Effects Of Different Forests On Water Quality And Its Prediction Model At Watershed Scale In Sub-tropic Region

Forest is not only an important resource for mankind survival,but also an important component of global environment.In the process of runoff flowing through forest,forest can weaken the toxicity and reduce the extent of the non-point source pollution in surface runoff and groundwater with physical,chemical and biological processing.After rainfall through forest ecosystem to form a runoff,forest brings a variety of impact on water quality of the surface runoff,the forest canopy,the vegetation layer under forest,leaves layer and soil layer have effects on water quality of surface runoff and groundwater runoff from physical,chemical and biological processing.The study on effects of water quality with different forest ecosystems is always through comparing of water quality of rainfall and water quality of runoff flowing from different forest ecosystems.The presences of forest vegetations have great impacts on suspended sediment, temperature,dissolved oxygen(DO),pathogens and chemical properties of surface and groundwater runoff.However,different forests or different distributions of plant species have different purification effects on water quality.It will provide guide for controlling agricultural non-point source pollution with identifying the best water purification forest and the best planting layout through our study.Nine runoff collection fields were set up at Luoxi small watershed in our experiment,we sampled waters after every runoff,and then measured mainly seven indexes of water quality: total nitrogen(TN),total phosphorus(TP),biological oxygen demand(BOD₅),pH,dissolved oxygen(DO),electrical conductivity(EC)and temperature,the experiment lasting for one year: from December 2006 to November 2007.Then,analyzing the data,we found out the purification rule of forests on the water quality:EC(us/cm):mixing forest(192.74)＜Chinese fir forest(196.90)＜waste land(217.62)＜adult pine(247.04)＜economic forest(276.22)＜bamboo forest(319.50)＜adult broadleaf forest(332)＜young broadleaf forest(421.00)＜middle-aged pine(526.7);TN(mg/L):mixing forest(0.159)＜adult broadleaf forest(0.174)＜Chinese fir forest(0.332)＜waste land(0.333)＜bamboo forest(0.336)=adult pine(0.336)＜middle-aged pine(0.393)＜adult broadleaf forest(0.417)＜economic forest(0.439).Other indexes were omitted here,in general,among the seven indexes,mostly water quality of surface runoff through the mixed forest was the best,and water quality of surface runoff through the economic forest was relatively poor.According to surface water environmental quality standard (GB3838-2002),their all values reachedâ… orâ…¡Class standards.Analyzing with single factor index method and comprehensive water quality index method (WQI),we found that single factor indexes' changing trend of surface runoff water quality from nine different forests were different,the test showed that in general the single factor indexes of surface runoff water quality through economic forest were all worst while single factor indexes of surface runoff water quality through adult broadleaf forest and mixed forest were all best.In another word,water quality of surface runoff through adult broadleaf forest and mixed forest were best while water quality of surface runoff through economic forest was worst.Moreover, analyzing with the WQI method,we found that the WQI of the surface runoff water quality form nine different forests were different:adult broadleaf forest(0.37)＜mixing forest(0.41)＜bamboo forest(0.46)＜middle-aged pine(0.49)＜young broadleaf forest(0.50)＜economic forest(0.51)= Chinese fir forest(0.51)＜adult pine(0.52)＜waste land (0.55).According to water quality index rating classification method,the WQI＜0.5 of the surface runoff through adult broadleaf forest,mixing forest,bamboo forest and middle-aged pine,in best level,and the 0.5＜WQI＜1 of the surface runoff through Chinese fir forest,adult pine,waste land,economic forest and young broadleaf forest,and they belonged to a better level.Appling the GIS software(Citystar)to analyze the land use type map of Luoxi watershed, also using direct extrapolation of scaling methods to scale from point to area to establish one forecasting model to predict water quality at the watershed Luoxi:Y = 0.4271X1+0.4425 X2 +0.0132 X3 +0.0384 X4 +0.0745 X5 +0.0043 X6(X1:Chinese fir water quality index;X2: adult broadleaf forest water quality index;X3:bamboo water quality index;X4:economic forest water quality index;X5:adult pine water quality index;X6:waste land water quality index).From average values of surface runoff of seven indexes on water quality and this prediction model,we could predict that indexes of Luoxi watershed were:EC:265us/cm,DO: 6.918mg/L,TN:0.264mg/L,TP:0.030mg/L,pH:7.883,temperature:20.134℃.According to the environmental quality standards on surface water in GB 3838-2002,water quality at Luoxi watershed accorded with theâ…¡grade.