Scale Effect On Soil And Water Loss In Small Watershed On The Hilly Loess Plateau

A basic requirement in the prediction of water loss and soil erosion should take watershed scale into consideration which is insufficiently understood. The data of water loss, soil erosion and environment factors in the hilly Loess Plateau region was gained by the approaches of historical data collection, sample plot investigation and location monitoring. On this condition, the paper illustrated fundamental characteristics, watershed scale effect as well as its influence factors of water loss and soil erosion at different temporal and spatial scale. The results could provide scientific basis for the assessments of water loss and soil erosion and transition between different scales. This paper proceeded as follows:1) In the Loess Hilly region, the runoff modulus which was unstable increased with the precipitation amount, while decreased with the implementation of protective measures. Meanwhile, the runoff modulus and sediment yield rate were both appeared to have a decline trend in amount and its range along with scale enlarged.2) It was shown that runoff modulus and erosion modulus take on a decline trend after the comprehensive soil and water conservation by analyzing the observed data in Zhifanggou and Yangou watershed ranging from 1997 to 2007 in the hilly Loess Plateau. Similarly, we found that the scale effect of runoff modulus and erosion modulus had a close correlation with the degree of comprehensive soil and water conservation. Compared with larger watershed, the temporal scale effect of runoff modulus and erosion modulus in small watershed were more obvious, due to the comprehensive result of rainfall characteristic and soil and water conservation.3) The study indicated that, with increment of watershed area, the response degree of watershed to the rainfall became smaller, the runoff modulus and erosion modulus of watershed had a decline trend and tend to be stable in different small watershed in Loess hilly plateau. However, the variation of runoff modulus and erosion modulus was comparatively larger when the watershed scale was small. The scale effect of runoff and erosion modulus could be fitted by the logarithm curve with the change of watershed area.4) The small watershed with 10 km2 and 50 km2 were the two turning points of the scale effect of runoff and erosion modulus. When the watershed area was less than 10 km2, the scale effect of runoff and erosion modulus were relatively larger, while the scale effects of runoff modulus decreased obviously when the watershed area was more than 50km2. The spatial scale effect of runoff modulus was larger than erosion modulus.5) Runoff sediment ratio showed a decline trend and tended to be stable with the increase of watershed scale. The duration of flood was longer with the increment of watershed scale, and the variation was larger when the area was small. The response of watershed to the soil and water loss influencing factors became weaker. When the area increassed, the watershed could weaken the degree of effecting factors of soil and water loss. The variation, representing the soil and water loss was small and to be stable.6) The spatial distribution of land cover showed the heterogeneity in different scale small watershed in Loess hilly plateau. The land coverage could be divided in to surface coverage, near-surface coverage and above-ground coverage. The influencing factors and degree of different land coverage was not the same. The road and residential zone erosion were unneglected element in the modern soil and water loss. There was difference about runoff and sediment yield in various courtyard types. Compared with brick-courtyard, the soil-courtyard was easier to flow, and both the runoff sediment ratio and the erosion modulus are larger. The maximum of sediment ratio of courtyard occurred in the initial stage during the runoff process.7) The soil and water loss in the small watershed in the hilly Loess Plateau was the comprehensive results of rainfall characteristic, land coverage, measurement of soil and water conservation, land use, geology, resident zone and road. The spatial heterogeneity of erosive rainfall, measurement of soil and water conservation, land coverage were the main influencing factors in the scale effect of soil and water loss in the small watershed.