Zero Net Water

A sustainable water development concept for the Texas Hill Country – and beyond

Imagine a water management strategy that would accommodate growth and development without unsustainably pumping down aquifers or incurring the huge expense and societal disruption to build reservoirs or transport water from remote supplies to developing areas. Welcome to the concept of Zero Net Water.

As the name implies, Zero Net Water is a water management strategy that results in zero demand on our conventional water supplies – rivers, reservoirs and aquifers. Under the Zero Net Water development concept, water supply is centered on building-scale rainwater harvesting, “waste” water management centers on project-scale reclamation and reuse, and stormwater management employs distributed green infrastructure to maintain the hydrologic integrity of the site. Together these result in minimal disruption of flows through a watershed even as water is harvested at the site scale and used – and reused – to support development.

The key is taking advantage of the difference in capture and distribution efficiency between a building-scale rainwater harvesting system and the watershed-scale rainwater harvesting systems that compose all of our conventional water supplies. The basis for this is illustrated in the schematics below.

The prevailing conventional water supply strategy – again, this is watershed-scale rainwater harvesting – is illustrated in this schematic. Typically only a very minor fraction of the total rain falling onto the watershed makes it into the “cisterns” of that rainwater harvesting system – the aquifers and reservoirs. The rest is lost to evapotranspiration, a “loss” which maintains the ecology of the watershed. Water that does make it into reservoirs is subject to high losses to evaporation, up to half of total inflow. So the inherent capture efficiency of this system is quite low.

Water supply produced from these watershed-scale “cisterns” is distributed to points of use – where some of the rain fell to begin with – a process which also suffers significant losses. Water industry standards recognize a 15% water loss in the distribution system as “good” performance, and many water systems have much greater losses. So here too we suffer an inherent inefficiency in turning rainfall into water supply that is available for human use.

The building-scale rainwater harvesting concept is illustrated in this schematic. Close to 100% of the rain falling onto a rooftop can be captured and converted into usable water supply. There will be some losses to cistern overflows in large storms or when there is an extended period of wet weather, so the actual efficiency will vary with weather patterns, but in a system properly sized relative to the water usage pattern, it will be consistently very high. The building-scale distribution system is typically – and can practically be – maintained “tight” so there would be negligible distribution losses.

This high capture and distribution efficiency allows the water supply to be “grown” in fairly direct proportion to water demand, one building at a time, thus rendering this a more sustainable water supply strategy. And because the water supply would be provided, and paid for, only to serve imminent development, this strategy is also economically efficient, and thus more fiscally sustainable.

An immediate, practically knee-jerk, objection to this water supply strategy is that harvesting rainwater off rooftops would “rob” the watershed of streamflow and/or recharge, and would thus produce no net gain in the available, usable water supply. As can be inferred from the illustrations above, this is not the case. When not directly harvested, a large majority of roof runoff would be abstracted in the watershed. In any case, when land is developed, the amount of rainfall that becomes quickflow – water that runs directly off the land – increases, and the amount that infiltrates is reduced. Because of the other impervious surfaces besides the rooftops that development adds, the volume of runoff would typically increase even if building-scale rainwater harvesting were to be implemented on all the buildings in the development, as noted in the illustration below.

Indeed, because development increases runoff, development regulations generally require that steps be taken to treat and detain this excess runoff. Broadscale practice of building-scale rainwater harvesting can actually reduce the magnitude of this problem. The net result in any case is that the post-development runoff volume is typically greater than the predevelopment runoff volume, thus there would be no “robbing” of flows into the watershed-scale water supply system, relative to the pre-development flow regime.

In any case, the water sequestered in the building-scale cisterns is not removed from the watershed. Its release back into the hydrologic cycle is simply delayed. Most of this water, once used in the building, appears as wastewater flow. As reviewed below, and illustrated in the schematic above, under the Zero Net Water concept this flow would preferably be used to defray irrigation demands, so doing a better, more targeted job of maintaining some of the plant life in the watershed. If instead the wastewater were discharged into streams (after treatment of course), the result would be to create a more steady flow of this water over time, as opposed to the “flash” hydrology imparted by direct runoff from the rooftop.

A simple way to encapsulate all this that we capture and utilize on site much of the additional runoff imparted by placing impervious surfaces over the land. We do this instead of allowing this additional runoff to become an increased quickflow that, if not mitigated in some other way, creates water quality, channel erosion, and flooding problems. So bottom line, broadscale practice of rainwater harvesting off all the buildings in a watershed would actually improve the overall yield from the watershed of water that would be directly usable by humans.

“Right-Sizing”

There is a caveat on the “zero” in Zero Net Water. The cistern in a building-scale rainwater harvesting system operates in the same manner as a reservoir in a conventional surface water supply system – it stores the water for future use. Just like a reservoir, a building-scale cistern has a “firm yield” that will cover a given water demand profile. The building-scale cistern is typically sized to cover most conditions, with imported backup supply added to get through the worst drought periods. 

Considerations of cost efficiency and the sustainability of the backup supply system lead to the concept of “right-sizing” of the building-scale rainwater harvesting system. This is the combination of roofprint and cistern volume relative to the expected water usage profile that would result in only limited backup supply requirements, needed only during the worst drought periods.

The backup supply would of course be drawn from the conventional water supply systems, from aquifers and/or reservoirs. So there would be some small draw of water from the watershed-scale system to get the building-scale rainwater harvesting systems through the droughts. The magnitude would depend on how well the building-scale systems were “right-sized” and on whether the users of those systems practiced “sufficient” conservation, and also of course on the happenstance of the rainfall patterns over the area. Still, modeling indicates that the vast majority of the water supply for these buildings would be provided by direct capture of the rainfall onto the building’s roofprint.

The “right-sized” facilities vary around the state, depending on the area’s climate. In the Texas Hill Country, a 4-person household which is “reasonably” conservative with their water use typically requires a roofprint of 4,500 sq. ft. and a cistern volume of 35,000 gallons. These sizes could be decreased if the users practice very good water conservation. Most cost efficiently incorporating “extra” roofprint, and perhaps integrating the cistern into the building envelope, are the province of building design concepts. It is suggested that efforts be made to formulate a “Hill Country rainwater harvesting vernacular” house design concept to address those matters. This needs to be taken up by architecture schools, working architects and homebuilders.

Wastewater Reuse

That “right-sized” system noted above will only cover interior water use. To supply landscape irrigation directly from the cistern would require either a significantly larger system or would incur significantly greater backup supplies. However, there is a flow of water right there, water that has already been provided for use in the house – the wastewater flow out of the house. This flow can be treated and dispersed in a subsurface drip irrigation field to defray landscape irrigation demands. Modeling shows that doing this, a sizable area of irrigated landscaping can be maintained without having to either upsize the cistern and roofprint or incur much greater backup supplies.

This strategy was reviewed in “Slashing pollution, saving water – the classic win-win (but ignored by society)”. As set forth there, this sort of reuse system has been implemented on the site scale for over two decades, and doing so will provide superior environmental protection, particularly in sensitive watersheds. It is a small step to do this same process on a project scale, if the nature of the development requires that it employ a collective wastewater system, rather than an individual on-site system for each house. That project-scale reclamation and reuse concept must be part and parcel of the Zero Net Water concept if irrigated landscaping is to be supported.

Stormwater Management

As noted previously, development causes an increase in quickflow runoff at the expense of infiltration due to some of the ground area having been covered with impervious surfaces. These impervious surfaces also increase levels of pollution entrained in the runoff. So development regulations typically require that methods be implemented to blunt both the pollution and the impacts of the additional runoff on downstream flooding and on channel erosion. The building-scale rainwater harvesting systems can help to blunt all these impacts by sequestering roof runoff in the cisterns.

Runoff from the rest of the development and any cistern overflows can, and should, be addressed using distributed low-impact development (LID) practices, focusing on intercepting and infiltrating an initial depth of runoff deemed to have entrained most of the pollution. The aim of the LID strategy is to restore the rainfall-runoff response of the developed site as close as practical to that of the pre-development site. This matching of runoff to pre-development conditions would maintain the hydrologic integrity of the site, and by a multiplicity of sites so treated, would maintain the hydrologic integrity of the watershed. This whole area of “green” stormwater management is the subject of a future entry on this blog. Suffice it here to note that it is an important element of Zero Net Water, as it holds more water on the land and thus blunts the “desertification” of the site that development typically imparts.

Confirmation – and Challenges

Modeling indicates that for all locations in and around the Hill Country, “right-sized” rainwater harvesting systems would not have required any backup supplies after the severe drought of 2010-2011 broke in late 2011, even though the general impression is that drought has persisted in this region. One indicator of this is that water levels in Lake Travis and Lake Buchanan remain very low. Indeed, it is reported that inflow to the lakes in 2012 was the 6^th lowest year on record and in 2013 it was the 2^nd lowest, above only the extreme drought year of 2011. It is noted that this occurred despite total annual rainfalls over the drainage basins flowing into the lakes having been generally around the long-term average rainfalls there over those two years.

This is simply a confirmation that the capture efficiency of the building-scale rainwater harvesting system is inherently much higher than that of the watershed-scale system. The low inflows to the lakes are a happenstance of rainfall patterns, failing to create the large runoff events needed to significantly raise lake levels. But those same rainfall patterns would result in high capture efficiency off of a rooftop, and so the building-scale systems would not be under the same stress that persists in the watershed-scale system.

Despite the overall efficiency of the building-scale rainwater harvesting system, the Zero Net Water development concept faces challenges to becoming commonly practiced. The building design issues were noted above. The large roofprint required to “right-size” systems in Central Texas would require “right-sized” lots to accommodate it. And two-story houses would clearly be problematic under this concept. Multi-family housing, as presently configured, would also be hard pressed to provide 

roofprint commensurate with water demand. Then too storage cisterns would take up space, unless they were integrated into the building envelope. All that would have implications for development style, and so would require some tinkering with prevailing development models.

On the other hand, with a typical occupancy of only 2 persons, water demand in seniors-oriented developments – which may be a considerable portion of new development in Central Texas – would be supported by the roofprint typically provided by a one-story house plus garage. Many commercial and institutional buildings would also have a favorable relationship of roofprint to water use in the building. Indeed, as asserted in “First ‘Logue in the Water”, these would be prime candidates for a Zero Net Water strategy. Employing some combination of building-scale rainwater harvesting, condensate capture, and project-scale wastewater reclamation and reuse, those types of buildings would draw no water from the watershed-scale systems. That would relieve a significant portion of demand due to growth, and as a bonus would also blunt stormwater impacts in those sorts of projects, which typically entail high impervious cover, the roofprint being a significant portion of it.

Cost is, of course, a primary consideration, for society at large as well as development principals. Where there is already a conventional water system nearby which has capacity to provide service to the development, the cost of the building-scale systems could not be justified relative to installing conventional distribution infrastructure. Wherever capacity is a problem, however, then the real cost of increasing capacity has to be figured in. Where those costs are very large – e.g., building a new reservoir or tapping a remote aquifer and building pipelines to deliver water from those to growth areas – then building-scale facility costs may be globally competitive. And as noted previously, building-scale facilities require money to be spent only to support buildings as they are built, while those area-wide strategies require huge investments up front of being able to sell the first lot, so the Zero Net Water concept is inherently economically efficient.

We must indeed consider costs globally, not just the immediately apparent costs of continuing with “business as usual”. This comes starkly into play in the Hill Country, where aquifers are under stress even at current usage rates, and serving considerable new development out of them will only “mine” them further. The drawdown created is drying up springs that historically flowed all throughout the Hill Country. That creates a cost to the local ecology, and will fundamentally alter the character of the region. The impact of this degradation was encapsulated in the title of an article appearing in the Texas Observer a couple years ago, “The End of the Hill Country”. This is not to mention reducing water availability from the rivers flowing out of the Hill Country, water which is depended upon for both water supply and ecological services all the way to the Gulf of Mexico. Thus the Zero Net Water development concept may be particularly valuable in the Hill Country.

While the Zero Net Water development concept faces fiscal and institutional challenges, the prospects for sustainably accommodating growth in a globally more cost efficient manner urge its consideration as a water management strategy for new development over much of Texas. Particularly in areas like the Hill Country, where aquifers are under stress and the only other option is a long distance “California style” water transfer from remote aquifers or reservoirs, which may entail both fiscal and ecological sustainability issues. The Zero Net Water concept offers a pathway toward sustainable water even where high growth rates are forecast. It remains only to address the challenges and to put it into practice.