Study Of Mechanism And Process Of Water Transmission On Water Resource Conservation Forests Ecosystem In Qilian Mountains

The study takes the water resource conservation forest in Qilian Mountains as the representative forest type in the northwestern arid and half arid area of China, and makes full use of the historical data of forest, which is based on long-term research and given by the Qilian forest ecological station. Combined with the experimental observation, in the size of the catchments, studying the water process and mechanism and discussing the theory and practice problem to resolve the relationship between forest and water from lots of aspects, such as precipitation, water transmission in canopy layer, water transmission in fog and litter layer, water dynamic in forest soil layer, rule and composing of the river runoff, the hydrological characteristic of frozen soil, etc. Then, we obtain the following achievements:1. In an experimental catchments, the precipitation has the spatial difference, on the shadow slope it is about 7% more than that on the sunny slope, and the precipitation increases by degrees from the lower altitude to the higher. In Pailugou experimental catchments, the hectometer average increasing rate of precipitation is 4.95%, while that of Tianlaochi catchments is 4.88%. The precipitation period could be divided into rainy and dry season due to the variation. The precipitation in the rainy season (from May to Oct.) approximately accounts for 87.2%; while that in the dry season (from Nov. to Apr.) is few, which only accounts for 12.8%. There are two forms of precipitation:snowfall and rainfall, the snowfall accounts for 27.8% and the rainfall is 72.2%. Snow is the unique'solid reservoir'of forest in Qilian Mountains; it plays good role to the drought in spring that occurred in the middle and lower reaches of rivers originated from the Qilian Mountains. The temporal-spatial variation and landform diversity are directly embodied in the variation of catchments runoff. In the Pailugou experimental catchments, the runoff in the dry period approximately accounts for 15.68% of the whole, while that in the wet period it is 84.32%. The result is in line with the rule of precipitation distribution.2. In the experimental catchments, the interception rate of Picea crassifolia forest is 26.6%-39.8%, the Cypress forest's is 23.0%～37.1%. And there are interceptive differences between the rainfall and the snowfall; the latter is 2 times as more as the former. The bigger forest canopy density is, the higher interception rate is. To the canopy density, when is up from 0.6 to 0.7, the interception rate will increases by near 9%; while it will increase by 3% when is up from 0.7 to 0.8. Most intercepted precipitation vaporizes except 0.3% of it enters into the forest in the form of stem flow. When the canopy interception alters the proportion of inside forest precipitation, it still accommodates the nutrition composing and its proportion.3. The average maximum water capacity of the moss and litter in Picea crassifolia forest is 36.4 mm, varies among 7.6～59.1 mm, and increases along with the increasing of stock of moss and litter layer. The water transmission in the vegetation coverage generally has three phases:interception, saturation by sopping up (non-saturation infiltration) and infiltration (saturation infiltration). In the interception phase, the through precipitation is entirely intercepted; in the non-saturation infiltration phase, there is water seeping out from moss and litter layer, but the layer doesn't reach the maximum water capacity, the infiltration is less than the precipitation; in the saturation infiltration phase, the filtration intensity is larger than rainfall's. Most rainfall couldn't make the fog and litter layer reach the saturation because of its strong water capacity; thus, there are commonly only two phases.4. In the experimental catchments, the evapotranspiration of the earth's surface varies with the seasons, and the forest evapotranspiration accounts is 80.46% of the whole in the growing period, and it is very small for only 19.54% in the dormancy period. The grassland evapotranspiration is 77.13% in the growing period, and 22.87% in the dormancy period. The soil water in the experimental catchments has the obvious spatial and temporal difference, the shadow slope contains more water than in the sunny slope; the containing water of the soil thaw layer represents the trend of decreasing with the increasing of soil depth. Compared with the soil that contains less water, the vertical variation of the more one is small. The seasonal changes of soil water could be divided into four phases:former storage, the losing, latter storage and stable. The influencing factors of soil water seasonal changes are quite complex, in which the precipitation and plants are two most important ones; the soil water containing amount from the surface to the deep increases with the rising altitude, the hectometer average increasing rate is 14.15%, the water containing amount of frozen soil layer is larger than movement layer. The infiltration, sluice and flow producing are main processes. To different soil types, their infiltration speeds are in great differences. The forest soil in Shidalong experimental zone is loose, with strong infiltration function. To the fog and Picea crassifolia forest soil A00, its initial infiltration rate is 214.4 mm/min, and the stable infiltration rate is 162.0 mm/min, those are larger than other soil surface's, for example, the stable infiltration rate of the Leymus Chinensis grassland is just 0.43% of the Picea crassifolia's, and the subsoil's is only 30%.5. There is more seasonal frozen earth, and some places are distributed with perennial frozen earth consecutive or in the shape of island in the water resource conservation forest areas in Qilian Mountains. The seasonal frozen earth begins freezing on Oct.20th in every year, and reaches the deepest on May 20th in the following year. In the lower altitude, it finishes the thawing on the next Aug.20th, but in the forest, it just wholly melts from Mar.10th to Oct.22nd. The annual changing rule of the frozen earth is generally divided into four periods. The initial and terminal times are different due to the different altitude, and the freezing and thawing alternate. The alteration between freezing and thawing of the seasonal frozen soil could enhance the runoff in late spring and early summer, resort the summer precipitation to autumn, increase the catchments sluice, influence the forming and distribution of river runoff, and accommodate the forming and stability of water resource. The influence made by the frozen soil on water distribution and transmission process, leads to form the runoff peak in the snow-melting runoff period, and its mechanism is the soil freezes lots amounts of water in the form of ice, then the water releases with melting, together with the role of snow, form that peak.6. In Pailuogou experimental area, the annual runoff is about 159.9mm,and the annual precipitation is 471.5 mm.The experimental catchments is not an even runoff producing area, on above the altitude 2700 m of where the precipitation is lager than evapotranspiration and interceptive evaporation, no matter it's on the sunny or shadow slope. It's provided with basic condition of producing runoff, and the potential runoff increases with the rising of altitude. In the place of same altitude in catchments, existing some different vegetation types. In the experimental area, the area in altitude above 2700m owns the water condition to form the runoff. The runoff is more with the area more in high altitude. The river runoff dynamical variation in the experimental catchmentss is the same, the minimum runoff is-during Jan. to Feb., and the maximum runoff is Jun. to Aug. The runoff is quite stable during Nov. to Mar., while it varies a lot during Apr. to May, when the flood peak occurs; but the biggest varying breadth appears during Jun. to Sep., when there're several flood peaks, and the runoff is quite big. There are three periods:the underground runoff period, the thawing water runoff period, and the rainfall runoff period in a hydrological annual runoff process by taking the month as the time unit. The three experimental catchmentss have quite different river runoff composing, and the rivers originated from the high altitude forest in Qilian Mountains are all the mixed supplying type.