Functional Assessment Of Soil Conservation And Water Supply In Small Watersheds In The Taishan District Based On The InVEST

Small watersheds are usually the basic units of soil and water conservation project construction planning,design and management.To explore the problem of soil conservation and water supply service functions and their spatial distribution patterns at the small watershed scale,based on the InVEST model and Arc GIS platform,a library of parameters required for the InVEST model was established to calculate the soil erosion and soil conservation and water supply service functions of different small watersheds in 2018,taking different small watersheds in Taishan District,Tai’an City,Shandong Province as the research objects.The spatial variability of soil conservation and water supply in different watersheds and under different land use,altitude and slope was analysed in combination with the division of small watersheds in Taishan District and the characteristics of their natural factors,and the two service functions were superimposed and compared through normalisation.The main results and conclusions of the study are as follows.1、Classification and basic characteristics of small watershedsWith reference to the technical standards and methods such as the Specification for the Classification and Coding of Small Watersheds,the whole area of Taishan District was divided into 24 small watersheds.The area of the different sub-basins ranges from 5km~2 to33km~2,and there are three types of sub-basins:intact,sloping and interval according to the form of the water system.11 sub-basins are intact and 13 are interval and sloping.2、Soil erosion characteristics of small watershedsThe average actual soil erosion modulus in the Taishan region is 430t/（km~2·a）.The actual soil erosion modulus of the different sub-basins ranges from 30t/（km~2·a）to 1455/（km~2·a）,showing a decreasing trend from the north to the south of the region.The sub-basins with higher actual soil erosion modulus are mainly located in the northwest and northeast of the region in the undulating low hill areas,while the sub-basins with lower erosion modulus are mainly located in the southwest in the pre-hill plain areas.3、Soil conservation function of small watershedsThe average soil retention per unit area in Taishan District is 15834t/（km~2·a）The average soil retention of each sub-watershed ranges from 452t/（km~2·a）to 63382t/（km~2·a）.The sub-watersheds with high soil retention are mainly located in the low mountainous areas in the northwest.The small watersheds with low soil retention are mainly located in the pre-mountain plain area in the south-central region of Taishan District.The soil conservation function of small watersheds is related to topographic factors（elevation,slope）and land use types.The soil conservation amount per unit area tends to increase with elevation and slope,and the soil conservation amount per unit area in different land use types is forest land>garden land>town and village land>cropland>grassland.4、Water supply function of small watershedsThe average water supply function（depth of water yield）in the Taishan region is 211.86mm,and the average depth of water yield in each sub-basin ranges from 168.51 to 241.42mm.The higher yielding sub-basins are mainly located in the south central part of the mountainous plain,while the lower yielding sub-basins are mainly located in the southwestern part of the hilly region.The water supply function is related to topographic factors（elevation,slope）and land use type.The depth of water production decreases with increasing altitude and slope,and the different land use types are in the following order:grassland>town and village land>cultivated land>garden land>forest land.5、Integrated features of small watershed functionsThe normalised values of the two ecological service functions were obtained through the normalisation formula and weighting process as the basis for comprehensive functional evaluation.In general,the normalised values of the two ecosystem service functions show a decreasing trend from north to south.The normalised values of ecosystem service functions in small watersheds in Taishan District range from 0.19 to 0.45.The higher values are mainly found in the hilly areas in the north-west,where the normalised values for both ecosystem services are mostly above 0.30.The lower values are mainly found in the southern pre-hill plains,where the normalised values for both ecosystem services are mostly below 0.25.