Optimization Of Forest Type Layout In A Small Watershed Based On Water Purification Function Improvement

The Middle Route Project(MRP)is an important part of the South-to-North Water Transfer Project(SNWT),which is a major strategic scheme to solve the problem of water shortage in the north of China.As the water source of the MRP,Danjiangkou Reservoir plays an important role in the SNWT.It's not only related to the water safety of people's living and production along the area,but also affects the social and economic development of the surrounding areas.This study selected the Jinzhuangyan small watershed in the Danjiangkou reservoir area of Hubei province as the research area.The remote sensing images were used and combined with the field survey to divide the small watershed into 14 sub-watersheds.On the foundation of remote sensing images interpretation and forest type landscape pattern analysis in small watershed,we collected water samples monthly from the surface runoffs of the 14 sub-watersheds.Concentrations of total nitrogen,total phosphorus,nitrate nitrogen,ammonia nitrogen and chemical oxygen demand of the surface runoffs were tested during the wet and dry periods from August 2018 to February 2019.The correlation between forest area proportion and landscape pattern index and water quality index was analyzed.According to surface runoffs comprehensive water quality,the sub-watershed type was classified.According to the difference of water quality,the sub-watershed forest type layout was optimized and benefit evaluation.The forest type layout optimization suggestion was put forward,in order to provide a scientific basis for improving the forest water purification function.Our research indicates that:(1)There is a big spatial difference between the forest type area proportion and the landscape pattern index among the 14 sub-watersheds.The spatial variation of coniferous forest area proportion,PD and LPI are 4.04%~61.64%,0.17~1.2/100 ha and1.95~28.39%,respectively.The spatial variation of broad-leaved forest area proportion,NP,PD,LPI,ED,and IJI are 0~48.66%,0~9,0.06~0.72/100 ha,0~13.72%,0~22.41m/ha and 0~87.63%,respectively.The spatial variation of coniferous andbroad-leaved mixed forest area proportion,PD,LPI,ED and LSI are 0~57.52%,0~1.34/100 ha,0~19.98%,0~31.06m/ha and 0~5.56,respectively.(2)Because of the serious excess standard of total nitrogen and chemical oxygen demand concentration,the runoff water quality of 14 sub-watersheds was under the surface Class III water quality standard,and did not achieve Class II water quality in Danjiangkou reservoir area.The concentrations of total nitrogen,total phosphorus,nitrate nitrogen,ammonia nitrogen and chemical oxygen demand in runoff water of14 sub-basins varied greatly.The concentrations of total nitrogen,total phosphorus,ammonia nitrogen and chemical oxygen demand were higher in the wet period than in the dry season while the nitrate nitrogen concentration was lower in the wet period than in the dry season.The spatial variation of total nitrogen,total phosphorus,nitrate nitrogen,ammonia nitrogen and chemical oxygen demand of the 14 sub-watersheds in the wet season were 0.44~4.85mg/L,0.0094~0.0564mg/L,0.15~3.20mg/L,0.33~ 0.55mg/L and 16 ~ 58 mg/L,respectively.While the spatial variation of total nitrogen,total phosphorus,nitrate nitrogen,ammonia nitrogen and chemical oxygen demand in the dry season was 0.20~2.65mg/L,0.0073~0.0535mg/L,0.1~2.51mg/L,0.14~0.45mg/L and 20~35mg/L,respectively.(3)Correlation analysis between the area proportion of different forest types and runoff water quality shows that the area proportion of coniferous forest and broad-leaved forest are significantly negatively correlated with runoff water quality index concentration,while the area proportion of coniferous and broad-leaved mixed forest is significantly positively correlated with runoff water quality index concentration.It indicates that the larger the area proportion of coniferous forest and broad-leaved forest,the better the runoff water quality;and the larger the area proportion of coniferous and broad-leaved mixed forest,the worse the runoff water quality.The correlation analysis between the landscape pattern index of different forest types and the concentration of runoff water quality indicators shows that the ED of coniferous forest,the LPI,ED and IJI of broad-leaved forest,LPI,ED,LSI and IJI of coniferous and broad-leaved mixed forest are significantly negatively correlated with runoff water quality index concentration,the ED and LPI of coniferous forest,the PDof broad-leaved forest and coniferous and broad-leaved mixed forest are significantly positively correlated runoff water quality index concentration.It indicates that it can be used to improve the runoff water quality by adjusting the landscape pattern index of different forest types.(4)Among the three types of water quality,light pollution and moderate pollution in the sub-watershed runoff comprehensive quality assessment,the sub-watershed with better water quality,coniferous forest area proportion,PD and LPI are 17.01%,0.3/100 ha,and 5.14%,broad-leaved forest area proportion,NP,PD,LPI,ED and IJI are27.11%,3,0.38/100 ha,9.71%,16.02m/ha and 76.45%,coniferous and broad-leaved mixed forest area proportion,PD,LPI,ED and LSI are43.42%,0.09/100 ha,11.98%,2.77% and 2.12,respectively.According to the sub-watershed forest type layout with better water quality,the sub-watershed with poor water quality is optimized,and the water quality improvement benefit optimized in the forest type area proportion is 22.58~46.07%,and the water quality improvement benefit optimized in the coniferous forest landscape pattern index is 8.06~25.64%,the water quality improvement benefit optimized in the broad-leaved forest landscape pattern index is 3.23~31.46%,and the water quality improvement benefit optimized in the coniferous and broad-leaved mixed forest landscape pattern index is10.13~28.09%.Moreover,the broad-leaved forests and coniferous forests are distributed on both sides of the river and the water outlets,which is conducive to the improvement of water quality in the sub-watershed,the coniferous and broad-leaved mixed forest is scattered away from the river and the water outlets is conducive to the improvement of water quality in the sub-watershed.