Effects Of Forest Ecosystem On Runoff And Water Quality In Medium-Altitude Mountainous Region Of Southern Slopes, Qinling Mountains

Water resources are not only shortage but also unevenly distributed in China. The hungriest region for water resources is Huabei region. The population in the region is 34.8% of the total in the country, and GDP is 32.3% of the total in China. So the social and economic development in the region is very important in China's sustainable development, but the problem of water resources shortage, the vital problem that has been preventing the sound development, in the region has not been solved completely now. In order to solve the problem in foot, Chinese central government made the decision of carrying out South-to-North Water Transfer Project. The middle line of the project will take water from Danjiangkou Reservoir in Hubei Province to supply metropolises, including Beijing and Tianjing, and other 17 big cities and 118 counties mainly for domestic and industrial use. The annual inflow of the reservoir is about 39820,000,000m~3, of which 77.92% comes from Hanjiang River, Shaanxi Province. The catchments area to the dam site is 80,000 km~2. 77.92% in Qinling and Bashan mountainous region. The region covers 54,800 km~2. The southern slopes of Qinling Mountain accounts for 55% more or less of it. Medium-altitude mountainous area is 50% more or less of the southern slopes and most of the area covers with forest. To study the effects of forest ecosystem on runoff and water quality in medium-altitude mountainous region of the southern slopes, Qinling Mountain, not only is of very important theoretical value but practical value in water supply catchments management.In the study, firstly, analyzing the precipitation, typical storm hydrograph and the mechanism of runoff producing and flow concentration in Huoditang forest catchments, which in medium-altitude mountainous region of southern slopes, Qinling Mountain, the characteristics of rainfall and discharge and the dominant factors that affect hydrograph in the research region, medium-altitude mountainous region of southern slopes, Qinling Mountain, were discovered. Secondly, analyzing the changes of rainfall water quality, stream water quality and the catchments discharge water quality monthly, the features of their changes with time were summarized in the research region, as well the dominant factors that affect water quality. Thirdly, investigating the relationship between water quality and its quantity, the ability of stabilizing water quality that forest ecosystem has was reproved. Fourthly, analyzingthe change of stream and catchments discharge water quality with months, the trend of water quality change was learned in the research region. Finally, according to the temporal and spacial orders of interaction between water and forest ecosystem, the ecosystem was divided into several layers and every layer water quality along rainfall-runoff process was studied by comparing, contrasting and mechanism analyzing. The dominant factors and critical parts that affect water quality in the forest ecosystem were advanced. The mainly conclusions in the study were as follows:1. Through statistical analysis of annual precipitation, rainfall distribution in a year, and the number of different intensity rainfalls occurred in different months, the characteristics of rainfall in research area were summarized. There are more storms and the precipitation is higher in summer than any other seasons;the number of storms that occurred in autumn is in second position and the precipitation is a little bit lower, but with more rainy days, the flow rate is relative high and stable;the annual precipitation changes greatly in different years. The characteristics can be used for reference for the storage operation management of Middle Line of South-to-North Water Transfer Project.2. According to forest soil layers formed naturally and their permeability, the subsurface flow was divided into two types occurred in horizon A and horizon B respectively first time. That forest soil is a principal factor that affects discharge in forest ecosystem was advanced. When precipitation and rainfall intensity are all high, subsurface flow may occur in soil horizon A and B respectively. Subsurface flow in horizon A moves faster than that in horizon B. When precipitation is high and the rainfall intensity is low, subsurface flow just may occur in horizon B. The condition that subsurface flow occurs in horizon A is that horizon B is saturated or rainfall intensity is greater than its percolation capacity. When heavy rainfall occurs, there will be two peaks on the hydrograph of the outlet of a drainage basin. The first one mainly caused by subsurface flow occurred in horizon A, the second one is mainly caused by subsurface flow occurred in horizon B. The former one has the characteristics of rising and falling all quickly, and the magnitude of the peak discharge is related to the antecedent soil storage, rainfall intensity and high-intensity rainfall duration. The later one rises and falls all slowly. Cathment storage is great in the later August and early September. Therefore, in this period, forest soil has a less attenuation effect. It is easier for a storm to cause a flood disaster.3. According to pH value, especially the changes of toxic elements or chemicals in rainfall with months, the water quality of rainfall is the best in autumn, worst in winter during the period from July to next January in the research region. Rainfall pH changing with months is mainly affected by precipitation and exhaust gas. The pH of rainfall is the lowest in August. Acid rainfall has occurred in some of the research region. The concentration of NH/ isconsistent with pH. PO43", K are the highest in September, but change a little in others months. Na, Ca, Mg are the highest in January. They are in the second position in autumn, lowest in November and December. Fe is lowest in autumn, highest in winter. Zn is contrary to that of Fe. Toxic elements Cd and Pb, affected by exhaust gas and precipitation, all are lowest in autumn, highest in November and December. Mn has the same changing trend as Cd and Pb.4. In medium-altitude mountainous region of southern slopes, Qinling Mountain, the water quality of forest stream and drainage basin outlet discharge change with months are same fundamentally. They are all qualified with Sanitary Standard for Drinking Water, but better in autumn, worse in spring and summer relatively. Discharge water alkalinity degree is higher in winter and spring, lower in autumn. Generally, the water is alkaline. Discharge water pH is correlated with exhaust gas, precipitation, temperature, soil and its parent rocks and litter decomposition. NO3" is higher in spring and early summer. That it is higher in spring is related with snow-melting water dissolving NCV from litter, and higher in early summer is related with last year's litter decomposition. Forest soil can absorb NO3" in water. NH/ is higher in winter, spring and June and September. PC>43' has two crests, which are in summer and winter respectively. K is lowest in January, highest in May, and other peaks in September and October respectively. Ca is the highest element in water. It is higher in June and October and consistent with the time of litter decomposition. Mg is higher in spring and summer. Fe and Zn are higher in spring, but less in autumn. Cd and Pb are less in autumn either. Mn is lowest in autumn.5. There is no notable relationship between discharge rate and its water quality. Correlation analysis proved that just PO43" and Mn correlated well with discharge rate in ecosystem. Other chemicals did not correlate with it. Mn and PO43" are very low in water. So it is impossible for them to have any impacts on water quality. Generally, water quality does not change greatly with the change of discharge rate. This further shows that forest ecosystem has a good function of stabilizing water quality.6. In medium-altitude mountainous forest region of southern slopes, Qinling Mountain, point source pollution is an important factor that could make water quality becoming worse. The increasing Pb in air could cause the increase of Pb in runoff in the region. NO3* and NH4+ in water are rising year by year in some area of the region. NO3" in discharge water is inversely correlated with the distance between the center of the watershed and the pollution source point. The falling of K, PO43' and Na year by year is related to the revegetation. The CO2 in air has a little impact on the rising of Ca and Mg in the water. But Ca and Mg are rising along routing. Cd and Pb are rising with years gradually. Cd is affected by local exhaust gas. Using gasoline without Pb has achieved a great success in controlling pollution. Butgenerally, controlling air Pb pollution that came from other regions was not that good. Mn is decline year by year shows that the function of decreasing Mn by forest ecosystem is enlarged. Fe has a rising trend, but Zn has a falling trend. pH is falling year by year.7. The water pH is well inversely correlated with NO3" concentration. NCV can be used for an indicator to indicate the degree of alkalinity of water discharged from the forest catchments. Recently years, the gradually increasing exhaust gas has had a notable impact on stream and discharge water pH although the forest cover is pretty good in experimental area. This shows that the function of forest ecosystem that has the ability to regulate water pH is not as good as imagined in the research region. Therefore, the most important way to control water acidification in forest water supply catchments of the research region should be controlling air pollution.8. Forest ecosystem has a function of regulating water pH. Canopy, litter, soil and rocks all have this kind of function, but canopy and soil do better. Also, forest ecosystem can reduce NO3", NIi?+, K and PO43" in water. It reduces NO{ and NR?+ by outputting them, and cuts down K and PO43' by absorbing them. The principle phase to reduce NCV in water is process of routing. It is forest soil that plays an important role to cut down NH4+, K and PO43' in water. From rainfall to the outlet discharge, Na concentration does not change much. Mg increases and Ca increases greatly. Canopy, litter, soil and rocks play an important role to increase Ca in water, but Ca mainly comes from soil and rocks. Canopy, litter, and rocks all can raise Mg in water, but most of it is from rocks. Forest soil can absorb Mg. Cd, Pb, Mn and Zn are mainly from rainfall input. Forest ecosystem has a good function of cleaning them up, in which canopy plays the most positive role. Fe is mainly from rocks and their debris in river. Fe increased by forest ecosystem is interest in improving water quality.There is no any relationship between pH of subsurface flow and the location the flow produced on slopes. From top part of slopes to the down, Zn has a decline trend, but NH4"1" and PO43* have not any regular change.