But wait a minute. Take a close look at the irrigation “system” in the picture. That 50-70% efficiency estimate is for a “designed” spray system – using rotors and spray heads, laid out in a pattern that provides head-to-head throw of water, uniformly covering the area to be irrigated, and hopefully with the spray arcs set so that very little water sprays over areas not intended to be irrigated, like sidewalks and driveways. What do you suppose the “efficiency” of the spray operation in the picture would be, spreading much of the water on the sidewalk and street? Maybe 20-30%?

Now sure this picture was selected exactly because it serves as a particularly bad example, but really it is not all that atypical. On a recent morning, I rode my bike throughout my neighborhood and took note of all the irrigation going on, at about 25 houses in all. Of those, only a couple were “solid set” systems using pop-up spray heads. The rest were hose-end sprinkler applications. And in only one of the operating systems was there no overspray onto pavement! Most of them were dropping A LOT of the water onto pavement, creating rivulets running along the curb. It’s a small sample of the entire city, to be sure, but it indicates that these low-efficiency operations are more common than well-designed spray systems. So it may be that converting those irrigation operations to subsurface drip would perhaps TRIPLE – or more – the efficiency. System-wide, that is a VERY BIG number.

This highlights that increasing the efficiency of irrigation operations could be a huge water saver. Not in one big fell swoop, but by the multiplicity of many, many small actions. And that is probably why irrigation efficiency has been pretty much neglected as a part of the city’s water conservation program – it WOULD require the stimulation of many individual actions, through education, incentives and/or mandates. The city no doubt considers that “too hard” – much easier to just build more capacity, which is under its unilateral control, they think, even though an ever-expanding supply is not sustainable – and may not be the most cost efficient strategy. So it’s clear we WILL have to take on measures like irrigation efficiency at some point. So why not now, BEFORE we put ourselves in hock for a billion dollars to expanded supply capacity?

The application efficiency – accurately routing the water onto the plants you want to irrigate – is only part of the overall efficiency. Other aspects must also be addressed to maximize the savings. One of them is the quality and depth of the soil. The more soil over the irrigated area and the higher its “sponge effect”, the more water it can hold, so more water would be held in the soil until the plant roots can take it up, rather than draining through the soil and being lost to the plants. Because more soil depth also holds more rainfall in the root zone, irrigation can be delayed longer after a rainfall, also saving water. As a bonus, improving the soil reduces stormwater runoff, so it’s a win-win-win sort of strategy.

Requiring a minimum depth of soil was urged by a Resource Management Commission resolution in early 2006, and was considered by the water conservation task force later that year, but it wasn’t included in the water conservation program, reportedly due to objections from builders. You see, builders are totally focused on the installation cost and aren’t impacted by the long-term costs of having to “over-water” because there’s not much soil there to hold the water. So the city, in its infinite wisdom, chose not to impose that cost on the builders, rather to in effect subsidize them by enduring the inefficient irrigation that results, so driving a perceived need to provide more water treatment capacity, for which the rest of us will pay.

Another aspect of irrigation efficiency is watering at the optimum time. You don’t want to lose water to leaching below the root zone because watering took place when the soil was still “too wet” – either because the area had been recently watered or because there had been recent rainfall. To maximize this aspect of efficiency requires either expert management, consistently applied – which simply does not happen, is not practical, for most irrigation systems – or using an irrigation control system which can sense when irrigation is needed. Means of doing this are readily available, and are cost efficient for high usage systems, where savings would be most significant.

All these factors also highlight the importance of good system design. As noted, it is likely that a lot of irrigation water runs through systems that are not designed at all, rather are simply a movable sprinkler at the end of a hose.

As noted, those hose-end systems may operate at very low efficiency. Look again at the picture, at the low regard for watering efficiency exhibited by setting the sprinkler on the sidewalk. Now I would speculate that the person who did this is not doing it because he is dumb, rather he is simply using the piece of equipment that he has to get water onto the parkway strip between the sidewalk and the street. And no one is telling him he should not be doing that.

The Water Utility’s propaganda does say that intentionally spreading water on pavement is considered illegal, so I don’t mean that no one is telling this person that setting the sprinkler on the sidewalk is not legal. I mean that he is not receiving any signal through either the billing system or through any incentive program that wasting water in this manner is not in the public interest.

This highlights the systematic neglect of irrigation efficiency on the city’s part. It seems rather basic that they need to examine the various means of increasing irrigation efficiency that were reviewed above. They need to come up with estimates of the system-wide water savings that could be attained by widespread application of those measures and of the costs of implementing those actions. This then would reveal the price of this “relieved capacity”, and that could be compared with the price to be charged for adding that same supply capacity to the system. Then the city could incentivize those actions at the level that reflects its real value to the overall water supply system. Or mandate those that are clearly fiscally efficient – like requiring drip irrigation in all new projects – to forestall having to spend a billion dollars to expand treatment capacity.

Note that relieved capacity obtained by increasing irrigation efficiency comes DIRECTLY off the peak demand, since that is driven almost exclusively by irrigation water use. And it is purported by the city that a growth in peak demand is creating the need for building WTP#4 sooner rather than later. So measures to increase irrigation efficiency would be particularly valuable to the overall system, yet these measures remain quite neglected, in terms of any programs explicitly aimed to stimulate, promote or require them.

So back to that person who set the sprinkler on the sidewalk, you’ve got to figure out if the fiscal signals you can “reasonably” send will influence this behavior in a meaningful manner, and if not, then how efficiency could be enforced in order to proliferate it. This is an effort that the city has so far chosen not to pursue, and so irrigation efficiency, despite that BIG NUMBER noted at the beginning, remains a neglected stepchild. Changing this might, by itself, allow WTP#4 to be delayed for several years.