| China is a big agricultural country.The development of agriculture needs a lot of water resources.However,in the process of agricultural development,the shortage of water resources has been restricting the development of agriculture.From the past to present,China has been committed to the construction of agricultural water conservancy projects.As a typical agricultural water conservancy project,ponds have a long history in China.However,in the process of agricultural development,many shallow ponds are mostly abandoned or converted into arable land,and their ecological functions such as flood storage,drought resistance,pollution control and decontamination and "ecological islands" can not be effectively played.Therefore,it is of great significance to carry out the restoration and optimal construction of ponds.On the basis of the existing research achievements on conservating water resource,reducing runoff speed,reducing peak discharge,reducing nitrogen and phosphorus output,this paper studies how to construct the farmland pond system and how it will affect the ecological environment after its construction in view of the current situation of the pond system.The results are as follows:(1)By exploring the characteristics of farmland and pond in farmland pond ecosystem,this paper provides a theoretical basis for the construction of farmland pond ecosystem.Two observation experiments were carried out to investigate the variation law of nitrogen and phosphorus concentration in field runoff with time in paddy field and dry field.The variation law of nitrogen and phosphorus concentration in paddy field and dry field experiments was similar,both rising first and then decreasing.Loss of nitrogen and phosphorus in the first two-thirds period of the rainfall accounted for more than 80%of the total wastage in rianfall period.This study provides a theoretical basis for the use of pond capacity to store runoff with high nitrogen and phosphorus concentration in premetaphase of runoff producing period.Besides,a year's observation was carried out to investigate the variation law of temperature and DO in different depths of the pond.Combined with rainfall characteristics and temperature tolerance of aquatic organisms in southern China,it was determined that the daily depth of the pond should be 1.5?2.0 m.(2)The farmland pond ecosystem is mainly composed of field ditches,pond and the ecotone.Combining with the geographical and climatic characteristics of southern China and the existing research experience,this paper puts forward the specific design methods of pond ecosystem,including the overall layout of the system,the construction of pond,the construction of field ditches and the construction of the ecotone.(3)A dynamic simulation model of pond water volume was established to simulate the process of daily variation of pond water volume in the system,and to deduce the process of storage and drainage of pond under different levels of years.It provides an effective method for later analysis of the impact of farmland pond ecosystem on reducing regional drainage and nitrogen and phosphorus output.(4)The specific application of pond ecosystem construction and its ecological environment impact after construction are analyzed by case study.Based on the above research results and the regional characteristics of the study area,it is determined that the area ratio of ponds in the pond ecosystem should be 2?3%.Based on the rainfall data of Yangzhou from 2013 to 2015,the hydrological impact model of pond ecosystem was used to simulate and calculate the regional drainage.During the period from 2013 to 2015,the rate of drainage reduction after the construction of pond ecosystem in the study area was 46.25%,100%and 22.48%.Combined with the nitrogen and phosphorus concentration observed in the rainfall runoff experiment,the reduction rates of TN,TP and TP from 2013 to 2015 were calculated to be 83.99%,100%,46.3%,68.27%,100%and 34.09%respectively. |
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