| The Loess Plateau in the middle reaches of the Yellow River of China is the most erosive area in the world. Severe soil erosion not only aggravates the fragile ecological environment, but also produces large amounts of sediment which deposits on the lower reaches of Yellow River, raises the riverbed, and threatens the security of that region. Since the 1950s a great number of soil conservation measures have been implemented to reduce soil erosion and increase rainfall use efficiency in the Loess Plateau. These measures are very successful and reduced sediment by 3.0×108 t yr-1 from the Loess Plateau to the Yellow River from 1970 to 1996. Meanwhile, they have resulted in great changes in streamflow regime.In this dissertation, 10 watersheds which are located in hilly and gully area of Loess Plateau were chosen as the research object. The impacts of conservation measures on runoff were analyzed from temporal and spatial scales. It could be useful to understand and evaluate the function of soil conservation measures which have been implemented, and guide the future construction of ecological resoration in this region. The main conclusions of this study are as follows:(1) Applicability of three kinds of methods, named PART, digital filter technique and smoothed minima method which all belong to automatic baseflow separations, were compared. It was concluded that the separation results of digital filtering technique was relatively stable, and the baseflow process preferable conforms to the characteristics of baseflow recession.(2)The baseflow accounted for a larger proportion of the total runoff, the average baseflow index was between 37% -64% for the 10 catchments. Strong downward trends were detected in the baseflow volumes which was affected by conservation measures, while the baseflow indices showed increasing trend in stages. Monthly baseflow displayed two peaks in March and August which were benefited from snowmelt and precipitation, and under the influence of large-scale conservation activities, decreasing trend was showed between the ages, with the most obvious changes occurring in summer and autumn.(3) Based on 50-year rainfall, runoff data of Lu River, the influence of soil and water conservation on runoff was analyzed. Compared with pre-treatment, annual runoff, surface runoff and baseflow were reduced by 60%, 66% and 56% respectively. Results showed that contribution rate of conservation measures was 81% and 89% for the annual total runoff and surface runoff changes. Conservation measures reduced the peak flow, so that the monthly change in runoff was more stable, runoff process also tended to homogenization.(4) By applying wavelet transform, the tendency and periodic variations of flow series and impact factors were studied. The standardized time series were decomposed by Multi-Resolution Analysis (MRA) using the db3 wavelet function. The reconstruction of the lowest frequency part showed the flow series descended more rapidly than precipitation. Continuous wavelet transform (CWT) was carried out to identify the periodic variation of the precipitation and flow series using the complex valued Morlet function. Results showed that all 4 series had significant periodicity which concentrated in the 30-31a, 21-26a, 9-14a, and 3-4a. But the dominant period for precipitation and flow series was different, localization of periodic variations exhibited the influence of conservation measures on runoff changes.(5) Responses of runoff to conservation measures were not consistent among 10 watersheds because of the differences in many factors, such as beginning time of soil and water conservation, spatial allocation of conservation measures, controlling area and underlying surfaces. But the similarity was that the surface runoff which were intercepted and stored by conservation measures did not compensate the amount of baseflow.(6) Through analyzing runoff coefficients changes in different catchments, impacts of catchment size on runoff effects were discussed preliminary. Results showed that the responses of runoff coefficient differed with different catchment area range. The change of runoff coefficient was mainly affected by the control degrees of soil and water conservation, its relationship with catchment size may be not significant. Keywords: Conservation measures, runoff, different scale, watersheds of the Loess Pleateau.
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