Design and practice of rainwater utilization system for roof garden

In recent years, urban landscape has changed from ground greening to the roof three-dimensional greening with the rapid development of landscaping in China, and a lot of roof gardens have been built to open up the new development space for the landscape in cities. Shortage of water exists in many cities, and the city water stress will turn out because of the increasing roof gardens. To solve these problem, taking the roof garden of Beidahuang group hotel as an example, we would intend to discuss the feasibility of using roof rainwater for irrigating roof garden, and design the collection and utilization of the roof rainwater following the plant evapotranspiration (ET), the rainwater collection quantity, the reservoir capacity and rainwater collection systems. Firstly, the plant water requirement in roof garden was calculated using Penman-Monteith formula and garden coefficient methods. The results showed that the total of ET was 3 521.78 m3 from May to October, and which was significantly different from only 361.8 m3 in October to 729.6 m3 in June. Secondly, the collection rainwater in roof garden and the next building roofs was calculated, it showed that the total collected roof rainwater was 2 179.3 m3. The reservoir capacity was designed at 131 m3 according to the stormwater runoff volume and the first-flush runoff quantity. It could save 61.88% of irrigation water if the collected rainwater was used to irrigate for roof garden. Therefore, the rainwater collection systems and the automatic irrigation systems for the roof garden were designed using engineering technology. The rainwater collection systems included rainwater collecting ports, transporting pipelines, split-flow pool, purifying systems and reservoirs to collect rainwater in the roofs of the buildings, split-flow runoff, purify runoff rainwater, and store rainwater in the reservoir to use. Automatic irrigation system, included sensor system, central control system, power system, transporting system and communication equipments, the central control system could control the turn-on time and the irrigation quantity to the irrigation system by analyzing the soil moisture signals collected by the sensor system. So the automatic irrigation system could achieve accurate and water-saving irrigation. Finally, taking advantage of this automatic irrigation system could save 67.20% of irrigation water and the rainwater utilization rate was up to 54.55% with the water balance and running results analysis. The new recommendable method of roof garden rainwater collection and utilization proposed in this paper have the important practical value and the application prospects to design the technology for collecting and utilizing rainwater in the north arid area.