Urban Irrigation

SESL Australia

Image source: California Department of Water Resources

Image source: California Department of Water Resources

By Alisa Bryce & Simon Leake

This modern, western world of ours is largely technocentric. The urban environment is testament to our industrial advancement, our ability to construct roads, towers, bridges and hard surfaces everywhere, completely altering the natural terrain, the soil and the way water is stored in the natural environment. As catchments urbanise the runoff coefficient rises steeply, from around 10-20% in forested catchments, 20-30% in a rural areas, 40-60% in peri-urban areas and as high as 95% in the centre of cities. This increasing runoff results in “flashiness” of the hydrographs in creek lines and their consequent scouring and destruction, and reduces soil moisture storage for urban vegetation. The only solution to this is to use science and technology as the necessary means to control runoff and reuse it to protect our urban environment.

Urban water reuse and its subsequent reuse for irrigation is an example of an emerging technology with the power to improve the environment of cities. Quite apart from beautification the use of irrigated vegetation in cities carries with it the advantage of reducing the “heat island” effect that will get worse with globally warming. 

Irrigation

To grow vegetation in cities it is necessary to use irrigation. How else could we grow a garden on a sky rise rooftop, or a thirsty landscape garden in the hot, dry, and alien environment of a concrete jungle without which there would be no evaporative cooling effect?  How does a sports field in western Sydney with 50 hours of (official) foot traffic survive a hot summer through to the under 12’s cricket grand final? – irrigation. We irrigate to maintain a satisfactory appearance during dry spells and droughts, and to create beautiful displays of plant life such as Floriade in Canberra. In the urban fringe remain many small-scale horticultural producers of vegetables, green-house crops, and nursery plants, most of whom still rely upon captured stormwater to reduce their use of potable water and prevent nutrient pollution running off site. These are all urban green spaces and all need water for irrigation.

Two elements (apart from the plants themselves) are essential to the establishment and preservation of urban green spaces – water and soil. Unlike cropping, which is somewhat limited in its irrigation options, urban spaces have a variety of irrigation water sources available. The drought that peaked in 2007 forced many land managers to rightly turn to recycled water keep their fields green. Urban irrigators can choose from:

  • potable
  • roof capture (domestic scale)
  • storm water capture (small catchment scale)
  • stream and river storm water capture (larger catchment scale)
  • ground water (only regionally available – some quality problems)
  • domestic grey water – detergent and hygiene issues to address)
  • sewer mining or effluent reuse schemes (needs dual plumbing)

Each have their own pros and cons, and a balancing act is required to ensure residents have enough water to drink, for hygiene, to cook, and industry and councils can operate and maintain the green spaces that keep our cities cool and vibrant. Water is, regardless of its source, a necessity for urban life.

Soil

The soil underfoot, an oft forgotten or ignored entity, is just as important in landscape success as water. Urban soils (or anthroposols as us soil scientists like to call them), just like urban irrigation water, are often recycled. Most of the time urban soil is simply what was left over after construction finished and the ‘soil’ (“it’s brown therefore it’s soil”) spread as the living layer and the plants expected to grow in it. Not surprisingly, in dysfunctional soil systems urban greenery can require a seemingly large amounts of water to keep the plants growing.

In some cases – such as high profile sports fields and golf courses – the soil is carefully designed and constructed, then manufactured to create a free draining medium that supports hours of foot traffic and can be irrigated up to 2 hours before kick-off. These fields are large water users as they were constructed with the purpose of maximizing foot traffic, not creating a soil that retains water and nutrients. Fields are often sand based, requiring frequent irrigation and frequent fertilization to keep the fields in optimal condition. Allowing a surface to go dry and hard not only causes turf quality to decline, but also presents a hazard for users of the field.

Soil purchased from a landscaping yards itself is not a ‘natural’ soil as you might expect in a broadacre cropping system. These soils are manufactured, pushed together with loaders and bobcats, often using a recipe of less than 50% soil, and the rest recycled materials such as compost, sand, ash, or other unwanted materials that needed to be disposed of. Rather than pay to send these materials to landfill, recycled organics businesses take these ‘wastes’ and transform them into a soil resource to be used in urban green spaces.

Greenhouses often use recycled water. This is applaudable, yet a big problem in greenhouse culture is that salts accumulate after long dry spells as the recycled water concentrates through evaporation. Quite often the soils are not drained properly to permit the leaching necessary to control this. Thus reusing water resources is a positive step, but to use them properly without damaging yields or surfaces, the soil beneath needs to be managed as well. Successful urban irrigation requires an equal understanding of the water and soil quality.

Evolving cities

Urban green spaces and irrigation highlight human ability to recycle – to capture, transform and move resources to where we think suits us best. Re-used materials together make some of the most ionic spaces in our cities – botanic gardens, sports fields, and racetracks.

As cities evolve and urban agriculture and food growing is making a come back, urban soil and irrigation will present an increasing demand for water.

The capture and reuse of stormwater runoff from the “flashy” catchments created by urbanization is a logical and sensible way forward to reduce reliance on potable water supplies while providing the irrigation water necessary to maintain our urban vegetation and peri-urban horticulture.

The process of planning to capture and reuse irrigation water has been held back for a number of reasons:

  • A desire by water engineers to treat such water to very high “potable” standards which requires very expensive water treatment works. This is not necessary.
  • The difficulty of fitting low-cost earth dam structures in urban environments and the high cost of the alternative “tankage”.
  • The myth held by many that damming creeks and catchments is bad and environmentally destructive. This may be so in a pristine catchment but not in a degraded urbanized stream, such permanent water bodies providing new habitat for aquatic and avian wildlife.
  • The myth promoted by many hydrologists that permanent water bodies cause a rise in the water table and salinisation. This may be so where saline water tables occur and dams are “leaky” but properly constructed dams that do not leak cause no such issues.
  • The cost of setting up “stand alone” local reticulation systems. This is significant but where a high water use facility occurs, eg a playing field complex it should be viable.
  • The myth that we should use less water in cities. To manage the urban heat island we should use more water (in our urban green spaces, and homes), not less.

In order to provide the irrigation water necessary for urban greening and to preserve peri-urban horticulture it is necessary that we find alternatives to the long-held reliance on massive storage structures like drinking water dams and develop ways of capturing the vast quantities of runoff created by the very process of urbanization. There is abundant runoff created by urbanization and if we are to create green cities we must find ways of storing it for dry times. By doing so we will improve the “flashiness: of the urban hydrograph and reduce the urban heat island effect. In order to have water for irrigation in dry times (especially important with Australia’s variable rainfall) we must develop ways to store and reticulate this otherwise wastes runoff.