- Urban landscape water use is increasingly a focus of water conservation efforts. This is especially true in the arid and semi-arid regions of the western United States where increased demand, environmental concerns, and extended periods of drought have created chronic water shortages. However, until recently, little attention has been paid to the design and management of water efficient urban landscapes and irrigation systems. Concepts and strategies have often been uncritically translated from the production agriculture context. For example, integrated water use efficiency is broadly defined as biomass production or yield per unit of water consumed via transpiration. In the context of crop irrigation, the definition also sometimes includes the water delivery efficiency of irrigation systems. These definitions can potentially be problematic in urban landscape contexts where plant biomass or yield are not necessarily the functional goals. Instead, traits such as aesthetics and plant health are often more important, but we don’t have a good understanding how those traits relate to water use for many types of urban landscapes. In addition, it is possible that individual management decisions might compromise the target design efficiency of irrigation systems to a much greater degree in urban residential contexts than in production agricultural contexts. In this dissertation, I look at three aspects of outdoor water use in semi-arid urban environments, including results of two separate studies looking at methods of increasing water use efficiency in created landscapes.
Chapter one provides a broad discussion of landscape water use in the context of extended drought conditions in the western United States, and suggests methods to increase conservation through design elements and management practices. I present a summary of landscape industry efforts to increase efficiency in outdoor water use through technological advances, educational programs, and methods to model the water requirement of urban landscapes, but conclude that current efforts may not be sufficient to address the growing need for water conservation in urban landscapes.
In Chapter two, I explore the historic development of urban landscape irrigation and discuss issues related to water availability and allocation that remain persistent and are reflected in current debates over water use. I found that urban landscape irrigation in the western United States developed from the initially agrarian oriented settlement of southern California and efforts to supply water to rapidly growing commercial agriculture in the arid west. Technological advances in water capture, transport and distribution for agricultural production were quickly adopted and modified for use in the expanding urban communities of southern California. The growth of urban demand for water fostered the development of dedicated sources of water and municipal water delivery systems that were distinct from agricultural delivery networks. Urban growth also catalyzed the development of an entire industry focused on urban landscape irrigation. The industry burgeoned after World War II, primarily in response to new materials and technologies developed as part of the war effort, and has experienced exponential growth since that time. Current work to increase irrigation efficiency has shifted from a utilitarian view to reflect more recent concerns about environmental quality, and suggest a subtle change in social values concerning water use. Recent efforts target increasing the efficiency of existing methods, and are generally reflected in a reduction of patent applications for profound technological changes in urban irrigation.
In Chapter three, I assess the correlation between xylem water potential of four xerophyte shrub species and soil moisture levels, reference evapotranspiration (ETref), and vapor pressure deficit (VPD), and determine the water saving potential of a native plant xeric landscape in an urban setting through evaluation of three common models of determining landscape water demand: Hunter Industries, EPA WaterSense, and SLIDE (Simplified Landscape Irrigation Design Estimator). Each approach relies on reference evapotranspiration (ETref) as the primary driver of landscape water use but differ in the level of complexity.
I found no statistically significant relationship between xylem water potential and ETref or VPD, but a strong correlation with soil moisture levels. Despite experiencing periods of extreme water stress, most of the xerophyte species in the study showed no gross morphological signs of water stress, and had positive annual growth. This occurred even though the species where in a highly modified residential landscape.
Results of the evaluation of the scheduling models suggest that while xerophyte species can be successfully used in urban landscapes, the common evapotranspiration based approaches for estimating landscape water needs are inappropriate for landscape types dominated by xerophyte species. In addition, the relationship between plant water status and aesthetic quality varies considerably among species. While some species maintained the same aesthetic quality independent of water stress, others suffered seasonally degraded aesthetic quality as a result of their specific drought response adaptations (i.e. leaf drop). These adaptive responses were reported by the homeowners as degraded aesthetic quality. This may affect the acceptability of reduced irrigation use in semi-arid environments. However, aesthetic value and acceptance of indicators of drought stress in a created landscape is shown to be highly subjective.
Chapter four presents a case study of the modification of an existing commercial landscape irrigation system in a semi-arid urban environment with the goal of achieving a minimum 30% reduction in water use. We found that irrigation water use levels can be significantly reduced in an existing commercial system through careful re-design and the use of currently available irrigation technologies. We achieved a 41% reduction in outdoor water use in one year through modification of an existing irrigation system at a commercial development in a semi-arid environment, but water use was still in excess of recommended application rates. Additionally, maintenance practices are shown to adversely affect the efficient use of water, negating benefits of the retrofit and diminishing the return on investment.
In summary, this dissertation found that modifying plant choice and the design of irrigation systems can improve water use efficiency in semi-arid urban landscapes. However, many of the current management recommendations associated with irrigation scheduling are inappropriately applied to the most xerophytic landscape designs. This partly reflects the agricultural legacy of urban irrigation designs and strategies. In addition, the water efficiency of even the best designs can by severely compromised by inappropriate day-to-day management. This can happen even on a project that was designed to showcase a water efficient design. Developing better management protocols and training that maximize the designed water efficiency of urban landscapes should be a priority.