Feature Of Runoff And Sediment And Nutrient Losses In Some Forest (Bamboo)+Grass Compound Patterns In Western Hills Of Sichuan Province

Based on the observation in runoff plots in the western hills of Sichuan Province in 2003 and 2004, the characteristics of vegetation growth and management, soil physical properties, and capability of above-ground water holding of four forest (bamboo) +grass compound patterns and slope farmland were compared and analyzed. More importantly, features of surface runoff and sediment yield, nutrient losses and factors which influenced these features were discussed. The results indicated that:1 Forest (bamboo)+grass compound mode was different from ecological forest and high-intensity disturbed slope farmland as well. Through planting trees or bamboos, growing forage grass under them and using grass to breed domestic animals, this mode produced good ecological benefits and meanwhile brought some economic effects into play. Although the disturbed degree of this mode was less than that of farmland, it would also be disturbed by people in a long run and therefore would have its own traits in producing ecological benefits.2 Compared with slope farmland, because of their own characteristics and different management, soil structure, porosity, water retention property and soil physical and chemical properties of these forest (bamboo) +grass compound patterns showed obvious difference. The capacity of soil water-holding in Arundinaria amara+Hemarthria compressa pattern was the largest, while Hemarthria compressa was the smallest.3 With vegetation growing and more litter layer retained in the forest land, the whole above-ground biomass would increase and the capability of above-ground water holding would correspondingly upgrade, although the biomass of forage grass decreased. The enhancement of water-holding capability would be beneficial to lessen surface runoff yield.4 Different patterns had different annual surface runoff and sediment yield. The feature of surface runoff and sediment yield was largely affected by the vegetation types and the means of land utilization. Soil and water losses could be effectively controlled by good vegetation restoration or increasing plant cover rate. In the early of the engineering of converting lands into forests, on account of its highest canopy density, most litter layer,least forage grass and least human-caused disturbance correspondingly, it was the pattern Arundinaria amara+Hemarthria compressa that had the least surface runoff and sediment yield. And at the same time, this pattern had a marked function of water conservation on the condition that rainfall was heavy or centralized. While for Betula luminifera+Hemarthria compressa pattern and Hemarthria compressa pattern, their ability for water conversation was even lower than that of slope farmland because of improper management. With soil less disturbed, annual soil erosion of these compound patters after converted into forest or grass was lower than that of slope farmland. 5 The peak of runoff yield and sediment yield was in accordance with the peak of rainfall. Monthly dynamics of surface runoff was mainly restricted by seasonal distribution of rainfall, and the surface runoff of various vegetations was mainly dependent on the comprehensive effects of crown, litter layer, soil layer and management. Because intense and big rainfall was mainly centralized in July and August in the western hills of Sichuan province, maintaining litter layer or ground cover in these two months would contribute to lessening surface runoff and sediment yield in this region. When the field was covered with pure forage grass or the canopy density was low, it was vital to pay attention to the time of cutting grass, which should be modified not to accord with the time of intense rainfall. But because rainfall was uncontrollable and forage grass needed to be harvested, to lessen soil and water losses, the measures of cutting grass should be altered. For example, cutting grass left with some height and letting fields inlaid with grass would be functional on soil and water conservation.6 In 2003, there was no obvious relationship in these patterns between surface runoff and precipitation in a single event. The causes resulted from two aspects. On the one hand, the increasing growth of vegetation and management caused the established systems instable. And on the other hand, rainfall intensity and content of soil water influenced this relationship. While in 2004, between surface runoff (R) and precipitation (P) in a single event, there was a regressive relation equation, R=aP2+bP+c. The relationship between runoff and sediment yield might be expressed better with linear function. Therefore, the measures of reducing surface runoff, such as increasing plant cover rate or retaining more litter layer, would distinctly cut down sediment yield.7 In lost nutrients of these forest (bamboo) +grass compound patterns and slope farmland, it was shown that TK>TN>TP. Due to the least surface runoff and sediment yield, it was still the pattern Arundinaria amara+Hemarthria compressa that had the least amount of nutrient loss. Monthly changes of nutrient losses in surface runoff were in accordance with monthly changes of surface runoff. When runoff yield was little, the average content of nutrient in surface runoff was high. When there was much runoff yield, the average content of nutrient in surface runoff was low. The mechanism of this phenomenon needed to be studied.Further study and perfection about the ecological management techniques of forest (bamboo) +grass compound patterns would be beneficial to enhance the capability of water conservation of these patterns, and it would be of great significance to instruct the execution on the engineering of converting lands into forests and vegetation management.