Many Denverties may be concerned that more green roofs could have a negative impact because too much water is needed and we live in an area prone to drought. We want to explain how green roofs impact water quality, and best practice irrigation methods to efficiently use water.
Although it is true that green roofs need a little extra water in the first few years of development, they also decrease runoff which helps manage stormwater and reduces flooding. Pervious surfaces (such as grass, dirt, and sand) retain water, while roads and sidewalks don’t causing water runoff. Green roofs also reduce water pollutants in our streams and lakes, as plantlife filters out pollutants. Water quality and quantity therefore will not be affected, as the positive and negative benefits impacts cancel out. There is also reduced energy usage and cooling reduction from green roofs, which provide water savings. Green roofs reduce urban heat island which can reduce the cooling needs of other buildings. And since most green roofs use sedums which are drought resistant, over time they do not require much water (1). Therefore, green roofs will not cause water issues in a high drought state.
Water used for irrigation is important to avoid replacing plants because of heat or drought, and to also provide fire protection. The amount of irrigation needed depends on the size of the project, the plants water needs, regulatory requirements, and growing media. Extensive green roofs with shallow growing media and drought resistant plants like sedums will need much less water than an intensive green roof with trees and diverse vegetation.
The type of irrigation method will ultimately affect water usage, but luckily there are many options that help conserve water. For smaller extensive green roof projects, you can use a manual irrigation (such as a water hose) that is low cost and uses small amounts of water when needed. If you are using this method, be careful of damaging plants while dragging a hose, and with temporary irrigation methods watering is often neglected. For permanent irrigation, low volume drip irrigation will provide water slowly and directly into the growing medium where it is needed. This is a highly efficient application that requires a higher water quality. Moisture retention layers are also a good form of passive irrigation that uses “egg carton” depressions to store small amounts of water when it rains. This of course relies on rainfall, only working in wet conditions and would be hard as the only irrigation method in a city like Denver, but could help offset some water usage in the long term.
There are also ways you can reuse water. Colorado has strict water rights laws and recently allowed a small amount of rain barrels, but only for residential (2). The current Green Roof Initiative will even need to change the wording around rainwater reuse, as it currently conflicts with Colorado law. That being said, you can integrate with other building systems and create a grey water recycling system to help irrigate a green roof. Grey water is reusing non-potable water from baths, showers, and sinks. Another popular way to reduce water usage is with HVAC condensation. This method uses condensation recovery to harvest water from your HVAC system. The Austonian has a green roof with pressurized irrigation from HVAC condensation and Austin, TX City Hall has built a scenic waterfall on their green roof with HVAC condensation (3).
To reduce your water usage on a green roof, ensure the design team knows your goals and can figure out location, access, plants and size accordingly. Make sure to include irrigation in your budget and maintenance plan, or plants will have a hard time surviving.