waterblogue

It is commonly reported that something like 40% of annual municipal water demand is typically used for landscape irrigation. This usage is also a major driver of demand peaking through the summer in our Central Texas climate. In Georgetown, for example, a recent Inside Climate News article (you can read that here) on the city’s efforts to obtain water supply from the Simsboro Aquifer to its east, the city manager is reported to have said “most” of that water will serve new residential developments and will be used “primarily” to irrigate lawns and other neighborhood landscaping. So it does seem that taking this irrigation off the potable water supply would be of huge benefit. Especially to cities, such as Georgetown, that are spending significant sums to import water from remote supplies. And in particular, shaving peak demands would be particularly beneficial, as peaking drives the costs of much of our water supply infrastructure. Like those hugely costly pipelines from the Simsboro.

Thus, it is reasonable to investigate, how might we accomplish taking irrigation off the potable water supply system?

Consideration of what this might entail reveals three main strategies that could be pursued:

1. Move to a more regionally appropriate landscaping ethic, minimizing turf and other landscaping that would need routine irrigation, prioritizing instead native plant landscapes, that would need far less irrigation – and would enhance the “sense of place” in the built environment.
2. Employ building-scale rainwater harvesting (RWH) to create the irrigation water supply, utilizing a resource that otherwise may become a stormwater management problem.
3. Create wastewater reuse systems to provide the irrigation water supply.

Along with all this, rendering landscape irrigation systems more efficient to reduce the demand to begin with, such as was reviewed on the Waterblogue here, must be an on-going effort, just as a matter of course.

On landscaping of a residential lot, simply because I am intimately familiar with it, I will use the landscaping at our house to illustrate the potential for measures 1 and 2. Here is the lot plan, showing the current configuration.

A street view of the current state of the front yard landscape is shown below.

And here is a closer view of the front patio area, the house entry visual.

And a bit of whimsy – SHARK!

When I moved in back in 1997, this front yard was rather typical for the neighborhood, two non-native trees and a swath of St. Augustine turf. Beginning in 2002, I started to transform it to what you see above, taking out a chunk of turf at a time, taking down the non-native trees and subbing in mountain laurels, a lacy oak and a cedar elm. This native landscaping is rarely watered. After the plants are well established, only during drought stress periods and sparingly then, so this landscaping incurs rather minimal irrigation water demand. The exception is the blackberries that I decided to try growing a couple years ago, that are in front of the raised planter with the accordion trellis on top of it. These plants do require routine irrigation to be viable, but the total amount of water they require is rather minimal.

In the back yard, we are doing some food gardening, in the raised beds, the in-ground potato bed, and blueberries in containers …

… which all does have to be watered frequently. But here again the areas are small, so the total water demand for the food crop irrigation is low, and generally all supplied from the rainwater tanks. We also have some citrus trees in containers – lemons and limes …

… and more blackberries in the back yard.

The sandbox, shown below, which my grandson has outgrown, is planned to become a milkweed bed to feed monarch butterflies.

As you can see in the picture above, the trellises along the yard walls, and in the picture below, around the veranda cover …

… are native plants, which are irrigated only during extended drought stress, when a bucket of water will be spread on them every so often. There are a number of hanging baskets and potted ornamentals that do require routine watering, but again the total amount of water required is very low.

The remainder of the back yard is covered with a small rain garden – you see that on the left edge of the picture above, it was featured previously on the Waterblogue here – with mulched beds containing native plants, and with non-irrigated “turf” areas. In the summer those “turf” areas become mainly populated with horseherb as the grasses fade – you can see that ground cover at the bottom of the picture just above. During long drought periods the horseherb becomes largely “dormant” – but always surges back green when the rains come.

You can see then that it is quite possible to move to a more regionally appropriate landscape style here in Central Texas, and not give up on having an attractive, interesting landscape. A landscape requiring minimal irrigation does not have to be, as an Austin city counselor once dimly put it, all rocks and cactus. While the turf aesthetic does still dominate the neighborhood, I have witnessed a number of similar transitions of front yards in the 20+ years since I started transforming mine. And the appearance of more and more rainwater tanks.

Leading us to strategy 2, using roof-harvested rainwater instead of potable water for irrigation supply. As shown on the lot plan and in the pictures below, we have a rainwater tank at each corner of the house, to capture all the roof areas.

The tank capacities, shown on the lot plan, total to 2,875 gallons of storage capacity. As you can see in the tank pictures, the shorter tanks are set on pedestals so that the tops of the tanks are all at about the same elevation. This was done because the tanks are tied together by 1-1/2” pipes running underground between them, so they will all operate as one hydraulic unit, all overflowing at the same elevation. That was done because each tank intercepts different roof areas, and so if not tied together, one tank could be overflowing before others are full, so this maximizes the collection efficiency off the entire roofprint.

So far, this RWH system has been efficient enough that over the last few years, since we started growing food as well as maintaining the visual landscape, only in the depths of the drought in the summer of 2024 did we have to use any significant amount of potable water for irrigation. You can see then that even such a limited RWH system can largely maintain a regionally appropriate native landscape, such as we have, as well as some food gardening.

Turning now to strategy 3, wastewater reuse. Here in the middle of the city on such a small lot as ours, reuse would generally not be feasible. Unless of course Austin Water were to install the “purple pipe” system redistributing reclaimed water from its centralized treatment plants into neighborhoods like ours, a very unlikely scenario, since that would be inordinately expensive. On any lot out in the countryside, however, that has a “septic” system – or OSSF (On-Site Sewage Facility) in regulatory system speak – folks could install a system that produces and reuses reclaimed water, as is reviewed on the Waterblogue here, and on my company website here. Really, just about every OSSF could be a reuse system, defraying whatever amount of irrigation may be needed on each lot. Cumulatively, over this region, that could conserve a lot of water.

In new development, however, a collective wastewater reuse system to defray irrigation demands can be quite feasible, even saving money relative to the cost of installing and running a conventional wastewater system to serve these developments. As was reviewed in “This is how we do it”, the dispersal fields of a decentralized concept wastewater system could be arrayed to irrigate front yards, parkways, parks, the margins of walking trails, etc. The illustration from that blog is shown below. By this means, a large portion of irrigation demands in new development – of any sort, not just residential projects – could be cost efficiently carried by wastewater reuse.

Before proceeding, it is noted that by integrating rainwater harvesting with the water quality management strategies that development rules in this region typically require, a further contribution to irrigation water supply can be realized in the sort of development illustrated above. This strategy was reviewed on the Waterblogue in “… and Stormwater Too”. The illustration of the strategy in that piece is shown below. The rooftop runoff, and perhaps the water falling on the driveway too, can be stored and used for irrigation water. Depending on the volume of storage provided, this could largely obviate using potable water for back yard irrigation, on top of the savings from wastewater reuse to irrigate the front yards. Between them, pretty much taking irrigation off the potable water supply.

Now let’s look beyond the residential lot level, to multi-family and commercial developments. Below is the street-facing landscape of an apartment village, a large area of turf.

And right across the street from that, we see the same thing, very large areas covered by turf, that appear to be well irrigated, given that they stay as green as they appear in these pictures through the Austin summer.

Note that these turf areas serve no purpose that would dictate that turf must be the landscape. No, this is simply an aesthetic, someone’s evaluation of what such areas “should” look like. Might such an evaluation be called to question, on sustainability grounds? Why could not such areas be instead native wildflower meadows? Looking something like this?

This sort of landscaping would present the project’s street view just as well, in any functional sense. Indeed, it would impart a “sense of place” to the site. And it would require far less irrigation to maintain it in “presentable” condition. It would also require much less routine maintenance, perhaps “trimming” a couple times a year, and no applications of fertilizers, herbicides or pesticides. So the labor and materials inputs to maintain such areas would cost the project owners less, beyond the avoided cost of irrigation water.

Now in such projects as those apartment communities shown in the above photos, it may indeed be reasonable to implement a point-of-use wastewater reclamation system – again in the same manner as was illustrated in “This is how we do it” – to produce whatever irrigation water may be needed to maintain such landscapes. As well as to irrigate the turf that “normally” covers the grounds of such projects, like we see in the picture below.

Being within the area already served by the conventional sewer system, such a project-scale wastewater system could provide the reclaimed water for irrigation on demand, and whatever treated water flow is not required for irrigation supply could simply overflow into the sewer, reducing the organic load on the conventional wastewater system.

It’s an open question of course what the payback on avoided irrigation water costs may be – given that water is currently priced well below the costs of replacement water supplies, such as importing water from remote aquifers – or what the operations and maintenance costs of such project-scale systems may be, understanding they are essentially “redundant” to the conventional system. These are matters that should be investigated, comparing these costs to the long-run marginal costs of other options for obtaining new water supplies, given the sustainability benefits of taking irrigation water supply off the potable water system. Noting that, while such actions might appear to be a stretch as retrofits of existing development, it may be quite cost efficient to design them into the very fabric of new development.

It is also noted in passing that covering such areas with wildflower meadows instead of turf would impart stormwater management benefits. This is because the wildflower meadow would have a somewhat lower propensity to produce runoff than would the same area covered by turf. “Curve Number” (CN) is the measure of how “absorptive” a soil-plant complex would be under the Soil Conservation Service method for evaluating stormwater drainage issues; the higher the CN, the more runoff would issue from the area, given the same rainfall. For a wildflower meadow on the areas shown in the pictures of the apartment communities above, CN would be about 65, while if covered by conventional turf it would be about 79. The wildflower meadow would start issuing runoff at a rainfall depth of 1.08”, while the turf area would start issuing runoff at a rainfall depth of only 0.53”, so on an annual basis the wildflower meadow would issue somewhat less runoff to be managed in stormwater facilities.

And these wildflower meadows shouldn’t need any stormwater quality management at all; they themselves might even serve as the stormwater quality management device for runoff from development upslope. Peak flow rates to be managed would also be somewhat reduced. Applied at scale in a watershed, this could significantly reduce the costs of installing and maintaining stormwater ponds, rain gardens, and such. Furthering the benefit of adopting the more regionally appropriate landscaping ethic.

Still, however, there will be turf to irrigate. In discussing this matter with the city councilman representing my district, he asserted that folks in an affluent area of town would not willingly part with their turf-covered yards. So let’s look at that sort of situation.

Looking at the area of town the councilman referred to on Google Earth, I see the lot shown in the picture below. Not at all atypical of that neighborhood, this one just happens to be fairly free of tree cover, making it easier to measure the area of turf and of roofprints available for rainwater harvesting.

Roughly measuring off Google Earth, I calculate there is about 3,500 sq. ft. of roofprint covering the house and the garage, and I calculate there is about 5,000 sq. ft. of turf landscape, and minimal other landscaping on this lot. So let’s look at how much water would be required to keep that turf looking lush, and the prospects for supplying it with other than potable water.

Inserting those areas into the rainwater harvesting model, programmed with Austin historic rainfalls over the period 2007-2023 and an irrigation water demand profile suitable for turf around here, and presuming a 10,000-gallon cistern were installed, RWH would have covered 62% of the total irrigation water demand over this period. In the severe drought year of 2011, only 24% coverage would have been provided by harvested rainwater. 100% coverage of irrigation demand would have been provided in only one of the 17 years covered by this model.

If a 20,000-gallon cistern had been installed, the overall coverage would have risen to 81%, with the 2011 coverage rising to 37%. 100% coverage would have been provided in 9 of the 17 years covered by the model. Increasing cistern size to 30,000 gallons, overall coverage would have risen to 90%, and the 2011 coverage to 51%. 100% coverage would have been attained in 11 of the 17 years.

It would of course need to be investigated where a cistern might be accommodated on the lot and what such a rainwater harvesting system would cost. Beside the cistern, the costs of proper guttering, collection lines, and a treatment unit – consisting of only a cartridge unit for this irrigation usage – and pressurization facilities, and system on-going operations and maintenance, would have to be calculated, to determine the amortized cost of this rainwater supply. Once again noting that payback may not appear favorable relative to the defrayed cost of the potable water supply for this irrigation, since the actual replacement cost of an equivalent amount of water supply from other new sources would no doubt be much higher than the current water rates charged by Austin Water. But it may compare favorably to the long-run marginal costs of other new supply sources.

Turning now to on-lot wastewater reuse for irrigation supply, as was noted above, installing an OSSF to produce reclaimed water would greatly defray draw on the potable water supply. As for what such an on-lot treatment unit might look like, we see such an installation on a lot in an urban fringe neighborhood in the picture below.

As you see, not really very “intrusive”, with only the top of the filter bed and the tank hatch lids visible on the surface. If this were deemed too visually intrusive, these facilities could be tucked into a corner of the lot and screened with plants, such as the Texas sage plants in the above picture.

How much irrigation water this system would produce would of course be set by the amount of wastewater produced in the house, which would be largely driven by the house population. Presuming a 4-person occupancy, the average daily flow would be likely be in the range of 200 gallons/day. With a 5-person occupancy, it might be about 250 gallons/day. Over the 5,000 sq. ft. of turf area on the lot we are looking at here, this would provide an irrigation rate of about 0.45-0.56 inches/week. That would be more than sufficient over 7 months of the year, but deficient in the other 5 months, which is the peak irrigation season, May thru September. In those months this reclaimed water flow would provide between about 1/2 to 2/3 of the total modeled irrigation demand. RWH could defray the rest.

Again, it would have to be investigated what such an OSSF would cost, including the subsurface irrigation system that would disperse the reclaimed water underground, to minimize contact hazard potential in such an “unsupervised” environment. This would set the cost of this new water supply, that could then be compared with the long-run replacement cost of water supplies by other means.

While it may appear questionable if providing new water supply on-lot with rainwater harvesting at the scale reviewed above or by installing an OSSF would “pencil out”, it is noted than no such options have so far been considered and evaluated by Austin Water as it conducts planning under its Water Forward program, touted as a 100-year water plan for Austin. Given that long time frame, it is called to question if failing to consider such “outside the box” options as these is missing a march.

Noting again that while retrofits such as reviewed here may not be deemed fiscally viable, the situation in new development, where these facilities may be designed in from the start, may be very favorable. Cities in situations such as Georgetown finds itself – and San Marcos and Liberty Hill and Dripping Springs, anywhere that growth is straining current water supplies – could go a long way toward blunting its problems by taking the 3 measures we’ve reviewed to pretty much take irrigation off the potable water supply:

1. Move to a more regionally appropriate landscaping ethic.
2. Employ building-scale rainwater harvesting to create the irrigation water supply.
3. Create wastewater reuse systems to provide the irrigation water supply.

These opportunities are there for the taking in many, many situations all over this region. Again, particularly in new development – understanding it is growth that is largely straining current water supplies – where these strategies could be essentially designed into the very fabric of the development, providing opportunities to actually save money while saving water. As has been argued in several other Waterblogue posts – see for example here, here and here.

Despite this, such strategies have not appeared in any of the regional water plans, nor as noted in the Water Forward process in Austin. It is suggested that water planners give these strategies a bit more attention than they have received so far. As was noted on the Waterblogue in this post, society would be well served by taking a peek down the road not taken … so far, as that could make all the difference. Are the water planners around here up to that task?