T E R R A . f l u x u s

Understanding Regional Ecoroof Irrigation

Always a fascinating topic, some of the more technical aspects of the irrigation of irrigation for vegetated roofs (aka ecoroofs) will be a topic of discussion at tonights Green Roof information Think-tank (GRiT) meeting, happening at the Lucky Labrador Taproom on 1700 North Killingsworth in Portland – starting at 6pm .  Should be a lively dialogue.

I’ve done some comparative analysis of cities before, but decided it would be interesting to update and expand on the data as the industry has grown.  Thus I captured precipitation data from a number of cities around the United States and the rest of the globe.   The table of values is found below (click on it to enlarge it to a more readable size).

The color coding gives high and low ends to precipitation, to show times of significant input and times of drought conditions.  Blue areas denote months that receive 4 inches or more of precipitation, and brown areas show months with 1″ or less of precipitation.  This shows our bookended precipitation scheme in the Pacific Northwest, with a distinct rainy winter and a comparatively dry summer.  Obviously the distinction is most common by comparing say Phoenix to Miami – but there are some interesting elements that emerge, particularly in comparisons of cities that have similar rainfall amounts as Portland, like Chicago and Washington D.C.  These may be the perfect analogue to a consistent 12 month rain period that is common in Europe (often leading to a common North American misconception that irrigation isn’t necessary).

Depends on where you live – and the disparity between precipitation and evapotranspiration in our summers is pretty immense, as shown in the following graph.

Our Portland area (and the western US in general) follows this interesting downward graph shape that is almost opposite to a summer rise in evapotranspiration.  A comparison of cities that are indicative of the trend:

The distinction is more obvious when there is some comparison between groups of cities with more stable precipitation through the year.  This hold true in the model green roof pioneering zones of Northern Europe:

And also in a range of cities throughout the United States, including some of the leaders in green roof implementation.

It is, of course, a macro-level analysis, but is really telling in fine-tuning large scale assumptions about green roofs to the more regional variations in climate and precipitation.  While the ever-evolving idea of sustainability is rightly generating an ideology of water-consciousness, there is an innate lack of homogeneity that exists throughout cities around the world and even within the US.   The goal is obviously not to pour lots of water on these systems, but rather to come up with a balance of how much irrigation (at strategic times during drought periods) is necessary to maintain ecological function and meet project specific functional and aesthetic needs.  There is a continuum of goals that must be taken into account that is misguided by blanket declarations of correct answers for implementation of any system.

Thus instead of a specific ‘no-irrigation mandate’ that is encourage by sustainable purists and/or oversimplified rating schemes, there needs to be a resource cost/benefit analysis of the value of resource to the overall collective benefits for a project and external benefits to the community at large.  Then we can begin to fine-tune the concepts of using only as much water as we need, test designs and efficacy of no-irrigation roofs, and refine technologies for re-purposing reclaimed rain and greywater, getting to a net-zero scenario that makes use the available resources.