Don't Be Afraid . . . It's Just ET

Or, not.  I'm not afraid, said young Luke.  'You will be,' replied Yoda.  'You will be.'

As everyone knows, plants take in water through their roots, transport it throughout the plant tissues via the xylem, and release moisture through pores in their leaves to make room in the pathway for a continuous stream of fluid. The sun’s energy drives this system as the plants capture its radiant energy as chemical energy through the process of photosynthesis. While the sun is shining, a certain amount of water also evaporates from the soil surface. ET—evapotranspiration—is the amount of water used by a landscape or crop in these natural processes. Although it is far from being a mysterious alien creature, ET is still regarded suspiciously by many due to its mathematic implications.

Reference ET (Eto) is used to determine the amount of water needed to grow a specific crop or landscape as the weather changes through the growing season or year. The state of California supports a statewide system of weather stations that measure and record Eto to guide irrigation use by farmers and landscape managers. For more information about this system, visit the CIMIS (California Irrigation Management Information System) website at www.cimis.water.ca.gov/. Click on the Eto tab for a comprehensive explanation of how this statewide weather service measures ETo. We have one CIMIS weather station in Orange County, located in Irvine near the El Toro marine base (latitude 33.689, longitude 117.721 for you GPS buffs). You can access daily and monthly Eto data for this station, like we do, at the CIMIS website.

It’s important to remember that any published Eto data is in the past.  Farmers start their irrigation cycle at what is called field capacity, the point where the soil is holding as much water as it physically can. From field capacity, Eto is deducted daily until a point is reached, depending upon the crop, when irrigation is required. In the case of landscape irrigation, yesterday’s or last week’s or last month’s or last year’s Eto is used both to allocate water to the user’s account, and as a basis for estimating the future water needs of the plants. Prediction of the weather is the realm of science, seers, and wizened farmers. We receive Eto predictions via our satellite weather service, and we refer to that for some guidance, although the predictions are not exceptionally accurate. We also rely on our own sense of historical weather patterns in the area, plant need cycles, the immediate past, and a magic eight ball to get an idea of what’s to come.

Why is Eto important to you? For one thing, its use as a means of allocating irrigation water to urban landscapes has been codified since 1990, when the Water Conservation in Landscaping Act (AB 325) mandated that localities statewide enact measures to regulate the design of new and rehabilitated landscapes within their permitting or design review jurisdiction. The “maximum applied water allowance” for landscapes watered with potable (drinking) water is 80% of local Eto. Landscapes using reclaimed water are given an allowance of 100% of Eto. Theoretically, the superior designs of landscapes and irrigation systems would ensure water savings. However, in practice and for the long term, maintenance of the entire system plays an immensely important role in realizing water conservation.

Changes to the WCLA were proposed this year, suggesting reduced budgets (to 70% by 2010 and 60% by 2015 of Eto) for potable systems, and no change (from 100% of Eto) for reclaimed systems. In my reading of the WCLA as well as the proposed amendment to it (AB 2734, which by the way did not get out of committee this year), I found no such budget mandate for existing, non-rehabilitated landscapes. Also, a recent study by the California Urban Water Association found enforcement of the 1993 local conservation ordinances to be “virtually nonexistent”, particularly regarding the post-construction period. So, since the potential conservation effects of the 1990 Act and the Water Efficient Landscape Ordinances it mandated have not been realized yet, it seems that amendment to the existing law is premature.

The WCLA does not address itself to the water providers, only to agencies responsible for land use planning. Even so, water providers have begun implementing water conservation policies and/or pricing structures that include establishment of allocations based upon Eto similar to those for new landscapes. An example of this sort of allocation:

    Month of April Eto at station 75 in Irvine was 4.04 inches, x 36.3 = 146.65 ccf     Possible budget for potable systems is 117.32 ccf (80% x 4.04 x 36.3)     Possible budget for reclaimed systems is 146.65 ccf (100%).

Your water provider may use a formula to determine your budget such as:Eto x Kc x 1.25 x 36.3 = allocation in ccf (hundred cubic feet billing unit) or some other combination of adjustment factors. Some water providers use historical or normal Eto (compiled from previous years’ data) to establish allocations. Your water provider may use their own weather stations, which theoretically would provide more accurate real-time measurement of local conditions. You could conceivably install your own weather station at your site, and use that data to establish your budget. However, the establishment of the billing criteria and % of Eto which equals the allocation is, I believe, solely at the discretion of the water supplier. We always suggest, as a starting point to any water and cost conservation effort, a thorough understanding of the water billing policies that apply to your site.

Clearly, the availability of Eto measurements statewide to help irrigators determine appropriate irrigation scheduling is good sense. Eto is a time-tested principal based on science and objective observation. It is important to be familiar with the process of data collection, how to obtain raw data, as well as the logic behind the way allocations or budgets are calculated for one to play an intelligent role in water use and management.