Study On Evapotranspiration Characteristics And Irrigation Management Of Extensive Green Roofs In Nanning

Green roof is one of the important technical measures to realize the construction of sponge city.Green roofs have functions such as alleviating urban waterlogging and urban heat island effects.So green roofs are widely adopted.The green roof evapotranspiration（ET）process plays a key role.However,on the one hand,evapotranspiration from green roofs is not only affected by local climate conditions,but also by its own structure configuration,which is a dynamic and complex process.On the other hand,due to its limited rainwater retention capacity,extensive green roofs（substrate thickness less than 20 cm）cannot avoid subsequent irrigation maintenance.Therefore,it is of great significance to study the influence mechanism of evapotranspiration of extensive green roofs in Nanning for the promotion and later irrigation management of extensive green roofs under specific climatic conditions.In this paper,two research methods of outdoor evapotranspiration model test and numerical simulation are used to study the influencing factors of extensive green roof ET in Nanning area.And the irrigation management design of extensive green roofs under different structural configurations is discussed.The research contents and results are as follows:（1）The outdoor evapotranspiration model test scheme of extensive green roofs in Nanning area was designed in detail,and the outdoor evapotranspiration model tests of extensive green roofs in Nanning area under different structural configurations（vegetation type,substrate type,substrate thickness and aquifer）were carried out.The hydrological characteristics,substrate soil-water characteristics and vegetation physiological indexes of green roofs are monitored.This study provides reference for the extensive green roof outdoor evapotranspiration model test device and monitoring method,and provides data support for the hydrological performance indicators of green roofs with different vegetation types,substrate types and aquifers.（2）Based on extensive green roof outdoor evapotranspiration model tests,green roof cumulative daily actual evapotranspiration(AET_cum)and substrate water content with drought time,effects of substrate soil-water properties and vegetation leaf area index（LAI）on green roof daily actual evapotranspiration rate（AET）,the correlation between substrate water content and green roof AET,the correlation between meteorological parameters with green roof AET and the seasonal variation of green roofs AET_cumwere analyzed.The results show that:The green roof ET properties of green roofs are jointly affected by the properties of the substrate（maximum water-holding capacity,water-holding capacity and permeability,etc.）.The ET rate of green roofs of different vegetation types is affected by the vegetation metabolism and LAI.The Sudem lineare green roof with sedum acid metabolism has a higher AET thanks to its higher LAI.In the early stage of drought,the aquifer had no significant effect on increasing green roof AET,while in the later stage of drought,the aquifer maintained a higher green roof AET by maintaining a higher water content in the substrate.There is a relatively strong positive correlation between the water content of the green roof substrate and the green roof ET,and this positive correlation is quite different due to the different configuration of the green roof structure.Green roof AET has a significant positive correlation with air temperature and solar radiation.Green roof AET has no significant correlation with wind speed and air humidity.And The difference in ET caused by green roof structure configuration is more significant in warm summer than in colder winter.This study reveals the influence mechanism of green roof structure configuration,substrate water content and climatic conditions on the ET of extensive green roofs in Nanning,and provides data support and scientific basis for the configuration parameter design of extensive green roof structure with similar climate conditions in Nanning.（3）Using the finite element software HYDRUS-1D to establish a green roof numerical calculation model,and the optimal design method for irrigation management of green roofs with different vegetation types and substrate thicknesses in Nanning area was studied using numerical models.The results show that:By weighing the total amount of green roof irrigation and the total number of days of water stress under the 9 irrigation schemes,Two optimal irrigation schemes were selected:15 mm after rain or irrigation every 3 days（irrigation scheme A）and irrigation to saturated water content every 7 days after rain or irrigation（irrigation scheme B）.Compared with irrigation scheme A,the Sudem lineare green roof and Ajuga repanms green roof implement the irrigation scheme B have higher irrigation efficiency,and there is no significant difference in irrigation efficiency of Ophiopogon japonicus green roof under the two irrigation schemes.The green roof with 5 cm substrate thickness implements irrigation scheme A has higher irrigation efficiency,the green roof with15 cm substrate thickness implements irrigation scheme B has higher irrigation efficiency.There was no significant difference in irrigation efficiency between green roofs with a substrate thickness of 10 cm under irrigation schemes A and B.This research explores the numerical modeling method of green roof irrigation management based on the HYDRUS-1D program,and proposes an optimal design scheme for irrigation management of extensive green roofs in Nanning.It provides a reference for the optimal design of irrigation management in Nanning and extensive green roofs with similar climatic conditions.