Seasonal Weather and Rainfall Patterns
When comparing the average monthly rainfall and daily average temperatures of various cities, the differences are amazing. In Phoenix, temperatures tend to be hot and what little rain falls is spread evenly over the year. In Portland Oregon, the temperatures tend to be moderate with wet winters and dry summers. In Chicago and New York the winters are cold, spring and fall are cool, and the summers are hot; precipitation is spread pretty evenly throughout the year. Miami is warm in winter, hot spring through fall, and has a defined summer rainy season where rain can fall in torrents. With this in mind, each climate will have different absorptive qualities. A city like Phoenix may experience nearly 100% rainfall retention, because it is typically hot and dry with little rainfall. Cities like Chicago or New York may see annual stormwater retention in the range of 60% to 85%. And a city where the rain comes in torrents will experience less annual rainfall retention. Retention will also vary somewhat from year to year as determined by the year’s weather.
Soil acts as a sponge in capturing and holding onto water. But, soil is less porous than a sponge, and will take longer to absorb and hold onto water. For this reason, if a rain event is very fast and intense, such as 1 inch over 15 minutes, a certain amount of water may sheet across the soil surface to the roof drains, before becoming absorbed by the soil. On the other hand, if a 1 inch rain comes in a gentle soaking drizzle over the course of a couple of hours, the efficiency of water capture is much greater.
Wind and Sun
Windy sites, particularly those that are windy during hot dry weather, will dry out more and retain more rainfall on an annual basis. Taller buildings will tend to be windier and those with reflected light or heat will dry out faster and therefore absorb more water as well. Of course, all things being equal, sunny areas are going to have less stormwater runoff then shady areas.
All roofs must slope in order to drain, and what most people refer to as a flat roof will have a slope of ¼” per 12’. Such roofs will retain more water than roofs with greater slope. This is because soil holds water by cohesion, but there is a limit to how much cohesive force soil can provide.
Practically speaking, soil acts as a sponge. If a moist sponge is angled upward, additional water will run out of it. The same is true of soil. The greater the angle, the less capable the soil is of retaining water.
Researchers Bradley Rowe, Kristin Getter, and Jeffrey Anderson have conducted studies regarding slope and water retention with inclines of 2%, 7%, 15%, and 25%, and found that annual water retention in Lansing Michigan ranged from approximately 85% with 2% slope to 76% with 25% slope.
One might logically presume that the deeper the soil substrate the more storm water a green roof will mitigate.
This is not necessarily true as independently discovered by Dr. Bill Retzlaff, et. al., of Penn State and Civil Engineer Drew Gangnes of Magnusson Klemencic Associates of Seattle have published their findings that a 4 inch soil depth performed optimally for storm water retention.
An extensive green roof with a 4 inch soil depth has the combined ability to hold water along with the ability to evaporate water between rain events, resulting in the least amount of runoff. In comparison, a 6 inch system might hold somewhat more water, but won’t dry out as effectively between rain events and a 2-½ inch deep system may dry faster but won’t hold as much rainwater.
Soil and Plant Characteristics
Plants and Soil also play a role in stormwater retention. Soil aggregate sizes and composition affect pore space, air space, and absorptive capacity. The volume and type of soil will also influence each system’s ability to absorb water.
And, while soil is mostly inanimate, it will gradually change over time due to natural freeze-thaw cycles that can alter particle sizes of some of the mineral components.
Neither is the organic component static and can either rise or fall and therefore affect the soil’s absorptive capacity. The process of plant growth and decay (leaves roots and stems) can contribute organic matter, particularly deciduous plants which shed their leaves to nourish the soil. Fully evergreen plants on the other hand may actually reduce organic matter from the soil. Plants also affect water retention and runoff by their use of water. They extract water from the soil and combine it with CO2 to make sugars, and they liberate water to the atmosphere by transpiration (similar to sweating) when it is sunny and there is sufficient water available.
In either case, plants help to keep rainwater out of the stormwater system and the more densely they cover the soil surface, the more absorptive capacity they have.