| Watershed water quality is an important element in evaluating regional ecological environment,among which total phosphorus(TP)concentration in surface water is an important indicator in the assessment of surface water in key ecological function areas,and TP concentration exceeding the standard is a common problem in small watersheds in northern Fujian.In order to clarify the relationship between rainfall characteristics and TP concentration in surface water of small watersheds,we selected Qingxi and Wutun small watersheds with different degrees of development in the upper reaches of Futun Creek in Glossy County as the study area.The percentage of forest cover in Qingxi small watershed was 88.54%,the population density was 10.17 persons/km2,and the proportions of paddy field,dry land,tea garden,orchard,forested land,shrub land,moso bamboo woodland,other forested land and non-agricultural land were 1.22%,0.08%,2.05%,0.37%,60.98%,2.46%,25.1%,0.91% and 6.82%,respectively;The forest coverage rate of Wutun small watershed is 57.98%,the population density is 86.69persons/km2,and the proportions of paddy field,dry land,tea garden,orchard,forested land,shrub land,moso bamboo woodland,other forested land and non-agricultural land are 8.38%,2.87%,1.95%,0.28%,47.99%,0.55%,4.87%,4.56% and 28.54%,respectively.By monitoring daily precipitation and day-by-day TP concentration data of surface water for 2019-2020 in the two small watersheds,the intra-annual distribution of precipitation,TP concentration,and the correlation between daily precipitation,≥5 mm rainfall,≥10 mm rainfall,and ≥20 mm rainfall and TP concentration in the two small watersheds were analyzed,and the results showed that:(1)During 2019-2020,the precipitation in the Qingxi small watershed during the abundant,flat,and dry periods was 3337.40 mm,1330.90 mm,and 236.10 mm,accounting for 68.04%,27.14%,and4.81%,respectively;the precipitation in the Wutun small watershed during the abundant,flat,and dry periods was 2747.4 mm,1449.7 mm,and 218.90 mm,accounting for 62.21%,32.83%,and 4.96% of the annual precipitation,respectively.(2)During 2019-2020,the average values of water quality TP concentration in Qingxi small watershed during the period of abundant water,flat water and dry water are 0.090mg/L,0.006mg/L and 0.003mg/L,respectively,and the extreme differences of TP concentration are0.540mg/L,0.079mg/L and 0.007mg/L,respectively;the average values of water quality TP concentration in Wutun small watershed during the period of abundant water,flat water and The average values of TP concentration in Wutun small watershed during the abundant water period,flat water period and dry water period are 0.083mg/L,0.068mg/L and0.007mg/L respectively;the extreme difference values of TP concentration are 0.237mg/L,0.257mg/L and 0.083mg/L respectively.(3)During 2019-2020,the correlation coefficients between TP and daily precipitation,≥ 5mm rainfall,≥ 10 mm rainfall,and ≥ 20 mm rainfall in the Qingxi small watershed during the abundant water period were 0.590,0.546,0.484,and 0.415,respectively,which were all highly significant and positive.The correlation coefficients between TP and daily precipitation,≥ 5mm rainfall,≥ 10 mm rainfall,and ≥ 20 mm rainfall in Qingxi small watershed during the flat water period were 0.320,0.319,0.561,and 0.561,respectively,all of which were highly significant positive correlations.The correlation coefficients between TP and daily precipitation,≥5mm rainfall,≥10mm rainfall and ≥20mm rainfall in Wutun small watershed during the abundant water period were 0.577,0.429,0.391 and 0.374,respectively,all of which showed highly significant positive correlation.The correlation coefficients between TP and daily precipitation,≥5mm rainfall,≥10mm rainfall and ≥20mm rainfall in Wutun small watershed during the flat water period were 0.619,0.711,0.681 and 0.682,respectively,all of which showed highly significant positive correlations.(4)Precipitation of different intensities can significantly affect water quality in watersheds with low forest cover.The water quality of Qingxi sub-watershed(88.54% forest cover)can remain stable under the influence of different precipitation intensity,and the difference between the highest phosphorus content and the lowest phosphorus content between 2019 and 2020 is only 0.08 mg/L,and most of the cases meet the national water quality standard of Class I,which can be used as a typical example of watershed management;while the water quality of Wutun sub-watershed(57.98% forest cover)fluctuates greatly under the influence of different precipitation intensity.The water quality fluctuates greatly under the influence of different precipitation intensities,and the difference between the highest water quality total phosphorus concentration and the lowest total phosphorus concentration is 0.27 mg/L during 2019-2020,and the average water quality is within the range of Class II water quality standards.The water resources in the watershed are subject to large changes and slow recovery from the influence of rainfall,and it is urgent to propose targeted measures to improve the water quality of the watershed.(5)The correlation between daily precipitation and phosphorus content of daily river water quality in Wutun small watershed is weaker than in Qingxi small watershed due to the influence of pollution from other surface sources;the correlation between total phosphorus concentration of water quality and daily precipitation is stronger in Qingxi small watershed during abundant water period and in Wutun small watershed during flat water period than in other periods.(6)By regression analysis of daily precipitation and total phosphorus concentration,the regression equations of total phosphorus concentration and daily precipitation in Qingxi small watershed during the abundant water period,Wutun small watershed during the abundant water period,and Wutun small watershed during the flat water period were derived,respectively,which showed that daily precipitation and daily total phosphorus concentration in the small watershed were positively correlated in the above three periods,while daily precipitation and daily phosphorus concentration in the two small watersheds during the dry water period were The correlation was very weak,which was related to the small number of samples due to very few precipitation days in the dry period.Based on the results of the study and the characteristics of the objects,a plan to improve the water quality of the sub-watersheds was proposed from various aspects such as forest stand renovation,joint control,and regulation development,which can be used as a reference for the future construction of ecological civilization and sustainable development in Glossy County. |
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