沙漠研究 24-1, 89-92 (2014)
Ἃ₍◊✲ 24-1, 89 -92 (2014)
Journal of Arid Land Studies
Journal of Arid Land Studies－ DT11 Refereed Paper －㸫DT11 Refereed Paper㸫

Alternative Water Sources for Urban Landscape Irrigation in Arid Regions
Raul I. CABRERA*1)
Abstract: Whereas irrigated agriculture is the largest water user in many countries, urban landscape irrigation is rapidly approachingthat distinction, particularly in highly urbanized regions, including those afflicted by desertification. Texas (in United States) is one
geographical region caught in this quandary, with about one-half of its 695,621 km2 area covered by semi-arid and arid ecosystems likethe Chihuahuan Desert. While the urban landscaped area of this state occupies only 1% of its surface, it uses approximately 13% of
the total annual water use by all activities, representing about 47% of the total water use within the urban and municipal sectors.
Fierce water competition caused by accelerated growth of urban population, industrial and energy-producing activities, along withagricultural production are challenging the sustainability of the current exotic and heavy water user urban landscapes. Along with achange to sensibly designed landscapes using water-conserving native and adaptive plant materials with weather- and sensor-guided
irrigation and deficit irrigation practices, the use of alternative irrigation water sources is imperative to minimize their dependence on
high-value potable water. Use of alternative water sources, including brackish, reclaimed, condensates and graywater, however,presents challenges to both plants and urban soils. Systematic monitoring and proper management practices are needed whenemploying these alternative water sources to minimize issues related to salinization and other undesirable effects to the immediate
urban and surrounding periurban and natural environments.
Key Words: Graywater, Irrigation, Reclaimed water, Salinity, Water quality

1. Introduction
While irrigated agriculture is the largest world water user,
an exponential population growth, primarily in urban areas, iscreating a huge demand for water resources, and therefore aserious competition and crisis for good-quality water resources(UN-Water, 2009). In developed countries (e.g. USA) urbanlandscape irrigation has become a major water user, using
considerable volumes of potable water, even in a state likeTexas, which has a large portion of its vast 695,621 km2 area
covered by semi-arid and arid ecosystems such as theChihuahuan Desert. Based on the recognized aridity index
defined as the ratio of annual precipitation to potential
evapotranspiration rates (Maliva and Missimer, 2012), about
one half of the Texas territory would be classified as semi-arid
or arid.
While the urban landscaped area of Texas occupies only1% of its surface (6,974 km2), it uses approximately 13% of
the total annual water consumed by all activities (Fig. 1), andrepresents ~47% of the total potable water use within the urban
and municipal sectors (Cabrera et al., 2013). An increase of
82% (from 25.4 to 46.3 million) in Texas population growth,
mostly in urban areas, in the next five decades is projected toalso increase municipal water demand by 71.4% over the same
period (TWDB, 2012). The 562 water projects recommended

Fig. 1. Relative water demand in 2010 for various activities in Texas.
About half of the water used by the municipal-urban sector(~13% of total) was devoted to landscape irrigation. Drawnfrom data in Cabrera et al. (2013) and TWDB (2012).

to supply these increased water needs include municipal andirrigation conservation strategies, along with use and reuse ofalternative water sources, new reservoirs and desalination.

2. Water Conservation Strategies
2.1. Selection of plant materials
The use of water-conserving landscape plants and suitable
designs for each eco-geographical region (i.e. soil and climate)
are the main components of urban water conservation.

* Corresponding Author: r-cabrera@tamu.edu
1619 Garner Field Road, Uvalde, Texas 78801 (USA)1) Texas A&M AgriLife Research and Department of Horticultural Sciences, Texas A&M University

㸦Received, September 14th, 2013; Accepted, February 10th, 2014㸧

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Agricultural research/extension agencies, municipal utilities
and water districts have preferred/approved listings of nativeand adaptive, resource-efficient landscape plants and trees
suitable for each region, and their use is often promoted withrebates and incentives (Cabrera et al., 2013).
Utilization of properly chosen native and adapted plants toeach region, with proper design and maintenance (including
soil conditioning and mulching) should ensure their survival
and ornamental performance within the limits of the expected
regional precipitation, with little to no supplemental irrigation.
2.2. Efficient irrigation technology and management
Irrigation management with smart controllers based on
evapotranspiration (ET) and soil moisture conditions,compared to traditional timers and calendar-based irrigation,offer the potential for significant landscape water conservation,
particularly with the development of more refined equipment
and sensors. The incorporation of deficit irrigation and crop
coefficients (Kc) into ET-based irrigation is a refinement that
increases the potential for more water savings while
maintaining the aesthetic quality and function of landscapeplants and turfgrasses (Pannkuk et al., 2010).

3. Alternative Water Sources for Urban Irrigation
Reserving the use of municipal potable water primarily fordirect human uses (drinking, cooking, bathing, laundry, etc.),
and employment of alternative water sources for landscapeirrigation and other industrial uses is a proposition that should
result in enhanced water savings and conservation efforts in theurban sector.
3.1. Rainwater and stormwater
For millennia civilizations have practiced and relied on
rainwater harvesting as a source of water for household toagricultural uses. Depending on the collection/storage system
this source offers the potential to have the purest, best quality,
low-salt content water (i.e. suitable water in Table 1).
Development of affordable, centrally treated and distributed
water in urban areas caused rainwater harvesting to beforgotten. A renewed interest has emerged in arid regions like
Texas due to drought, scarcity, competition and rising costs,
and it is being actively promoted by water purveyors (TWDB,2005). Rainfall frequency and limitations in storage capacity
are two of the conditions that might be restricting a more
extensive use of this source in residential landscape irrigation.
Reuse of stormwater can also be seen as an option for
urban landscape irrigation and other non-potable uses. Withfew exceptions, however, stormwater collection, treatment,
storage and uses are typically handled by local municipal

Table 1. Main chemical quality parameters in suitable irrigationwater for ornamental landscape plants, slightly brackish
and reclaimed water in Texas (Data from Anderholm andHeywood, 2003; Cabrera et al., 2013; Duncan et al., 2009;
Farnham et al., 1985).
PARAM ETER

Suitable
WaterpH
EC (ds/m)

6.0 – 8.0
<1.0

Na (mg/L)
Cl (mg/L)

<70
<110

B (mg/L)HCO3 (mg/L)<1.0
120 - 180

Slightly
Brackish
7.3 – 8.3
1.6 – 4.7
50 – 560

Reclaimed
7.0 – 8.0
1.0 – 2.030 – 510---

90 – 120
120 – 160
0.5 – 1.0

80 –250

180 – 200

utilities. Current stormwater management practices are
considered to be ahead of research, as there is a lack oftechnologies designed specifically for stormwater recycling,with more attention being directed to pollution control (Hatt etal., 2006). There are some urban design propositions that callfor retaining water in the urban landscape through stormwater
harvesting and reuse in urban vegetation to reduce heat islandeffects through enhanced evapotranspiration and surface
cooling (Coutts et al., 2013).
3.2. Brackish and saline waters
Brackish groundwater, from naturally saline aquifers or
those affected by coastal saltwater intrusion-aquifers areabundant in Texas (TWDB, 2013). While TWDB indicatesthat brackish water with electrical conductivity (EC) up to 4.7
dS/m could be employed for irrigation, this salinity levelexceeds the maximum level of 1.5 dS/m recommended formost landscape plants (Table 1), in addition to having highconcentrations of ions like sodium (Na) and chloride (Cl),which are particularly toxic to many plants (Farnham et al.,1985). The growth and quality of plants irrigated with saline
water suffer significantly, particularly shrubs and trees,
expressed in scorched, necrotic and chlorotic leaves, ultimately
leading to plant death (Niu and Cabrera, 2010). A higher
degree of tolerance to overall salinity and specific ion
concentrations can be observed in turfgrasses and some annual
plants (Duncan et al., 2009). Blending of brackish and saline
waters with other high quality water sources, along withsuitable irrigation and leaching practices and salt-tolerant
species, could be successfully used to grow and maintainaesthetically pleasant urban landscapes.
3.3. Reclaimed water
Municipal reclaimed water has also been considered aviable alternative for landscape irrigation, being extensively
used in golf courses, large corporate and municipal parks andlandscapes in arid urban areas across the western US, including

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Texas. Depending on the degree of treatment, however,reclaimed waters could have similar drawbacks as brackish
water, with relatively high levels of total salinity andundesirable specific ions (Duncan et al., 2009). It should be
noted that the quality of the reclaimed water produced by theSan Antonio Water System (SAWS), the largest municipal
water treatment facility of its kind in the US, is fairly good,with an average EC of 1.1 dS/m, 180 mg/L of alkalinity(HCO3-), 145 mg/L of Cl and 98 mg/L of Na, all levels thatwere slightly to moderately higher than those recommendedfor woody ornamental shrubs and trees, but still adequate formost annuals and turfgrasses (Duncan et al., 2009; Farnham etal., 1985). Availability and supply of reclaimed water isunfortunately limited, as its collection (i.e. original raw sewageeffluent), treatment and subsequent distribution are strictly
regulated, and employ a separate pipeline system accessible to
only few large end-users (SAWS, 2006). Depending on the
final quality of reclaimed water, use in landscape irrigation
could require the use of modified sprinklers or drippers tominimize direct contact with the foliage of plants, to reduce salt
scorching. These precautions are also suggested to minimize
the risk of human exposure to the recycled water, due toconcerns with pathogenic microorganisms and other chemicals
that could still be present in undesirable concentrations(Duncan et al., 2009). As with brackish water, successful useof reclaimed water calls for use of salt-tolerant plants (Farnhamet al., 1985; Niu and Cabrera, 2010), suitable irrigationequipment and management, leaching requirements, and shortand long-term management of urban soils and their associatedwatershed to minimize salt accumulation and undesirable
effects on the urban ecosystem (Duncan et al., 2009).

3.5. Graywater
Graywater, defined as residential wastewater from laundry,
showers and bathtubs, is an additional alternative water sourcewith potential for residential landscape irrigation. Graywater
can constitute as much as 60% of the total wastewater from ahousehold, and might yield up to 114 m3 per year for anaverage US family (Roesner et al., 2006). Graywater fromlaundry (clothes washing machines) constitutes one-half of thetotal household graywater, and could potentially provide up to130 mm of annual irrigation to an average-sized landscape.
The routing of the drain hose from washing machines to asimple drip irrigation set-up would be a relatively inexpensive
option to reuse this graywater compared to plumbing retrofits
to reroute, capture and use graywater effluent from bathtubs
and showers. Graywater reuse could represent a substantial
saving of potable water supplies if coupled with awell-designed low-pressure drip irrigation system and using
native and adaptive (resource-efficient) plant materials(Cabrera et al., 2013). A convenient feature of reusing
laundry graywater is the ability to reroute or reconnect thewashing machine effluents back to the sewer system when notneeded due to rainfall or low irrigation demand. Among theissues that discourage an extensive and permitted use ofgraywater for landscape irrigation is a lack of documented
knowledge on the short and long-term effects of graywater onplants and soils. Furthermore, as with reclaimed water, thereis the need to identify any and all associated microorganisms
and chemicals that are of concern for public health, plus theirrigation equipment considerations and practices needed tosuccessfully manage and apply graywater (Roesner et al.,2006).

3.4. Air conditioning condensates
Another potential source for landscape irrigation is thecondensate water from air-conditioning (A/C) systems (Guz,2005). This is more viable in sites with a relatively large
indoor footprint vs. landscape footprint (i.e. commercialbuildings), offering the possibility to be self-sufficient for theirlandscape irrigation needs. The quality of condensate water
can be very good, and requires minimal treatment for storage
or immediate use. Condensate recovery systems in San
Antonio (Texas) have worked so well that it became the first
US city to require its new commercial buildings to design drainlines to readily capture A/C condensate water (Guz, 2005).
There are still design and engineering issues being addressed
for the successful and cost-effective implementation, and in thecase of landscape irrigation applications, these include storage,treatment (like chlorine injection to prevent bacterial growth)and hook-up to irrigation systems.

4. Concluding Remarks
Population and economic growth, competition and climatechange (i.e. drought) are placing significant pressures in thewater balance (demand-availability) across the planet, moreevidently in arid regions, and where agriculture “clashes” withurban development. While agriculture is by far the largest
user of water, the increased growth and economic developmentin urban areas across the world are shifting the balance ofwater use and allocation patterns (UN-Water, 2009). The
literature indicates significant advances in water use efficiency
achieved by modern irrigated agriculture, i.e. crop yields
produced per unit of water used (Wagner, 2012). The urban
sector is, however, severely criticizing their volumetric wateruse, but often loses track of its own deficiencies, for instancethe significant use of potable water resources devoted toirrigation of exotic ornamental landscapes. The use ofalternative irrigation waters, along with smart-irrigation

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technologies and landscape designs based on water-conserving
native and adaptive plants and turfgrasses, could significantly
enhance water conservation and use efficiency while sustaining
the many societal and environmental benefits of urban
landscapes. A remarkable urban water conservation effort isthat realized by the San Antonio Water System over the lasttwo decades, basically using about the same amount of waterthat it used in 1984, despite a 67% increase in population - or
dropping the per capita water use by ~40%, from 840 to 515
liters per day (Cabrera et al., 2013). The bulk of the water
conservation efforts and subsequent accomplishments by thiscity have revolved around improving landscape water use. It
is contended that sound research-based results and outreacheducation efforts are still very much needed to help urban
communities in arid regions to achieve substantial progress intheir urban water use efficiency and conservation goals.

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