Greening initiatives that hit home

One size does not fit all when it comes to urban greening initiatives. This is the message from KAUST scientists Pei-Ying Hong, Himanshu Mishra, Daniele Daffonchio and Matthew McCabe in a recently published comment in the journal Nature Water.^[1] The authors highlight the critical role of water in restoration of degraded lands while proposing a pathway for greening initiatives in a Middle East context.

“Urban greening will need to be a major part of efforts to deliver more sustainable water use in the Middle East while also enhancing livability,” says Hong, who leads a team developing energy efficient wastewater-treatment processes.

In many parts of the world, large-scale afforestation can be used to restore degraded landscapes. However, in the Middle East — one of the most water-stressed regions in the world — the use of large-scale tree planting is limited by the availability of fresh water.

The authors argue that, in this environment with large urban populations, directing greening efforts towards urban settlements is a more strategic and sustainable restoration approach than afforesting large areas of uninhabited arid land.

Challenges for implementing any urban greening plans in the Middle East include the energy intensive nature of water production, the arid and hot conditions, and the limited rainfall and high evaporative losses.

The authors emphasize that sustainable urban greening must maximize the potential for desired outcomes, such as enhancing livability, producing urban cooling and providing extreme weather buffers while simultaneously minimizing water input and the associated carbon footprint.

They argue that rapidly depleting levels of nonrenewable groundwater across much of the Middle East make it unfeasible to use groundwater for greening initiatives.

“With many major cities in the region located close to the sea, using desalinated water may seem a good option, but water produced by current desalination technologies has a high energy cost, with most of that energy derived from fossil fuels,” says Hong.

“It defies logic to emit CO₂ to generate water for plants being grown to combat the effects of climate change,” she says.

While using brackish groundwater or partially desalinated seawater to irrigate salt-tolerant plant species is one option, Hong says an even better solution is the use of reclaimed wastewater.

The complete capture of wastewater generated by 34.3 million people (the population of Saudi Arabia) would provide about 9 million cubic meters of treated water per day. Anaerobic-based technologies, such as the process developed by Hong that can clean wastewater at potentially zero energy cost, have shown promise as a more sustainable way to generate reclaimed water for nonpotable reuse.

The authors note that in urban areas with access to treated wastewater, plants that take up nutrients and contaminants can be used in nature-based biofiltration systems or constructed wetlands. The wastewater that flows through the biofilter can be collected for other reuse purposes, such as flushing toilets, or maintaining recreational water features in urban parks.

By identifying the right plants and integrating them into water-conserving architectural forms, like blue-green walls or tree-pit gardens, cities can benefit from enhanced livability and urban cooling while being buffered against extreme weather conditions.

The authors also consider sustainable soil amendments in the sandy soils of the region, such as superhydrophobic sand applied as a mulch to reduce evaporative losses and boost plant health.

Director of KAUST’s Climate and Livability Initiative Matthew McCabe says that the benefits of any greening initiatives in terms of improving the sustainability and livability of urban environments need to be quantified to measure their success.

“This requires ongoing monitoring and data collection, including an accurate lifecycle assessment that covers water generation, distribution, tree planting and plant growth and maintenance,” he says.

“Long-term studies to determine the impacts on the soil ecosystem and city-climate feedbacks are also needed.”

While noting the many potential benefits of urban greening initiatives, the article is a timely reminder that any large-scale implementation of urban greening needs to be operated within a sustainable water–energy–environment framework.