Experiment And Simulation Of Water Evaporation In Bioretention

The bioretention is an important sponge City treatment facility,which has prominent rainwater detention and purification capacity.In the process of operation,the moisture of the packing layer of the bioretention is constantly changing,and the water content of the packing layer directly affects the stagnation efficiency of the bioretention.Therefore,it is very important to study the water evaporation of the bioretention for the objective evaluation of the hydrologic effect of it.In this paper,an artificial climate box was used to study the effect of environmental factors and bioretention structure on the evaporation efficiency of bioretention.And a model of water evaporation resistance in bioretention was established.The main work and results were as follows:Design and establish a water evaporation test system for bioretention.The system consists of an artificial climate box,a bioretention cell unit,a weighing unit,a water supply unit and a signal acquisition unit.It can simulate the water evaporation of the bioretention under the conditions of different temperature and humidity and with or without submergence area.And it can measure,collect,process and preserve the evaporation data in real time.The evaporation capacity of the bioretention under different temperature and relative humidity was analyzed.The results showed that the water evaporation in the bioretention was in accordance with the soil evaporation,which could be divided into three stages,namely,the constant rate stage,the falling rate stage and the residual stage.In the first two stages,the environmental factors were the main factors affecting the water evaporation of the bioretention.The evaporation strength at the initial stage of evaporation is close to the maximum evaporation capacity under the environment,the surface water content is higher than 10%,and then the evaporation strength decreases rapidly.After 15 hours,the evaporation strength decreases more than 60%,and the surface water content decreases to less than 4%,and the higher the temperature is,the faster the evaporation strength decreases.The evaporation strength and surface water content of the third stage are both low and stable.The evaporation strength is about 1mm/d(0.82~1.1mm/d),the surface water content is less than 1%,and the surface layer forms dry soil layer.The results show that the cumulative evaporation of the bioretention with the submerged area is obviously higher than that without submerged area,and the cumulative evaporation of the 50 hours is 16.48% higher.In the second stage of evaporation,the decreasing amplitude of the evaporation capacity of the bioretention with submerged area is also relatively slow.And the water in the bottom submerged area is recharged the upper layer by the capillary action and the water vapor diffusion.Based on the ? model of evaporation resistance,a model of water evaporation resistance of bioretention was established.The evaporation resistance includes aerodynamic drag and packing layer resistance.The input parameters of the model are temperature,relative humidity,surface moisture content and wind speed and the output parameters are aerodynamic drag,packing layer resistance and evaporation scapacity.According to the test results,the simulation of the first stage of evaporation is modified and corrected.The calculated results of the model are in good correlation with the measured data,which is 2.3% higher than the measured results.The evaporation model of bioretention is used to consider the hydrological effect of the bioretention calculated after the effect of water evaporation,which is more consistent with the actual measured runoff reduction.The research results can be used to evaluate the hydrology effect of the bioretention and can also be used to guide the design of it.