Exploring the Feasibility of Rainwater Harvesting in Southern California
Following the evolution of municipal separate storm sewer system (MS4) permitting in California reveals the growing number of regions within the state that currently or are soon to face mandated “retain/reuse” requirements as part of any future development or redevelopment project. The retain/reuse aspect is interpreted in most instances to be satisfied by management approaches such as infiltration (groundwater recharge), evapotranspiration, and rainwater harvesting. However, southern California’s development future involves much more redevelopment and infill, as opposed to construction within greenfields. These infill areas are invariably in proximity to dense existing infrastructure and/or poorly draining soils. The risk of incorporating low-impact development (LID) measures such as bioretention and permeable pavement in these types of areas would typically be mitigated through the use of subdrain systems and other methods necessary to make the members of our geotechnical and legal communities sleep better at night. These types of approaches are now considered at odds with the goals of retain/reuse. Because infiltration is, in many instances, either not possible or not advisable, by default the designer is left with implementation of rainwater harvesting.
Rainwater harvesting is a great concept. People in various regions around the world have been utilizing it for literally thousands of years. The approach has been documented to have been used long ago in the Middle East to support agriculture. More recently, organized rainwater harvesting programs are flourishing in areas within the United States such as Texas, Ohio, Hawaii, Florida, and even Arizona.
Southern California faces unique challenges not found within any of the regions mentioned above. These challenges are largely a function of climate and precipitation patterns, but also include an inherently pronounced economic disincentive compared to other water reuse alternatives, as well as a possible contradiction with other stormwater discharge requirements related to hydromodification. Hydromodification is the environmental result of an artificially altered rate of erosion or deposition within a natural streambed. It is typically a result of urbanization but can also be caused by a range of other human activities such as dam construction, dam removal, or channel hardening. These obstacles are all beyond the “normal” challenges faced in regions that utilize rainwater harvesting.
In the process of performing the analysis necessary to answer the questions regarding southern California’s ability, given limited participation, to satisfy typical annual water balance needs and the financial feasibility and investment on return, an apparent contradiction with the fundamental intent behind local hydromodification standards became apparent.
Rainwater Harvesting: Annual Balance Issues of Southern California
San Diego County, the second most populous area in southern California, receives about 10.6 inches of rain in a typical year. About 83% of that amount occurs within a five-month period ranging from the end of October to the beginning of April. During this rainy season, this works out to about 1.76 inches on a monthly basis, and on a weekly basis 0.41 inch. In contrast, cities currently using rainwater harvesting, such as Austin, TX, and Sydney, Australia, receive about 33 inches and 43 inches of rain per year, respectively. Austin and Sydney both have a much more even precipitation distribution of rainfall throughout the year compared to San Diego.
The amount of precipitation that can be harvested is a function of a number of physical properties that relate to precipitation intensity as well as the efficiency of the collection system itself. Long-term records show that a substantial percentage of rain days during a typical year in San Diego, or any area of southern California, do not produce runoff. All factors being equal, “harvestability” of rain on a per-inch basis is more difficult in San Diego and other coastal and arid regions of southern California when compared to other locations that are using rainwater harvesting.
During the rainy season of 1966–67, Lindbergh Airport in San Diego recorded 10.86 inches of rain that occurred over 44 days of measured precipitation. In 1972–73, 10.99 inches were measured over 57 days. In 2002–03, 10.31 inches were measured over 31 days. These three years provide good insight into what a typical rainy season looks like from an event-based perspective. This equates to 0.24 inch as a daily average. In typical storm events, the relationship between rainfall and runoff is extremely difficult to correlate. The influence of antecedent moisture is a large contributing factor. However, it can be assumed that many of the 0.24-inch-and-less events do not contribute to runoff. These would account for roughly two-thirds of the precipitation events measured in these three rainy seasons. On a typical residential lot that is 50% impervious, optimal results would be to harvest 75% of the total precipitation volume. The reality is that harvesting would result in capturing less than 75% of the runoff, dependant on antecedent ground moisture and the inherent inefficiencies of the collection system and limitation in volume of the storage tank.
Assuming that a typical household has three people and is irrigating 4,000 square feet of pervious yard area, an average daily total water demand of approximately 395 gallons per household would exist. This would correlate well with my personal historic records after adjustment for variation in lot size and household members. Billing records from my own local water company indicated that last year my family of four was using 299.2 gallons per day of water. This covers both indoor and outdoor use. The outdoor use is essentially all irrigation necessary to sustain the turf landscaping that makes up roughly 50% of my lot. Indoor needs include laundry, cooking, dishwashing, showering, and drinking. A good rule of thumb is that turf within the southern California region requires about 1 inch per week to maintain a healthy look. It is reasonable to expect some seasonal variation in the rule of thumb, but again, speaking as a homeowner in southern California, we cannot eliminate sprinkler use during the rainy season. So, given the 10.6 inches of rain that occurs typically in San Diego as well as the typically imbalanced distribution of that precipitation, a monthly water balance analysis shows that it is impossible to completely satisfy normal domestic use with harvested rainwater–even without any physical limitation in storage. Figure 1 illustrates that the typical supply of harvestable rainwater in San Diego covers only about 28% of the total annual domestic demand on a single-family lot, with demand having to be met almost exclusively by other sources during the months of May, June, July, August, and September.
Austin and Sydney, given their substantially greater and more evenly distributed rainfall, are positioned much better to meet similar domestic demands. In the case of Austin, there was enough harvestable supply to meet nearly all of the demand on an annual basis, although January, February, and March typically produce the lowest monthly precipitation values. In these months, a small supplement from another source might be expected (Figure 2).
Sydney, not surprisingly, has the best overall results. As shown in Figure 3, Sydney’s harvestable supply on an annual basis equates to about 247% of the indoor and outdoor residential needs. Because of the relatively consistent distribution on a month-to-month basis, supplement from other sources (i.e., domestic or reclaimed water systems) would not be necessary.
The “unlimited storage” scenario discussed above provides good insight to supply-side limitations of rainwater harvesting. The physical reality is that all rainwater harvesting systems require storage, typically in a tank. This not only brings into a play a cost issue, but also further limits the percentage of total precipitation that can be captured. By recognizing this issue, it becomes apparent that there is a critical three-way relationship between total precipitation, typical “inter-event” duration, and drawdown (demand) rate. For any given storage volume, as either inter-event duration or drawdown rate decreases, there is increased likelihood of system overflow. When considering all of the above, San Diego is simply too challenged to consider using rainwater harvesting to handle all domestic needs. A much more modest goal would be to attempt to satisfy just non-potable indoor needs, which would typically equate to about 15 to 20% of the total domestic requirements.
Figure 4 shows a similar monthly water balance estimate under the “indoor-use only” scenario. By trial and error, it was determined that a 10,000-gallon tank was necessary to meet indoor needs throughout the year, without supplement from another source. An examination of the graph shows that in this particular case, the overall storage need was dictated by the ability to meet indoor demand during the month of October, after which the beginning of the rainy season would cause supply to replenish within the tank. A 10,000-gallon tank would be considered to be beyond the threshold for what is considered spatially obtrusive on a single-family residential lot. However, meeting just a portion of these indoor needs would be a positive result, especially for the southern California region. The designer would have to weigh the costs and benefits and other intangibles on a project-by-project basis.
Not all development would be single-family residential projects. Development forecasts for southern California all indicate an increased trend in high-density residential development. Considering collectively the anticipated need for commercial, institutional, and high-density residential projects, it would be reasonable to assume that a good percentage of future development projects in southern California will involve relatively high percentages of impervious area. Additionally, water conservation requirements at both the state and local levels are being implemented through updates to landscaping ordinances. These ordinances require or encourage the use of drought-tolerant species and xeriscaping. For most types of projects, water-consuming turf is already on its way to becoming a thing of the past. The xeriscape plant pallets in our local climate can be expected to consume roughly one-quarter of the water required by turf. The achievable water balance scenario given increased tendency for impervious coverage combined with xeriscape looks more favorable, especially if did not involve an attempt to satisfy indoor use.
Figure 5 illustrates the annual water balance condition for a commercial scenario (80% impervious) that utilized xeriscape, with no indoor demand applied.
For comparison purposes, the analysis was done on the same 8,000-square-foot basis as for the residential scenario. In this instance, the tank required was 4,000 gallons–compared to a 10,000-gallon tank on a residential project serving only indoor needs. Despite the apparent reduction in required volume as compared to residential use, this result is not reasonable when scaled up to the acreage of most commercial projects. Neither scenario (impervious coverage) could reasonably be expected to simultaneously meet the demands of both indoor and outdoor use.
The results of the indicated commercial scenario summarized do not account for daily precipitation patterns typically observed in San Diego and other parts of southern California. Similarly to how the annual precipitation is clustered disproportionately into several months, a substantial percentage of the monthly precipitation volume is bunched into multi-day events, with intermittent (less-than-48-hour) pauses in rainfall. Within the context of mandated use provisions within a growing number of MS4 permits within California, this natural rainfall pattern has dramatic repercussions. The use mandates are applicable for everything up to and including the 85th percentile event. Within the coastal area of southern California, this has been determined to be about 0.6 inch over 24 hours. From a regulatory perspective, the performance of a rainwater harvesting system must increase from merely meeting intended physical demands (be they indoor or outdoor). They must also capture the regulatory event, which has been defined as a 24-hour event. True 24-hour storms are rarely experienced in southern California. In reality, the 85th percentile event is a multi-day scenario, with a volume that often exceeds 0.6 inch. In considering differences in drawdown (use) rate, it becomes apparent that the required storage starts to become inordinate compared to an identical scenario in which the goal is just to meet long-term physical demand. A daily analysis of the same commercial scenario illustrates this point. Figure 6 shows how a 4,000-gallon tank would perform, using daily rainfall data from Lindbergh Airport in San Diego for the period from November 20 to December 7, 1966. Prior to November 20, during the first two months of the rainy season, the tank would have become virtually full from four rain events totaling about 1.57 inches. Because of the relatively low drawndown rate of xeriscape, the runoff from events on December 3 and 4 would have overflowed from the system, despite the fact that both 24-hour totals were less than the defined water-quality event (0.51 inch and 0.16 inch).
A review of daily computations from the remainder of the 1966–67 rainy season shows frequent bypass of the 4,000-gallon tank when subjected to events of less than the water-quality event of 0.6 inch. The question arises as to how much more storage is required to satisfy both the physical irrigation demand and the regulatory requirement to reuse all storms up to and including 0.6 inch. Approximately 40,000 gallons for this scenario is expected. This indicates that satisfying the regulatory requirement takes 10 times the storage as satisfying the physical requirement. This equates essentially to storing the entire rainy season supply of rainwater. In the future, there may be potential to use computer-controlled bypass systems to alleviate this issue (especially on larger projects). However, utilizing them efficiently would require redefining our treatment standards from a daily basis to something seasonal or more reflective of actual storm
occurrences.
In considering the two alternative design objectives of physical demand versus regulatory requirement, it is clear that accomplishing one efficiently prohibits efficient accomplishment of the other. Satisfying the regulatory requirement efficiently requires the fastest possible drawdown rate. Preventing bypass for all the water-quality or smaller events can happen only when our “bunched up,” or clustered, series of multi-day rainfalls is accompanied by a condition in which the storage tank is mostly, if not entirely, empty. The most efficient way to accomplish this would be to landscape business parks and subdivisions to look like a Brazilian rainforest. However, satisfying the physical requirement is best served by landscaping with highly water-efficient xeriscape. That is the best way to stretch seasonal harvest all the way through to the end of the typical “dry” season at end of October. It would appear that the regulatory requirement, which is rooted in water-quality objectives, and the physical requirement, which is rooted in a desire for water conservation, are mutually exclusive. Although possible to meet, neither objective can be satisfied practically from either a spatial or economic perspective. Meeting future regulatory requirements in a practical manner will, in most instances, involve a multifaceted approach incorporating a combination of management techniques.
The Market Value of Rainwater Harvesting
People want to act as stewards of the environment. The question is, at what price will incorporation of rainwater harvesting come in a typical residential scenario in southern California? To answer this question, it’s important to refer back to the single-family case study discussed earlier that would involve a 10,000-gallon tank. In this scenario, you can either address only the typical indoor needs or just a small percentage of outdoor needs. Concrete and fiberglass are two of the more commonly used tank materials. Most cost estimating data indicate that tank construction ranges from $1.00 to $2.50 per gallon. Construction of the tank represents the overwhelming majority of initial cost. Other related improvements might account for 10% or less of the system, and will vary depending on the intended application. These may include improvements to expand or redirect the roof and yard collection system. Roof washers will be required to prefilter particulate matter that would otherwise clog drip irrigation. If the intent were to utilize the system for potable or contact use, then additional measures for secondary filtration and disinfection would be necessary. Secondary filtration is most frequently accomplished using activated carbon cartridges or commercially available membranes that operate through reverse osmosis. Typical disinfection techniques include dosing harvested rainwater with chlorine tablets or exposure to ultraviolet light. Because a residential system in southern California could not realistically meet most normal outdoor demand, any attempt to supplement irrigation from a public supply would involve a cross-connection with a service from a domestic or reclaimed line and, consequently, some sort of backflow prevention. A need for a small pump system capable of delivering the water to the end use would certainly be expected in virtually all situations. The bottom-line initial cost for all of the above would be on the order of $15,000 to $25,000 per household. Routine maintenance costs would be minimal, perhaps in the range of several hundred dollars annually depending upon the type of system.
The actual market value of harvested rainwater can be defined by comparison with an equivalent volume obtained through retail consumption. It varies to some degree by location, amount used, and type (i.e., potable versus reclaimed). Southern California has some of the most expensive retail water rates anywhere in the United States, meaning that the economic incentive to utilize harvested rainwater to reduce retail consumption would be expected to be higher than in most other locations. But “higher” is a misleading description. The cost of my personal domestic consumption over the past year works out to about $1.79 per 100 cubic feet (“unit”). The 10,000-gallon tank in the previously mentioned residential case study was estimated to collect about 5,400 cubic feet of rainwater on an annual basis. This means that for the San Diego region, the “market value” of the harvested rainwater on an annual basis would work out to slightly less than $100. Contrast that with Sydney, where market value is more along the lines of $1.15 per unit and Austin, where a unit costs approximately $0.75.
Unintended Consequences of Good Stewardship
Assuming you were able and willing to bear the cost of a rainwater harvesting system, there are other issues pertaining to the use of rainwater harvesting worth considering, particularly if you imagine large-scale application in southern California within areas slated for future development.
The local southern California streams are ephemeral in nature and have been formed by a pattern of long-term rainfall, considered relatively minimal by most standards. However, dating at least as far back as 1959, there has not been a year in which the San Diego region was completely shut out from rain. Consequently, our local streams have been flowing with water, at least seasonally, on a regular basis. The use mandates contained in several draft MS4 NPDES permits could threaten the natural pattern of ephemeral flow in some watersheds where rainwater harvesting may be applied on a large scale.
A long-term continuous simulation of this potential effect was modeled using the San Diego Hydrology Model (SDHM), which is a localized adaptation of the more commonly recognized Hydrological Simulation Program Fortran (HSPF). In this simulation a 1-square-mile case study area was developed to simulate use of our locally defined water-quality 24-hour event of 0.6 inch. The simulation was based upon hourly rainfall data from Lindbergh Airport from 1959 to 2003. Analyzing the effect of mandatory use in conjunction with development (condition of 70% imperviousness) involved creating a theoretical storage area featuring two separate outlets. The lower outlet was used to represent use and was set up to drain 0.6 inch over the 1 square mile (32 acre-feet). This requires an average discharge rate of 16.1 cubic-feet per second.
Above 32 acre-feet of storage, discharge from a second outlet was triggered. The second outlet was oversized to pass incoming flow freely and represented runoff from storms beyond the water-quality event that would flow to receiving streams. The model was configured such that once the second outlet was discharging, the first one stopped. This was done to simulate a halt to use after the 0.6-inch threshold had been reached. Discharge from each outlet was connected to a separate node of analysis. Flow at the first node represented the total amount of use during the 44-year simulation. Flow at the second node represented the remainder that would be available to feed receiving streams and maintain their natural shape and alignment. The comparative analysis between the pre-and post-development conditions was performed for four different combinations of soil type and topographic slope. The results of the scenario that involved type “B” soils with a flat grassy cover in the existing condition illustrate the typical extent to which annual runoff to streams would be eliminated. Results were compiled for 1966–67, 1972–73, and 2002–03. As mentioned previously, these three rainy seasons had been deemed to be fairly typical (i.e., approximately 10 inches of precipitation) for the San Diego region. For 1966–67, the predevelopment scenario generated runoff during 96 days. For 1972–73, the number of days in which runoff was generated was 129, and for 2002–03, runoff was generated during 92 of the days. The post-development conditions for these three seasons, which included use of the first 0.6 inch occurring within 24 hours, generated runoff for only five days total. In fact, most scenarios resulted in an annual “zero discharge” condition during about half of the 44-year simulation.
Applied at large scale in a newly developed watershed, the environmental impacts are unknown. A permanent disconnection of such a large percentage of the natural surface water balance could lead to altered rates of sedimentation and channel aggradation. The current hydromodification standard already being applied in San Diego County limits increased flow duration within a range of events deemed “geomorphically significant.” To adequately protect a natural stream from altered rates of erosion and sedimentation, flow duration should be neither increased nor decreased. It would appear that pending use mandates may be at odds with at least the intent behind the current hydromodification standard. This is a potential impact that requires a much more in-depth analysis before it can be dismissed as an environmental issue.
It is clear that MS4 stormwater use mandates are coming very soon to most of California, and ultimately to other areas within the US. In many instances, compliance with this particular objective will require, by default, incorporation of rainwater harvesting because of limiting factors within the soils that make infiltration into the subgrade either impossible or highly unadvisable. Rainwater harvesting in southern California is not likely to compete economically with retail water supply anytime soon, nor can it physically satisfy a major portion of either our typical indoor or outdoor needs. In some instances, when applied on a large scale in new development, it may contribute to the effects of hydromodification rather than serve as a means to mitigate it.
The challenge is whether we can, as an industry, clearly develop and adapt this approach to suit our local environment in southern California before the objectives are more clearly defined and limited. As we can see by case analysis in Austin, Sydney, and San Diego, rainwater harvesting, as with almost all green practices, is context sensitive. Our solutions, mandates, and regulations all need to consider stormwater management techniques that go beyond invoking images of the one-size-fits-all philosophy and delve into the unique characteristics of each region.
