‘D’ word (drought), but no doubt you have noticed all the extra watering your garden has needed.    How water efficient is your garden?  What can you do to conserve water in your landscape?

Water Use and Sustainability

Conserving water in your garden is perhaps the most significant way you can be more sustainable in the landscape and there are a wide variety of things you can do.  A good start is proper soil preparation to improve your dirt’s water holding capacity.  Plant selection is a big factor, and the use of native plants or those from the Mediterranean climate zone reduce the amount of water needed.  Making use of water sources other than drinking water is another important aspect, such as rainwater harvesting, wastewater reuse, and stormwater. Lastly, the way in which you get water to your plants is just as important, so the design of the irrigation system plays a big role.

Landscape water use is so important here in California that there are specific laws and regulations in place.  Like everything legal and political, the rules are complicated, but anyone with a garden should understand the basic concepts in order to make the best use of our precious water resources.

California Water Use Regulation

California specifically regulates irrigation water use through Assembly Bill 1881 which became effective at the beginning of 2010.  In general, small-scale landscapes of less than 2,500 square feet are not required to conform to the regulations.  However, each municipality has adopted their own version of the state law, in the form of a Water Efficient Landscape Ordinance, so the specifics vary by City or County.  And it seems likely that these requirements will become stricter in the future.

When a project is required to conform to the Ordinance, the landscape water use must be documented in detail and approved by the local agency as part of the project’s permitting before construction starts.  You’ll need a Landscape Architect and sometimes an irrigation specialist who can design to meet these requirements and provide the appropriate drawings and calculations.  But all gardeners should understand the basics of these calculations because they can help with even the smallest decision of what plant to buy next for your yard.

This law provides a standardized way to make two calculations – estimated total water use and maximum water allowance.  In order to be approved, a proposed landscape must have an estimated water use that is less than the calculated water allowance.  The estimated total water use (abbreviated as ETWU) calculates the landscape water use of a planting design based on several factors including plant water use classification, planting square footage, local evapotranspiration, and irrigation efficiency.  The water allowance, officially called Maximum Applied Water Allowance (MAWA) is calculated using similar factors without the plant specifics.

Water Use Ratings for Plant Species

For the home gardener, perhaps the most interesting thing is the water use classification of specific plant types.  Each plant species is categorized as high, moderate, low or very low water use in a document called the Water Use Classification of Landscape Species (see references below).  This categorization translates into a Plant Factor number in the water use calculations.  For example, the high water use Birch tree (Betula species) has a multiplier of 0.9 while the very low water use Coast Live Oak (Quercus agrifolia) has a multiplier of 0.1.  When thinking about what plants to put in your garden, you should refer to these classifications, which were decided by a panel of experts that has just been reviewed, expanded and updated.  The web site now includes a searchable database that is pretty handy, if you have a good handle on the specifics of what you're looking for.

Evapotranspiration

One of the key variables in the water use calculations is the evapotranspiration rate, abbreviated as ET.  This is defined as “the loss of water to the atmosphere by the combined processes of evaporation (from soil and plant surfaces) and transpiration (from plant tissues).”  The ET rate varies according to the local climate, and in California we have a network of weather stations that record local weather data to calculate evapotranspiration.  With  such big variations in climate across the state, and even within the Bay Area, it is important to factor in how quickly water is lost when you select your plants.

Hydrozoning

From the perspective of irrigating the garden, it is important to group your plantings by water use, known as hydrozoning.  By referring to the water use classifications for specific plants, you can select compatible plants for each watering zone in your garden so that there is an equal water demand.  Otherwise the tendency is to overwater some plants to keep the thirsty ones happy.  There are ways to compensate for this in drip irrigation, by putting higher flow drip emitters in the root zones of plants that need it, but this adds a level of complexity that may be hard to keep up with.

Irrigation Efficiency

Another factor in water use calculations is the efficiency of the irrigation system.  There are a great number of irrigation products available that can very precisely deliver water exactly when and where it is needed most.  There is, in fact, so much to discuss that we will have to get into that in a future posting.  For now, start thinking about the plants you choose, and how you group them to minimize your water use.  Not only are you helping the environment by preserving water, you will save money on your water bill!

References:

The California Department of Water Resources website has a huge amount of information on these issues as follows:

California Department of Water Resources, general information: http://www.water.ca.gov/wateruseefficiency/

University of California Cooperative Extension hosts the newest version of the Water Use Classification of Landscape Species:  http://ucanr.edu/sites/WUCOLS/

California Irrigation Management Information System, Department of Water Resources, Office of Water Use Efficiency:  http://wwwcimis.water.ca.gov/cimis/infoEtoOverview.jsp