Richard Martinson

John Lambrinos

The efficient use of water in urban landscapes is a critical subject for many municipalities throughout the western United States. The distribution and allocation of water as a limited resource is becoming increasingly important as long-term drought conditions continue to affect the availability of water. Municipalities are responding these challenges by looking at options to reduce urban water consumption. Landscape irrigation is often a target of conservation efforts, but few case studies exist documenting the real potential of water savings using design considerations and technological advances in the irrigation industry.

This study quantifies actual outdoor water savings from retrofitting an existing commercial landscape irrigation system to a more water efficient design. This provided a unique opportunity to study the effects of irrigation system modification on outdoor water use in a realistic urban environment. We compare average annual landscape water use data collected by the city of Bend, Oregon at Summit Park, a commercial low-income housing development, with results of a redesign and modification of the existing irrigation system. We achieved a 44% reduction in irrigation water use through utilization of current available technology. Landscape maintenance decisions are discussed as factors contributing to decreased benefits of the irrigation system modification.

Key Words: irrigation, irrigation scheduling, urban landscapes, water-use efficiency

Introduction

Increasing demand for freshwater as well as supply disruptions have increased the frequency and spatial extent of water scarcity such that 4.0 billion people now experience severe water scarcity at least one month of the year (Mekonnen and Hoekstra 2016). The World Economic Forum has cited the increasing frequency of water scarcity as one of the greatest systemic risks facing global society (World Economic Forum, 2017). These risks have spurred considerable interest in developing strategies to reduce consumptive water use and improve water use efficiency (Hoekstra 2014). In urban environments, a particular focus of these efforts has been the water used to irrigate residential, commercial and public landscapes. This is because landscape irrigation accounts for an average 45 – 55% of urban residential water use (Brookshire 2002; Hilaire et al. 2008; Mayer et al. 1999). Perhaps because landscape water use is the least valued use of water by many people (Stoutenborough and Vedlitz 2014), it has been a focus of public concern as well as municipal regulation and policy development. This has been particularly true in the western United States where long-term drought conditions have increased concerns over water availability in rapidly growing urban areas (Hornberger et al. 2015).

These municipal efforts take the form of a range of regulations, economic incentives, and educational outreach designed to drive the adoption of technologies and landscape designs that improve water use efficiency. These strategies usually focus on improving the efficiency of the water delivery (irrigation) system and to changing landscape designs to include more xerophytic or water use efficient vegetation types. However, few studies have quantitatively evaluated the actual water savings accrued from the adoption of these strategies in a realistic residential setting. This study quantified actual outdoor water savings from retrofitting an existing residential landscape irrigation system to a more water efficient design. This provided a unique opportunity to study the effects of irrigation system modification on outdoor water use in a realistic urban environment.

Summit Park

Summit Park is a low-income housing complex covering approximately 2.1 hectares (ha) in Bend, Oregon, a city of ~80,000 at 1,115 m elevation (44° 4’ 11.92” N, 121° 16’ 30.96” W). Summers are typically warm and dry with an average daytime temperature of 25 oC (June – September). Winters are normally cold, with most annual precipitation falling as rain or snow. Average winter daytime temperature is 5.97 oC (November – February). Average annual precipitation is 289 mm (Figure 1).

The City maintains ownership of the land and subsidizes a portion of the rent. The developer owns the structures and manages the property for low-income residents (Bend 2015). The complex was originally constructed in the mid 1990’s and includes 7568 m2 of irrigated area: 6556 m2 of turf and 1012 m2 of planted shrub beds (Bend 2016). Sod covers most of the property outside the buildings. A one meter tall berm planted with a mix of conifer and deciduous shrub and tree species runs the length of the east side of the property. Two large berms (>2 meters tall) run north-south in a common area between two multi-unit buildings, and narrow planting beds with mature conifer shrubs exist along the foundation of each building that faces the parking area. The plantings on each side of the entry drive are predominantly lawn with planting beds of aspen (Populus tremuloides), redtwig dogwood (Cornus sericeus), barberry (Berberis sp.) and Oregon grape (Mahonia aquifolium). Sod composition on site consists of three grass species; Lolium perenne, Poa pratensis, and Festuca spp (Baker et al. 2014). Shrub beds are limited to small linear plantings between sidewalks and building foundations (Figure 2).

Outdoor water at the site is metered separately from indoor use, providing detailed landscape water use data on a weekly basis. City of Bend records include water use data from 2011 through 2016. Records provided by the city document a mean annual application rate (2011-2015) of 11.7 x 106 liters (Bend 2016).

Project description

This project was initiated by the building owners as a way of reducing operating costs at Summit Park, primarily through a reduction of outdoor water use. The original design included removal of over 75% of the sod and shrubs, replacing them with native xerophytic species, modification of the existing irrigation system to increase application efficiency, and capture and reuse of storm water from the roofs of all twelve buildings on the property. Property owners identified a target reduction of 30% below annual mean outdoor water use. The city supported the project by providing partial project funding, supplying water use data, and monitoring results.

The project included four distinct steps: 1) an irrigation audit, 2) review of audit results and final designs for modification, 3) implementation, and 4) monitoring. The design for the project went through several modifications to meet budget constraints. Modification of the exiting irrigation system was the primary method to achieve a 30% reduction in water use, but replacement of high water use plants with native xerophytic species was completed for approximately 10% of the site.

Irrigation audit

An irrigation audit is a procedure used to collect information about the overall condition of an irrigation system (Irrigation Association 2014; Wilson 2009). Formal audits are completed by a certified landscape irrigation auditor adhering to guidelines developed by the Irrigation Association (IA). Irrigation audits normally include 1) a visual inspection of the irrigation system; 2) evaluation of distribution uniformity (DU) – a measure of how evenly the irrigation water is applied to a landscape; 3) a determination of precipitation rate (PR) – the amount of water emitted from an irrigation nozzle; 4) determination of a landscape water budget (LWB) – the amount of water necessary for effective irrigation based on existing irrigation models; and 5) development and review of an irrigation schedule based on precipitation rate and landscape water budget (Irrigation Association 2014).

Two formal audits of the irrigation system have been completed. The first was completed by the city in 2013, the second was contracted as part of this project in 2015. We collected the precipitation rate for each head type and nozzle from manufacturer specifications and totaled precipitation rate by zone (Figure 2). The sum precipitation rate for the property was compared to records from previous years provided by the City of Bend Public Works department. Recommendations for retrofitting the existing system was included in the audit results, but the development of an irrigation schedule or landscape water budget was not completed as part of the 2015 audit.

Audit review

The irrigation audit completed in 2015 identified 18 separate irrigation zones, 17 of which were functional. Each zone contained a variety of irrigation heads, nozzle sizes, and precipitation rates (McDowell 2015). Thirteen zones were identified as irrigating turf, two were identified as shrub zones, and three were mixed turf and shrub beds. Water use data from 2011 through 2015 show a linear reduction in water use (Figure 3). The 2013 audit recorded an annual irrigation water use of 13.8 x 106 liters (Bend 2016); the 2015 audit estimated annual irrigation water use at 13.2 x 106 liters (McDowell 2015). Records for 2012 indicate a significant drop in irrigation water use for that year, possibly reflecting a change in management practices or recording error.

We identified inconsistencies in irrigation head type, nozzle type and size, redundancies in head placement, and broken or leaking irrigation control valves as primary factors affecting the efficiency of the existing irrigation system (McDowell 2015). The system appeared to have been modified over time, with multiple heads irrigating the same area on multiple schedules and a variety of head types by different manufacturers installed on the same irrigation zone. A determination of the efficiency of each zone was determined through analysis of spatial distribution of each head within the zone, the type of head or nozzle and associated precipitation rate, and the uniformity of distribution between existing heads. Recommendations for replacing or moving heads to increase distribution efficiency in the existing system were developed in response to our review of the audit. In some cases, recommendations were not adopted in the final plan because underlying geology prevented moving pipe, or because of budget constraints.

Calculations completed as part of the irrigation audit process indicate an expected 51% decrease in water use from simply changing the type of irrigation body and head. The expected reduction in water use was determined through analysis of precipitation rates from manufacturers specifications for individual irrigation heads and nozzles and replacement of those heads by MP Rotators. The total water use after modification of the irrigation system was compared with the target irrigation rate determined by the City of Bend.

Irrigation Scheduling

Various irrigation scheduling models have been developed to assist in maximizing irrigation efficiency in urban landscapes. Many are created by irrigation suppliers or industry professionals, but several are city, state, or federal efforts. All are based on reference evapotranspiration rates (ETref). A typical approach estimates water demand as a fractional proportion of ETref. The proportion is estimated using adjustment factors to account for species differences in water demand as well as a range of other processes and factors, such as microclimate, that potentially modify water flux from the landscape (Grabow 2013; Pannkuk 2010; Radwan 2010).

At Summit Park, the reference ET rate is 188.98 mL/month. The equation used by the City to determine recommended irrigation rates modifies this figure with an adjustment coefficient of 0.83 to achieve a target reduction of 17% in irrigation water use, resulting in a modified base rate of 156.85 mL/m2 This figure represents the estimated landscape water demand for each m2and is used to develop an irrigation schedule designed to apply that quantity of water per m2/month. However, best management practices suggest seasonal adjustment of irrigation rates to reflect changes in monthly ETref values (Table 1). The irrigation schedule developed as part of this study for Summit Park adopted the use of seasonal adjustments.

System modification planning

Retrofit of the system focused on two primary targets: 1) increasing efficiency in precipitation rates and distribution uniformity through improving consistency in nozzle choice, type of irrigation head, and spatial patterning of irrigation heads, and 2) addressing grading concerns and improving storm water collection and use. To affect the most change within budget constraints, recommendations for modification of the existing system focused on zones with a mean precipitation rate over 2.54 cm per hour (Figure 3).

We reviewed precipitation rate data for each head in each zone and recommended changes in irrigation body type, nozzle size, or head location to increase application efficiency. Consistency in coverage was addressed through adjusting irrigation head placement or removing redundant or ineffective heads. In some cases, the spatial distribution of heads was adjusted to ensure consistent coverage from the replacement irrigation heads. Precipitation rates were standardized through consistency in irrigation head and nozzle choice during the system modification. Variation in water pressure inherent in long irrigation runs was mitigated through the use of pressure regulating irrigation bodies.

Recommendations to increase efficiency in precipitation rate and distribution uniformity through system modification also considered saturated hydraulic conductivity (Ksat) values. Ksat values quantify the potential for subsurface infiltration and are useful when designing irrigation systems for deeply rooted species (Watt et al. 1995). Ksat values were completed for a depth of 60 cm to determine the ability of soils at Summit Park to provide plant available moisture at deep soil layers, and ranged from 26.7 to 92.0 µm/sec.

Methods

The primary method to reduce irrigation water use was to replace each spray head with MP Rotators ™. MP Rotators are trademarked by Hunter Corporation and feature a multi-trajectory rotating stream system delivering multiple streams of water at a steady rate. Slower application rates typical of MP Rotators allow more efficient irrigation with an expected 30% decrease in water use compared to traditional spray irrigation (Hunter 2016). Reported efficiency rates of MP Rotators is 80% with a consistent application rate of 9.9 mm per hour (Hunter 2016). This is within reported Ksat values for soils in the project site, indicating application rates will not exceed absorption rates (9.9mm/hr = 2.75 µm/sec).

Each zone was designed and modified individually. For example, Zone 16 exhibited an average degree of variation in precipitation rate of all zones in the study area. Prior to modification, this zone included 23 individual irrigation heads by three manufacturers with 10 different nozzle sizes applying a total of 127 liters per minute. Replacing these heads with pressure regulating bodies and MP Rotators sized and spaced for the specific application radius of each head resulted in a total precipitation rate of 49.3 liters per minute, a 61% decrease in precipitation rate for that zone (Figure 3).

Storm water capture and reuse was improved through grading changes that created small swales in each shrub beds immediately adjacent to interior facing residential units (Units A – J, Figure 2). All roof runoff was directed to swales, and all spray irrigation within the shrub beds was changed to 13 mm in-line drip system (Netafim ™) with 1.8 liter/hr emitter rates at 46 cm spacing. Existing vegetation was removed and replaced with native xerophytic shrub and forb species.

Results

Standardizing precipitation rates, improving spatial patterning, and utilizing recent innovations in irrigation technology resulted in a 43.6% reduction in annual irrigation water use within the first season at Summit Park, although audit calculations indicated an expected 51% average decrease in water use. Data collected by the city of Bend show a mean annual irrigation (2011-2015) volume of 11.7 x 106 liters. Data provided for 2016 report annual irrigation volume at 6.6 x 106 liters, exceeding our 30% target reduction (Figure 4).

However, data for 2017 show a 9% increase in outdoor water use over 2016 figures, reflecting management decisions and the level of active landscape maintenance at Summit Park (Figure 4). Irrigation records for 2018 suggest the lack of seasonal adjustment in irrigation scheduling, resulting in excess water use (Figure 5), although complete seasonal records for 2018 are not currently available.

Discussion

The Summit Park case study was designed to achieve a minimum reduction of 30% in seasonal outdoor water use. We exceeded expectations by achieving a 44% reduction from the 2011-2015 mean documented by the City of Bend. These findings support industry claims concerning technological advances designed to improve efficiency in irrigated landscapes through focusing on distribution patterns, precipitation rates, and water infiltration. However, long-term monitoring of the project suggests that maintenance practices can affect the degree of benefit gained through efforts aimed at improving water use efficiency in urban landscapes. For example, the failure to incorporate seasonal adjustments in scheduling from April through July 2018 at Summit Park resulted in an average application rate 27% higher than recommended by the city (Figure 5).

The decision to improve water use efficiency is often made by owners or managers responsible for fiscal oversight. Continued drought conditions in much of the western U.S. has stimulated the development of incentive programs aimed at reducing water use in urban landscapes (Hilaire et al. 2008; Shandas and Hossein Parandvash 2010). Incentives often take the form of scaled pricing for water; costs per unit increase with the volume of water used (Brookshire 2002). The decision to improve water use efficiency is frequently made in response to incentive programs by owner or managers responsible for fiscal oversight, but the management of modified systems and the responsibility to properly implement strategies designed to maximize the return on investment (ROI) typically falls on a landscape maintenance company with little or no incentive to monitor use. The gap between intent and implementation is a substantial barrier to addressing long-term water use concerns in managed landscapes.

But there are no easy solutions. Companies providing landscape maintenance at Summit Park have changed three times since 2015 (Wooden 2018). The lack of consistency in maintenance can result in large swings in water use, and may be a factor contributing to the significant reduction of water use documented at Summit Park in 2012 (Figure 4). Although the city has an active program in “waterwise” landscaping, involvement in that program is voluntary and the adoption of recommendations developed through the program by landscape maintenance companies is unknown (Buettner 2018). The responsibility to educate maintenance companies on current water use issues and irrigation scheduling is often left to homeowners or property managers. The education of the maintenance companies at Summit Park is unspecified and oscillates between the city’s Department of Public Works, the property owner, and us, resulting in a general lack of understanding by maintenance workers of the intent and design of the modified irrigation system.

In their defense, landscape maintenance companies are expected to preserve a level of aesthetic value critically important in urban landscapes (Hayden 2015), and reducing water use often generates questions of aesthetic value. Although aesthetics is a subjective value (Thompson 2000; Thorne and Huang 1991; Tyrväinen et al. 2003), the appearance of a healthy landscape is the primary goal of most landscape maintenance companies. Maintenance practices designed to meet that goal often include the application of water at rates exceeding plant physiological requirements (Martinson et al. In-Press), or base irrigation schedules on experience or intuition (Harris et al. 2012) and fail to consider the effect on long-term water conservation objectives.

The modification of the irrigation system at Summit Park documents the potential for increasing irrigation efficiency and reducing water use through careful design and application of existing technology. However, maintenance practices have the potential to significantly affect the degree of success in efforts to reduce landscape water use, identifying a need for consistency in educational opportunities for landscape contractors and maintenance companies. The development of educational programs targeting outdoor water use reduction in response to growing concerns over long-term drought and water availability in the western United States must address regional environmental conditions, be responsive to local values, and be collaborative between municipalities, the landscape industry, and educational institutions.

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