| Soil and water loss from sloped lands in the Loess Plateau is a serious issue in China.Slope type and slope gradient are two important factors on effecting rainfall infiltration,surface runoff,runoff hydraulic characteristics and soil erosion on slopes.Composite slope is one of the main slope types in nature.Therefore,deep understanding of soil erosion mechanisms on different slope types and slope gradients are an urgent demand to protect water and soil resources.In view of the insufficiency research on runoff hydraulic characteristics,soil erosion processes,and nutrient loss on composite slopes,the rainfall infiltration,runoff,runoff hydraulic characteristics,soil erosion,and nutrient migration processes on loessial composite slopes were carried out by soil flume and rainfall simuluation experiments in laboratory.The effects of polyacrylamide?PAM?application on surface runoff,soil erosion and nutrient loss also were tested.The main results are as follows:?1?Runoff velocity on uniform linear slope increased with flow path and progressively tended to be steady,and it increased with the increased slope gradient or runoff rate.Runoff depth on uniform linear slope decreased with flow path and tended to be steady,and it increased with the runoff rate or decreased with slope gradient.The runoff velocity on downslope segment of convex slope was greater than that on the upslope segment,while the runoff velocity on downslope of concave slope was smaller than that on the upslope segment.The runoff depth on downslope segment of convex slope was smaller than that on the upslope segment,but an opposite tendency existed for the concave slope.The Reynolds number?Re?of runoff almost was a constant along with flow path,and it increased significantly with the increase of runoff rate but was less affected by slope gradient and slope types.The Reynolds number?Fr?on concave slope was a little less than that on convex slope.Runoff belonged to laminar flow when runoff rate was smaller than 25 L/?min · m?,otherwise it was turbulent flow.The Froude number of runoff increased with flow path,and it increased significantly with the increase of slope gradient or runoff rate.Froude number on the downslope segment of convex slope was greater than that on the upslope segment,and the runoff on upslope segment of convex slope was subcritical flow while that on its downslope segment was torrent flow.The Froude number on the downslope of concave slope was smaller than that on the upslope segment,and the runoff on concave slope usually belonged to torrent flow status for most experimental treatments.?2?The time of runoff initiation and runoff volume on composite slope with a big upslope gradient??15°?was shortened as compared with uniform linear slope.The runoff coefficient on convex slope was increased with the increase of slope gradient,and it was increased persistently with time during rainfall process when slope gradient was greater than 10 degree.The runoff coefficient on concave slopes increased progressively with the rainfall process and then tended to be steady.The greater the gradient difference between upslope and downslope was,the bigger the runoff coefficient on concave slope was.The sediment concentration in runoff on convex slope at a big upslope gradient??10°?increased sharply at some time during rainfall process because of rill erosion occurrence,and the soil loss rate of other treatments,besides the convex slope with a big upslope gradient,was decreased with rainfall process and processively tended to be steady.?3?The NO3-N was the main form of nitrogen loss in runoff.The concentration of NH4+-N and NO3--N in runoff was to be steady processively with time during rainfall process.The total NO3--N loss in runoff was far more than that in sediment.The concentration of NO3--N in sediment was decreased and then increased with time during rainfall process,but an oppsite tendency existed for PO43--P in sediment.The slope type was significantly affected and changed the rule of the concentration of NO3--N in sediment during the rainfall time.The effect of slope type on PO43--P in sediment was not obvious under experimental conditions.?4?Polyacrylamide?PAM?application significantly shortened the time of runoff initiation averagely by 63.7%-71.6%.The effects of slope type and slope gradient on the time of runoff initiation were not obvious.Polyacrylamide application increased runoff coefficient as compared with control treatment.Runoff coefficient increased with the increase of PAM application rate.Under experimental conditions,when PAM application rates were 0.5,1.0,and 2.0 g/m2,total runoff volume increased by 16.6%,69.3%,and 108.9%on convex slopes and 55.7%,100.2%,and 147.3%on concave slopes compared with control treatment,respectively.The total runoff volume on convex slopes was generally greater than that on concave slopes by 14.7%-31.8%.?5?Polyacrylamide application significantly increased sediment concentration in runoff and soil mass loss from concave slopes or the convex slope with the small upslope gradient of 5°?P<0.05?.Polyacrylamide application played a remarkable control role on rill erosion occurrence and no obvious rill erosion occurred after its application.Consequently,PAM application resulted in the decrease of soil mass loss from convex slope when its gradient was greater than 10°.The effects of PAM application rate on soil mass loss of convex slope was not obvious,but the soil mass loss on concave slope was increased with the increase of PAM application rate.After PAM application,slope gradient did not obviously affect soil mass loss for convex slope and concave slope.When PAM application rates were 0.5,1.0 and 2.0 g/m2,the averaged soil mass loss was 1.34±0.30,1.89±0.77,and 1.53±0.57 kg/m2 for convex slopes and 0.91±0.38,1.52±0.59,and 1.56±0.49 kg/m2 for concave slopes,respectively,and the former was about 20%greater than the later after PAM application for all experimental treatments.This study showed that the runoff hydraulic characteristics,rainfall runoff,soil erosion and nutrient loss on loessial composite slopes were significantly affected by slope type and slope gradient.The results presented in this dissertation can serve as an important background for the future studies in related fields,and also provides scientific theorety and practical guidance on deep understanging of soil erosion under natural slope conditions and effectively prevention of agricultural non-point source pollution for loess area. |
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