Analysis of possibilities for rain water harvesting from green roofs as a part of sustainable water management in Lithuania

Europe faces extreme climate change; these changes are fixed as longer drought periods, not just in springs or the middle of summer but all over the vegetation period. Long droughts mean water shortages in rural areas, reduced river water flows, and limited opportunities to use this water for irrigation. Attention should be paid to different rational water management technologies and systems with particular emphasis on rainwater harvesting. Sustainable development goals targeted for the water sector in Lithuania can be reached underwater by harvesting from roof systems using water tanks or harvesting rainwater in green roof systems. One method meets farmers’ requirements, another— sustainable development goals. Lithuania is in a zone of excess moisture, and by accumulating this water, it is possible to compensate for the lack of water reserves in topsoil in the dry season. This study aims to compare two technologies, water storage in green roof structures and water storage in rain tanks. The comparison is based on a field experiment study, when the dynamics of rainwater runoff from an experimental green and bituminous roof was recorded in 2022–2023 two 1 m² stands were affected by open field climatic conditions. One stand was covered with traditional bituminous tiles and the other was covered with a “green roof” coating. Meteorological data were obtained from the Kaunas City meteorological station. The study confirms that a green roof can reduce the load on rainwater systems and retain precipitation in the roof layers. During the first months of observation, it recorded the tendency that rainwater runoff is slow in the case of a green roof, but it continues even after the intense rain has ended. This confirms the claims of foreign authors that the green roof accumulates, retains rainwater, and thus reduces the runoff peak during rain. This means that the water from the roof system accumulates in rain tanks giving the possibility to reuse it for the future and bigger agglomeration.