The Hydrological Effects Of Forest/Vegetation And Simulation In The Small Watershed Of Honggou, Liupan Mountains

In order to evaluate the eco-hydrological effects of forest and vegetation in the arid areas of Northern China, a serial of experimentations focused on forest vegetation characteristics, hydrological process and runoff were performed during the growing seasons (from May to October) of 2006 and 2007 in the small watershed of Honggou, which is located at the south side of Liupanshan Mountains with a semi-humid climate. With these works it is expected to understand the hydrological mechanism of forest/vegetation more deeply and also to provide the theoretical and technological basis for the management of water-retention forest in the Liupanshan Mountains and similar regions.As a representative mountainous and headwater area in the Loess Plateau, several branches of the Yellow River are started here, such as Jing River, Qingshui River and Hulu River. Liupanshan Mountains is very important to maintain the regional ecological balance and provide water resource. Xiangshuihe watershed is a representative mountain forest small watershed, of which the area is 10.49km2 and its elevation ranges from 2040m to 2942m. With a coverage of 65.5%, the forest vegetation consists of many vegetation types. The soil is embedded many stone fragments. The annual mean precipitation is 591.6mm.The content of the dissertation is comprised of analysis upon the distribution characteristics of soil physical properties, the forest vegetation characteristics and special distribution, the main forest hydrological processes, runoff dynamics, and the application of hydrological model. The main results are shown as follows:1. Spatial patterns and characteristics of vegetationBased on sampling-line investigation upon typical vegetation , the following five main types of vegetation in Honggou watersherd are identified: Larix principis-rupprechtii, Pinus armandii, Betula albo-sinensis, Populus davidiana and shrub. Regarding species composition of the arbors and shrub layers, dominant species within arbor layer are Larix principis-rupprechtii, Pinus armandii, Betula albo-sinensis and Populus davidiana. The dominant species of shrub layer are Prunus virginiana, Rosa omeienis var. pteracantha, Berberis cirumserrata, Cotonedster acutifolius, Hippophae hamnoides and Fargesia nitida.The results about the spatial distribution law of vegetation showed that the range of vertical distribution of L. principis-rupprechtii was from the elevation of 2200m to 2500m; Pinus armandii is distributed from 2040m to 2942m, and the most suitable range is from 2100 to 2200m in shady slope and half-shady slope. B. albo-sinensis is distributed mainly within 2200～2300m in shady slope and half-shady slope. The shrub is distributed above the elevation of 2300m in sunny slope and half-shady slope, which reveals the relationship between vegetation pattern and human activity to some extent.2. Distribution characteristics of soil physical propertiesBased on field investigations, the variation of soil physical characteristics with soil depth, slope aspect, slope position, and vegetation types within Honggou watershed was studied. The result shows: The soil bulk density and stone content increase with increasing soil depth; while total porosity, capillary porosity, non-capillary porosity, saturated water content, capillary soil water content, and field capacity of soil decrease with increasing soil depth.All the soil physical properties measured in this study are relatively better on the shady slope than on the sunny slopes and all other half-sunny slopes. With lowering slope position, the soil thickness, bulk density, stone content, and non-capillary porosity are gradually increasing; while the total porosity, capillary porosity, and all water-holding capacity indicators gradually decreasing, with an exception that the water-holding capacity on the up-slope is higher than that on the slope top.By comparing the averaged soil physical properties among five main vegetation types, the soil bulk density showed the following order: shrub land (0.98) > the Populus davidiana (0.91) > Pinus armandii (0.89) > Larix principris-upprechtii (0.83) > Betula albo-sinensis (0.73). The stone content of soil in the P. davidiana (9.33) is the least among all vegetation types. The capillary porosity of the soil in different vegetations showed order as follows: P. Armandii (43.40%) > B. albo-sinensis (42.73%) > P. davidiana (40.88%) > L. principris-upprechtii (40.35%) >shrub(34.36%) ; non-capillary porosity showed the following order: L. principris-upprechtii (21.69%) > B. albo-sinensis (20.19%) > shrub(17.56%) > P. armandii(16.05%) >P. armandii(13.65%).3. The main forest hydrological process of watershedThis study discusses the relationship between canopy interception and the precipitation during one precipitation event in the Honggou watershed during the growing seasons from 2006 to 2007. The results show that the the canopy interception ratio to total precipitation is 100% when the precipitation is less than 1 mm during one precipitation event, and it declines to 15% when the precipitation is 10 ⁵0mm. But when the precipitation is over 50mm, the rate of canopy interception is basically lee than 15%. The canopy interception is more relative to the level of total rainfall, i. e. the higher the total rainfall is, the lower the interception ratio is, and the more the holding capacity of interception is.The paper studied the spatial variability and minimum sampling area of litter mass. The result shows that litter mass of two plots are not uniform in the spatial distribution, and the variation coefficient is 0.41 and 0.56 respectively. The litters mass and micro-landform of the two Larix principis-rupprechtii plots are related linearly, and the correlation coefficient is 0.444 and 0.677, respectively. The semivariograms of litter mass in two Larix principis-rupprechtii plots were best described by spherical model, and spatial correlation is moderate. Statistical analysis showed that 9 is the least adequate sampling area (at least 9m2) to denote the average total litter mass of Larix principis-rupprechtii plot in liupan mountain.The rainfall interception processes were studied under artificial simulated rainfall within the normal rainfall intensity range of this region (< 30 mm/h). The results showed that the interception capacity of litter increases with rainfall intensity. A mechanism-based model for describing the rainfall interception process of litter of the two species was developed:The model can reflect the effects of water content, rainfall intensity,and duration of rainfall simultaneously. The model parameters were calibrated based on the observed data both from the soaking experiment and simulated rainfall interception experiments.The diurnal variation of stem sap flow of Larix principis-rupprechtii on clear days in the whole growing season can be described as diurnal alternating fluctuation. The activity of stem sap flow can be divided into three phases:"relative rest stage"(20:00～next day07:00)with sap flow velocity less than 0.05μl·cm^-2·min^-1 , and"active phase"(07:00～20:30)with sap flow velocity less than 0.25μl·cm^-2·min^-1.The soil evaporation of internal forest was similar to that of external forest. The maximum evaporation rate of soil outside and inside of forest is 0.697mm/hand 0.505 mm/h respectively at 12 :00～14 :00. The environmental factors influencing soil evaporation include solar radiation, temperature, relative humidity, air temperature and wind speed and its equation follows: E=-0.468+0.205SW+0.01557SR-0.0137RH,R²=0.8732.According to observed data of soil infiltration under different vegetation types in Honggou watershed, the initial velocity of infiltration range from 8.23mm/min to 62.96mm/min and the velocity of stable infiltration range from 7.55mm/min to 40.91mm/min. The velocity of infiltration increased with the time passing, and trend to be a constant finally. The soil infiltration rates of typical plots of vegetation are simulated by Kostiakov model, Horton model, Phillip model and Fangzhengsan model and the results show that the Horton model, of which the parameters have clearly physical meaning, is a relatively acceptable model for soil infiltration.4. The analysis of watershed runoffInfluenced by temperature and precipitation,the runoff varies between seasons.The runoff volume is concentrated in the summer (September, August). The runoff is mostly consist of basic flow from November to June the next year. The Minimum Smoothing Method was used to separate the runoff component of Honggou watershed during growing seasons of 2006 and 2007, and the calculated BFI is 0.039. The investigation about the relation between runoff(m3) and rainfall(mm) indicates that they having a significant function relationship(Y=145.11X^0.5977,R²=0.6499。).5. The effect of vegetation variation on water balance of watershedIn order to understand the mechanism of the interaction between forest/vegetation and hydrological processes, and to quantify the effect of the heterogeneity of physical conditions on the hydrological processes and water balance in watershed,as well as to evaluate the hydrological response to vegetation transition, especially the watershed function of water yield, the hydrological model SWIM was used in Honggou small Watershed of Liupan Mountains in this paper. Calibration and validation of SWIM model was done using the meteorological and hydrological data observeded at the outlet of the watershed during 2006 and 2007. Based on the simulation of the hydrological processes, the water balance on slope as well as the contribution to watershed runoff generation of different watershed parts was analyzed. Seven scenarios with different forest coverage of Larix principis-rupprechtii were established. Then the responses of hydrological processes of the watershed for these vegetation scenarios were simulated, and the effect of the vegetation transition on water balance of watershed, especially on runoff generation, were calculated.