Posted tagged ‘drip irrigation’

Irrigation Efficiency – a new “reservoir” for your city

March 11, 2013

Let’s start this one with a BIG NUMBER. To quote the web site of the Alliance for Water Efficiency, “The efficiency of overhead irrigation, such as rotors and pop-up sprayheads, is typically 50 percent and rarely exceeds 70 percent. The efficiency of a well-designed drip irrigation system can reach nearly 100 percent.” This indicates that irrigation efficiency could be as much as DOUBLED by converting to drip. Or, to put it more graphically, the same amount of irrigation would be accomplished using HALF THE WATER! System-wide, that would be a VERY BIG number.


But wait a minute. Take a close look at the irrigation “system” in the above picture, a not at all atypical scene along the streets of my city, Austin. 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%? Or less?!

Now sure this picture was selected exactly because it serves as a particularly bad example, but as noted it is not all that atypical. One morning, I rode my bike through my South Austin neighborhood and took note of all the irrigation going on that day, 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, just like we see in that picture above. 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, VERY BIG number! As the title of this piece notes, it would be sort of like a whole new “reservoir” for your city’s water supply.

A “reservoir” of relieved capacity at just time it is most needed! That’s because water savings obtained by increasing irrigation efficiency comes directly off the peak demand, since that is driven almost exclusively by irrigation water use in this region. And it was purported by the City of Austin that a growth in peak demand created the need to build the new water treatment plant it is presently constructing sooner rather than later. So, as is no doubt the case in many cities, measures to increase irrigation efficiency would be particularly valuable to the overall system, allowing sufficient service to be provided without having to increase their peak supply capacity. Yet these measures generally remain quite neglected, in terms of any programs explicitly aimed to stimulate, promote or require them.

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 aggressively pursuing irrigation efficiency has been pretty much neglected as a part of most city’s water conservation programs – it would require the stimulation of many individual actions, through education, incentives and/or mandates. The city bureaucracies no doubt consider 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, as just noted, is unlikely to be the most cost efficient strategy. To move toward sustainable water, it’s clear we will have to take on “distributed” measures like irrigation efficiency at some point. So why not now, BEFORE we put ourselves in hock for expanded peak supply capacity that could be avoided?

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 depth of good quality soil also allows more rainwater to infiltrate and holds more rainfall in the root zone, irrigation can be delayed longer after a rainfall, also saving water. And because improving the soil reduces runoff, so blunting stormwater management problems, it’s a win-win-win sort of strategy.

Requiring a minimum depth of soil was urged by a resolution of Austin’s Resource Management Commission 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.

This illustrates the insidious nature of allowing today’s first cost issues to dominate what should be a long-term strategy. This is a ubiquitous problem plaguing many efforts to instill deep conservation practices.

Another aspect of irrigation efficiency is watering at the optimum time. You don’t want to lose water to runoff or 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 real-time 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. “Smart” irrigation control systems that can do this are readily available, and are cost efficient for high usage systems, where savings would be most significant.

All these factors 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. He is doing this, so gratuitously wasting a lot of water, rather than financing a highly efficient drip irrigation system, which is exactly the best way to water areas like the parkway strip between the sidewalk and the street in that picture. And no one is really telling him he should not be wasting water like that.

The Austin Water utility’s propaganda does say that intentionally spreading water on pavement is considered illegal, so I don’t mean that literally no one is telling this person that “irrigating 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, that it is so economically inefficient, that he – and all the rest of us – are paying for his wastefulness by financing increased water supply capacity, needed only for peak demands that are driven by that wastefulness.

It is also notable that considerable efficiency may be gained simply by better educating people who are using well-designed systems about the actual need for irrigation. One city’s conservation department compared actual irrigation rates to ET (evapotranspiration) rates obtained from weather stations and found that most users were drastically over-irrigating. This launched an effort to educate their irrigators, which reduced irrigation water usage city-wide substantially.

All this highlights the systematic neglect of irrigation efficiency on the part of most cities. 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 deep conservation actions, like highly efficient irrigation systems, at the level that reflects their real value to the overall water supply system. Or they could mandate those that are clearly fiscally efficient for the end user (despite perhaps being more costly to the builder) – like requiring drip irrigation in all new projects – to forestall, or even avoid entirely, having to do things like spend a billion dollars to expand treatment capacity.

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 most cities have 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 have, by itself, allowed Austin to delay construction of its new water treatment plant by a decade or more. How many more cities could, in essence, gain a new reservoir’s worth of capacity simply by investing aggressively in irrigation efficiency?