Study On Hydrological Processes Of Chinese Fir Plantations In Huitong At Different Age Classes

The study takes the representative forest type in subtropical area of China, the Chinese fir plantations, as research object. To discuss the theoretical and practical problems and to resolve the relationship between forest and water from lots of aspects, combined with the experimental observation in the 8 small catchments, based on the data collected for 25-years successive observation at Huitong National Key Field Scientific Experiment and. Research Station, the coupling rules of eco-hydrological processes in Chinese Fir Plantations at different age classes were investigated, such as precipitation and water transmission in canopy layer, interception, evapotranspiration, runoff changes, and water balance law. We obtained the following achievements:1. The annual rainfall was 1287.2 mm in the study area from 1982 to 2006. The times of rainfall was evenly distributed, and the amount of rainfall was mainly concentrated from April to August every year and the rain intensity was mainly belongs to small strength. There was a little more rainfall upon the canopy than the outside stand about 200 m away. And there were different rainfalls in different places of the same catchment. The relationship models were built up among rainfall in the foothill, slope and outside the stand, and their accuracy were all above 95%. The maximum aridity index (AI) appeared in January, and the minimum AⅠappeared at May and June. Those kinds of rainfall and AI characteristics were valuable to the soil infiltration and were advantageous to the exploitation to humidity of the stand, and were promoted to the growth of the Chinese fir plantation.2. Along with the age increasing of stand, the canopy interception continuously enhancing their ability. During the age classⅠ,Ⅲ,Ⅳ,Ⅴ, the average canopy interception rates were 25.7%,28.6%,30.1% and 32.9%, respectively. The canopy interception rate decreased rapidly with the increasing intensity of precipitation. For example, when the daily precipitation were less than 0.5 mm·d-1, the canopy interception rates were more than 98%, and when the daily precipitation were 0.5 to 1.0 mm·d-1, the canopy interception rates were 90% above; but for the showers of rain (daily precipitation were 60 mm·d-1 above), the canopy interception rate reduced to 10% or so. The amount of interception increased with the amount of precipitation increased, but the amount of interception increased in a limit of about 7-12 mm in a one time precipitation. Monthly change trends of the canopy interception were higher monthly precipitation with higher canopy interception, but with less canopy interception rate. In the winter, the average canopy interception rate was more than 30.0%, while from May to August the average canopy interception rate was less than 28%. Canopy interception was also closely related to the growth of the forest:the higher growth of the forest, the higher ability of the interception. In the same small catchment, the ability of the canopy interception order from higher to lower was:valley, downhill, and hillside, the same law to the productivity of the stand. The results of stepwise regression analysis indicated that the dominant factors, which impact to the canopy interception of the one-time precipitation were the amount of rainfall, stand age, precipitation intensity, the latest rainfall factor, the time to the latest rainfall. The established three polynomial regression model, exponential function equations could be used to forecast the amount interception in one-time precipitation in the different age class. Generally, Chinese fir plantation could not effectively decrease the rainfall kinetic energy only in minimal rainfall when most of the rainfall was intercepted by the canopy and in the greatest rainfall when large diameter rainfall was separated by impinging against the branches.3. The surface runoff in the stand at the age from 1 to 5 year (age classⅠ) was the smallest among all age classes and the annual average surface runoff coefficient accounted for 0.0071, due to tending treatment (e.g. site preparation and weeding). However, the surface runoff increased with age class increase after tending treatments and reached the maximum value at the stand age from 10 to 15 year (age classⅢ), with annual average surface runoff coefficient of 0.0220. A decline in surface runoff was found at the stand age from 15 to 20 year (age classⅣ). The annual average surface runoff coefficient was 0.0093 and approximated to 50% of the stand at the age classⅢ. The underground runoff gradually decreased with stand age class increase. The highest underground runoff coefficient of 0.3012 occurred at the age classⅠ, which was about 2 times of that in the mature stand before clear cutting (0.1577). The underground runoff coefficient at stand age class V declined to 0.1577. The decrease of annual average underground runoff coefficient as stand growth was attributed to increase in interception and transpiration because of canopy development, and improvements in soil structure. The surface runoff only accounted for 2.3% to 8.7% of the total runoff and underground runoff is the dominated pathway of watershed. The monthly runoff of all the age classes was proportional to the mean monthly rainfall. The rainfall during the period from April to August was 62.2% of the total annual rainfall and the runoff was amounted to 71.7% of the annual runoff. The average runoff coefficients from April to August were estimated to be 0.3566,0.3278,0.2907,0.2123 and 0.1751 for the stand age classⅠ,Ⅱ,Ⅲ,ⅣandⅤ, respectively. The result of stepwise regression indicated that the monthly precipitation and stand age of the stand influenced the monthly runoff. The established multinomial regression equations could be used to forecast the monthly runoff.4. Soil moisture change trends in the stand of Chinese fir plantation can be divided into three stages:stage of water storing, stage of water losing, and stable stage of water storing. The changing trends of the soil moisture were the elder of the stand the higher of the soil moisture. Vertical stratification dynamics of soil moisture showed the laws that the soil moisture content (SMC) was lower in surface soil (0-15 cm) and deep soil (60-90 cm), and the SMC was higher in the middle soil (15-60 cm), which were helpful to the roots to absorb and utilize the soil water. The result of stepwise regression indicated that the SMC was influenced by the stand age, effective rainfall factors, humidity, continuous drought factors, precipitation and atmospheric temperature. The SMC could be predicted accurately by using multiple linear regression equations.5. The evapotranspiration increased with age class increase and reached the maximum value at the stand age from 20 to 25 year (age class V), with annual average evapotranspiration coefficient of 0.841. The amount of evapotranspiration from June to September is much bigger in every year, and the amount of evapotranspiration was above 51.4% of a whole year. In the same catchment, evapotranspiration capacity of upper canopy was the highest and evapotranspiration capacity of lower canopy was the least. In different catchments, the evapotranspiration capacity was higher in the stand of better growth conditions.6. By selecting DBH and height of single tree as units, and using the multi-model optimization, the forecasting models of the amount of water storage in the 5 components (trunks, barks, branches, leaves and roots of a single Chinese Fir tree) had been established, which improved the predictive accuracy of the amount of water storage in a single Chinese Fir tree. The dualistic amount of water storage tables of the 5 components had been made, which can be used directly in the study of coupling laws of hydrological process and growth of Chinese fir plantations.7. By means of water balance method, water balance tables of Chinese fir plantation ecosystem at different age class were established. The results in those tables indicated that the rainfall was the input water items of the small catchment with annual average rainfall of 1295.4 mm (the 3rd catchment, from 1984 to 2006), and the evapotranspiration and runoff were the main output water items with annual average output water of 1294.7 mm, about 99.95% of the rainfall, and about 0.05% of the water were stored in the small catchments in the form of solid organic fixed in organisms.