| Soil water is the crucial factor connecting climate change and vegetation dynamics as well as a key factor impacting vegetation restoration. Vegetation, as one factor of underlying surface, influences runoff significantly. Adequate vegetation covereage can alleviate direct wash to soil surface caused by rain, and benefit soil and water conservation. It is important to study of soil water and runoff practically and theoretically. Therefore, the study of soil water change as well as understanding of erosion rule can provide science references for vegetation restoration and control of soil erosion in the Loess Plateau. Wangdonggou watershed was chosen as the experiment site of this study. We investigated soil water dynamics and characteristics of slope runoff.The results showed as follows:(1) In Beishan, the variability of soil moisture in 0~30 cm layer was moderate. Soil moisture of sunny slopes was lower than that of semi-sunny slopes. The change of soil moisture was agreed with the change of rainfall: in the 0~10 cm layer, it has a good synchronization with rainfall, but in the 10~20 cm and 20~30 cm layers, the synchronization between them was poor. It was observed that the higher vegetation coverage, the higher mean soil moisture. When the vegetation coverage was up to 40%, the decrease of soil moisture with depth was more obvious in soil profiles of middle down slopes. The mean soil moisture gradually increased form slope top to bottom.(2) In Shaoguwan, the variability of average soil moisture soil layer (0~100 cm) was moderate. The sequence of variation coefficient was uphill>middle slope>downhill, indicating that soil moisture increased gradually from uphill to downhill. The average moisture of 0~100 cm soil layer was "S" curve.Variation of soil moisture and rainfall had similar trend.Soil moisture increased significantly with heavy rain, while it did gradually with small rainfall.(3) The change of coefficient of variation of soil moisture uder the different vegetations was moderate. Compared with the deep soil layer, soil moisture in the shallow soil layer changed significantly with seasonal variations.Temporal variability coefficient of each layer soil moisture manifested middle level under different vegetations. While, shallow soil moisture change tended to be significantly seasonal relative to deep soil. Under the dry and wet conditions, semi-variance optimal models of soil moisture (0~100 cm soil layers) were all Gaussian models; compared with the wet condition, sill and range were higher, and the ratio of gold nugget and sill was lower under the dry condition.(4) Using the method of grey rational degree, we analyzed the different factors influencing on runoff. According to the contribution to runoff, the descending order of the factors was: rainfall, maximum rainfall intensity, rain intensity, vegetation coverage. According to contribution to erosion, the descending order was: rainfall, rain intensity, maximum rainfall intensity, vegetation coverage. The relationship between vegetation coverage and the volume of runoff and erosion was that: when the coverage is less than 25%, the volume of runoff was unalterable, when it was greater than 25%, the volume of runoff was reduced with increased coverage; when the coverage was less than 50%, the erosion trend was was clearly up with decreased coverage, when it was greater than 50%, erosion tend to decline slowly as the coverage increases.(5) Different vegetation coverage could decrease the surface runoff obviously. For the different vegetation, the amount of runoff and erosion decreased as the following sequence: bare land>Arborvitae forest>Pine forest>Locust and Arborvitae forest>locust and Sea buckthorn forest>Locust>Sea buckthorn forest>Pine and Locust>Grassland>Pine and Sea buckthorn forest. The grassland with good coverage could obviously decrease the runoff and soil erosion. Sea buckthorn and its mixed forest with other tress had the most prominent function decreasing soil erosion, the second was the mixed forest of locust and pine, and the fuction of the arborvitae forest was relatively bad. The runoff had the significant positive correlation with the precipitation and maximum rainfall intensity in 30mins. The main factors affecting the erosion amount was PI30 which was the Product of the precipitation and maximum rainfall intensity in 30mins, and I30max. Both the PI30 and I30max had no correlation with rainfall duration and the average rainfall intensity. I30max had the significant positive correlation with both runoff and erosion amount.(6) Both the changes of runoff and soil loss were stable on the slope of 0°, 1°, 2°, compared with other slopes. Runoff and erosion amount produced on the slope of 35°were obviously large. So it was concluded that the higher slopes, the more runoff and erosion. There were no significant differences between 9 m and 20 m slopes when vegetation coverage was greater than 60%; while the runoff of 20 m slope was significantly greater than that of 9 m slope when vegetation coverage was below 60%.(7) From four years'data of Wangdonggou watershed, it was found that runoff, sediment yield and rainfall occurred averagely seven times per year, and the average of rainfall was 327.2 mm for the four years, accounted for 61.9%~84.4% of the total rainfall during the flood season. Although the rainfall varied from year to year, we still found that more than half of the rainfall could cause soil erosion in the flood season on the Loss Plateau. Just for the rainfall that led to runoff, the rainfall events with amount of 10mm to 40 mm accounted for 65.2% of total rainfall, and rainfall events with 10 mm~20 mm, 20 mm~30 mm, 30 mm~40 mm took the proportion of 21.7% of total rainfall, respectively. The rainfall events with amount of 60 mm~70 mm, 70 mm~80 mm and over 80 mm accounted for 8.7% of total rainfall. |
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