Study On The Hydraulic Property Of Surface Runoff In Typical Forest Sites Of Jinyun Mountain, Chongqing City

Four kinds of land use types of bare field, conifer broad leaved mixed forest, evergreen broad-leafed forest and bamboo forest were selected and scouring experiments of five different flows of 0.12,0.18,0.24,0.30,0.36L/min were used to study the Litter water holding characteristics, soil infiltration rate, the hydraulic parameters and roughness coefficient of surface runoff under saturated storage in Jinyun Mountain, Chongqing City. The results showed that:(1) The order of litter water holding capacity in three different forest types was:evergreen broad-leafed forest, bamboo forest, conifer broad leaved mixed forest. At the first half hour of the experiment, litter water absorption capacity was relatively strong, and then gradually weakens. Under the same flows, the order of soil infiltration rate with four land use types were:conifer broad leaved mixed forest, evergreen broad-leafed forest, bamboo forest, bare field. The relationship between soil infiltration rate (Y) and time (x) with four kinds of different land use types was power function, and it was closed to the Kosjiakf formula model.(2) Darcy-Weisbach coefficient of different forest stands decreased with increasing flow rates; it is indicate that organic matter and roots on the ground can increase the roughness and surface resistance effectively. As the flow increased, the reinforcement effects of shallow root system against runoff would enhanced significantly. However, the amount of coefficient of woodland with deep roots gradually approached to the amount of bare field. The velocity of four kinds of land use types has positive correlation with flow rate. Litter and roots can slow down the runoff velocity, especially in bamboo forest while with higher flows. As the amount of flux increases, the Reynolds number increased and runoff characteristics transformed from laminar to turbulent; Froude’s number decreased to a minimum value and then increased, the minimum value was gained at the amount of flux between 0.18 and 0.30L/min. Under the same flow, the order of Reynolds and Froude’s number were:bare field, conifer broad leaved mixed forest, evergreen broad-leafed forest, bamboo forest.(3) Under the same flow, the surface roughness coefficient of conifer broad leaved mixed forest was the largest and bare field was the smallest. The surface roughness coefficient of bamboo forest was higher than that of evergreen broad-leafed forest while the flow rate was 5mL/s, in other cases, the surface roughness coefficient of bamboo forest was lower than that of evergreen broad-leafed forest. Under the same land use type, surface roughness coefficient increased with the increase of flow rate. The quantitative relationship between the surface roughness coefficient n (y) and the flow Q (x) was the closed to exponential relation, and it was y=0.004e0.788x, R2=0.985. At the same flows, the surface roughness coefficient and the Darcy-Weisbach coefficient of bare field was lower than that of the other three types of land use. It was showed the positive correlation relationship between surface roughness coefficient and Darcy-Weisbach coefficient.