Temporal And Spatial Distribution Characteristics And Interrelationships Of Runoff, Sediment And Total Phosphorus In The Mainstream Of Chishui River

The Chishui River is a first-level tributary on the south bank of the upper reaches of the Yangtze River.Human activities have changed the runoff and sedimentation processes of the main stream of the Chishui River,thereby affecting the water quality.This article is based on the collection of the precipitation,runoff,sediment and water quality monitoring data of important representative stations in the upper reaches of the Chishui River(Chishuihe Station),middle reaches(Moutai Station),and downstream(Chishui Station)from 2003 to 2016,and water quality from 2012 to2016.According to precipitation,runoff,and sediment data,the main stream of the Chishui River is re-divided into the wet season and the dry season.Analyzed the relationship among the change trend of the upper,middle and lower reaches of the Chishui River,the change trend of sediment,total phosphorus,and the temporal and spatial distribution of the three.the result shows:(1)The wet season of the main stream of the Chishui River is from May to September.Taking Chishui Station as an example,from 2003 to 2016,the precipitation,runoff,and sediment transport in the wet season accounted for 66.6%,65.3%,and 94.8%of the annual total,respectively.The data of the long series(1956～2008)are consistent.(2)Runoff and sediment transport have a similar change process.The runoff at the Chishuihe Station and Maotai Station during the dry season has a significant increase trend(p<0.10);the sediment delivery during the wet season and the annual sediment delivery at the Chishuihe Station and the Chishui Station have a very significant decrease(p<0.05);the Chishuihe Station The average sand content of Chishui Station during the wet season and the annual average sand content have a very significant decreasing trend(p<0.01).The Sen slope estimates of the relative change rates of annual runoff at Chishuihe Station,Maotai Station,and Chishui Station are 0.0273/a,0.0282/a,and 0.0162/a,respectively;the relative change rates of annual sediment discharge at Chishuihe Station and Chishui Station are-0.0268/a,0.00113/a;the relative change rate of the annual average sediment concentration of Chishuihe Station and Chishui Station are-0.0375/a,-0.210/a,respectively.(3)The time distribution law of water quality is that the dry season is better than the wet season,the spatial distribution law is that the upstream is better than the downstream,and the downstream water quality has a significant deteriorating trend(p<0.10).(4)The annual total phosphorus flux is concentrated in the wet season.Except for a few years,the total phosphorus flux in the wet season accounts for50.48-88.07%of the annual total phosphorus flux.Taking Chishuihe Station as an example,the total phosphorus flux during the wet season in 2015 was 213,000 tons,accounting for 57.8%of the annual total(368,000 tons).(5)The correlation coefficient between discharge and total phosphorus concentration is small,only the Chishui station in the wet season reaches 0.35,which is moderately correlated;while the correlation coefficient between runoff and total phosphorus flux is larger and positively correlated.The Chishuihe station in the dry season after the flood,the Moutai station in the dry and wet seasons before the flood,and the Chishui station in the dry and wet seasons before the flood all reached 0.35,and the largest reached 0.58,showing strong correlation.The proportion of sediment transport from upstream is higher than that from downstream,and the proportion of total phosphorus flux from downstream is higher than that from upstream.Taking2014 as an example,the sediment transport from Chishuihe Station is 72.5×10~4t,accounting for 42%of Chishui Station;the total phosphorus flux is 36.8t,accounting for only 18.5%of Chishui Station.(6)The rank correlation coefficient of total phosphorus flux-sediment volume during the wet season is greater than that in the dry season.Taking Chishuihe Station as an example,the coefficients of the wet season,post-flood dry season,and pre-flood dry season are 0.307,0.136,-0.075,respectively.The rank correlation coefficient of total phosphorus flux-sediment transport in the downstream is larger than that in the upstream.Taking the wet season as an example,this coefficient at Chishui Station is0.466,which is 0.159 higher than Chishuihe Station.