Changes Of Riparian Vegetation And Its Driving Factors At The Middle And Lower Reaches Of Shiyang River Basin

The riparian vegetation character, aspect of precipitation, river change, runoff, water quality, annual change of ground water, soil and relation with vegetation at middle and lower reaches of Shiyang river basin were discussed by a method of time and spetial series, remote material, investigation results and correlation analysis on vegetation, soil and hydrology at up and lower zone of Shiyang river body with runoff or not by a bound of Hongya reserveoir. The spatial and time change of vegetation and hydrology and coupling relation with environment were studied by hydrological process, existence and lose of wholeness. The aspect of soil of riparian zone and relation with vegetation were measured and analyzed at line and cross direction. In addition, the function, change of river with desertification was studied. Then theory according and advice were given for ecological improvement of middle and lower of Shiyang River basin. The main conclusions as follows:(1) The width and abundance of vegetation of right bank is larger than that of left bank. The minimum width of riparianvegetation is about 30m and maximum width of 3000m. It is extremely serious that the riparian vegetation is effected by artificial activity, and then the types of riparian vegetation on the bank of main body of Shiyang River are artificial forest of Populus spp., Elaeagnus angustifolia, Tamarix spp+Salix spp.+Elaeagnus angustifolia, Tamarix spp+Artemisia areanaria and Nitraria tangutorum+ Elaeagnus angustifolia along with bank at different altitude. The gradient change is obvious at broadwise of typical riparian vegetation.(2) There are 14 species of woody plants and 22 herbs of riparian community at middle and lower reaches of Shiyang River basin. The dominant species with more frequency are Artemisia areanaria Nitraria spp., Tamarix spp., and Phragmites communis. The distribution of dominant population is aggregative. The vegetation is desert vegetation type group, and there are 5 vegetation types,6 second vegetation types,12 vegetation form groups,20 vegetation forms and 30 associations. The community change of riparian vegetation tends towards simple and retrogradation status. The dominant population of riparian is desert population.The largest area of plant coverage is 15%≤fg＜30%, and the relation between coverage and distance is exponential function.(3) The difference of average precipitation from middle to lower reaches is 60 mm, but it is not obvious that precipitation effects riparian vegetation. The difference of distance that river effects ground water is obvious at different river section. The affect distance to ground water by river is 5m at valley of Hongshui River with season runoff. And the width is 1000m that river effects ground water at flood plain of river part with rich water, while respond width of vegetation to the excudation of river water is larger than that of ground water table.There is much moisture of soil at 500m distance from river bed without water at lower reaches of Shiyang river basin. It will be known by the future of different vegetation located bank with water or not. With river escape, the evolution of vegetation is swamp to meadow and saline meadow; desert riparian forest change into bush to desert shrub and small shrub desert and annual vegetation.(4) The large area with aquatic mark and meadow remains were found at Shiyang River basin with the hydrological change. The direction of "Caiwan" is with the direction of Daxi River, and the "Caiwan" is the riparian forest that grown at bank of Daxi River.Acorrding to hydrological process of Shiyang river at recent year, the amount of the river at middle and lower reaches was high flow with globe warm; but the used water was increasing, the quantity of flow from up reaches was decreasing, it is not obvious that the riparian vegetation was effected by runoff.(5) The coverage and richness of riparian vegetation at middle and lower reaches of Shiyang river accord with hydrology and texture and chemical character of soil, but the maximum of coverage and richness is not corresponding with soil character. The coverage and richness of riparian vegetation is positive correlation with moisture and chemical character of soil.(6) The affect factor to riparian vegetation main is rate of flow and climate, but stream morphology, soil character and artificial disturbance promote the form of riparian vegetation. The riparian vegetation at middle and lower of Shiyang river basin is divided into 6 types:small shrub with season runoff at valley, saline grass meadow with spring water at valley, desert riparian tree with rich water at flood plain, shrub riparian vegetation with rich water at flood plain, shrub at channel without runoff, small shrub at channel without runoff.(7)The river bed and relic of river bed is located oasis border, and give a function without replacement. There are many ecological function of riparian vegetation, which hold out region diversity, provide species source and perch place, adjust micro-region climate, buffer function, protected bank, stabilized river bed, decreased harm of wind-sand and flood, protected environment of residence. The riparian is a landscape corridor and juncture. Riparian supply matter organic and increase system energy, in addition riparian is a good open place for entertainment.(8) At present, the riparian vegetation at middle and lower reaches of Shiyang River Basin is a degeneration station which is affected by natural factors of disease and insect, morphology, abnormity climate and so on. It is the dominant factor that the flux of runoff decrease, ground water table goes down continually and make up large density forest with extensive management to arise ecological problem. The riparian vegetation is protected and raised to defend region desertification. The desertification is decreased by the process to improve the riparian vegetation. The advice on revegetation and decreasing desertification is given by riparian vegetation character and station of desertification. The investment of education will be increased, established the monitor system, and the management of riparian vegetation should be enhanced. The water resource advantage of bank will be used to makeup riparian vegetation, and accelerate restoration of degenerate vegetation for establishment of protected system. The water will be transferred to supply lack of water, reasonably used the water resource by all of river basin. The land reclamation should be forbidden, and enhance the region improvement to contain region desertification.