Green Roof Water Harvesting and Recycling Effects on Soil and Water Chemistry and Plant Physiology

There has been an increase in the utilization of green roofs in the United States due to their abilities to improve the urban environment. Alternative irrigation sources have received a lot of attention over the last decade due to the demanding pressures put on fresh water supplies in urban ecosystems. Alternative irrigation sources can include grey water, sewage effluent (black water) and harvested rainwater which can be a) water captured from an impervious roof and b) stormwater captured from impervious surfaces both of which are stored for later use. This harvested rainwater serves as a freely obtained source of water and can be used for irrigating green roofs. Four intensive green roofs that utilized harvested stormwater and building grey water for use as irrigation in the Houston, TX metropolitan area were examined for their water and soil chemistry and plant physiology. All drainage water or runoff from the roof was captured and returned to the irrigation supply tank so that a recycling system was in place. The green roofs were similar in construction and their water recycling system except that one green roof (roof 1) added the aerobically treated septic discharge onto the roof in addition to the recycling system; while a subset of roof 2 (roof 2B) was unirrigated. A bituminous conventional roof was also included in the investigation to compare runoff quality from green roofs and conventional roofs. Irrigation water, cold-water extracts of growing media, and roof drainage samples were analyzed for their salt and nutrient content. In general, drainage water from the conventional roof had the lowest salt concentrations while roof 1 had the highest salt concentrations in its drainage water. The recycled irrigation water quality was similar across all green roofs and also similar to municipal tap water from the area. The chemistry of irrigation water samples remained similar throughout the investigation while quality of growing media extracts and drainage water samples from the green roofs showed a strong correlation with the amount of recent precipitation. Containers with identical growing media planted with Trachelospermum asiaticum (Asian jasmine) were installed on each green roof to address the concern about different starting media and plant composition on each green roof. Plant water potential, specific leaf area and rates of photosynthesis, transpiration, and conductivity were not significantly different between roofs which helped to further conclude that water quality parameters remained within acceptable boundaries for soil and plant health, regardless of roof age. Container media extract concentrations showed some significant differences for electrical conductivity (EC), percent organic matter, and concentrations of TDN, Mg^(2+), and Ca^(2+) among roofs yet evidently not enough to influence physiological measurements of plants.

For a study to measure the effect of vegetation and climate on green roof media erosion, green roof modules were installed on a roof in College Station, TX. The presence of vegetation reduced erosion after initial instrument installation yet had little effect once the media settled. Neither windspeed nor precipitation rate had a direct effect on erosion; however, under conditions of high wind and high precipitation an accumulation of media was observed.