5. CONCLUSIONS
The flow duration curve (FDC) is a hydrologically representation of the statistical distribution of daily streamflow. The complexity of processes contributing to the FDC introduces challenges for the direct exploration of physical controls on FDC. (Pumo et al., 2013; Yokoo and Sivapalan, 2011). In this paper, we used 56-year continuous daily precipitation from 698 weather stations across China and streamflow from more than 200 hydrological stations in the middle and lower Yangtze River basin, proposed a new framework to analyze the physical controls of PDC and FDCs applicable to the catchments in the middle and lower reaches of the Yangtze River, which includes dividing the total flow into fast and slow parts, constructing the FDCs by combining FDCs of fast and slow flow (i.e. FFDC and SFDC, respectively) and performing curves fitting and parameter estimation. We quantified the dependency structure between precipitation and flow to measure the correlation and interactions between different duration curves, which can be controlled by climatic and catchment characteristics.
1. The shape parameters α of PDC are related to CIM while the scale parameters β of PDC has a high degree of correlation with precipitation and precipitation percentiles (The correlation betweenβ and 99th percentile wet day precipitation reaches 0.9816), that is, spatial changes in scale parameters are significantly affected by precipitation patterns, especially by extreme precipitation.
2. The values of parameters p0 and α of PDC and FFDC have similar spatial patterns, and those of TFDC and SFDC also exhibit similar spatial patterns. but there exists a certain degree of dispersion between PDC and FFDC. The Spearman’s rank correlation coefficient between α of TFDC and SFDC is 0.9351 while it is 0.5455 between α of PDC and FFDC because many factors may also have an impact on the correlation between PDC and FFDC, including terrain effects,vegetation coverage, gradient, and soil characteristics.
3. The similarity between PDC and FFDCs’ lower tail is weak,FFDC is steeper, and the value α of FFDC is generally lower than that of PDC. Similarly, for the similarity between TFDC and SFDC, the parameterα of SFDC is generally greater than those of TFDC, because higher flow is separated and the duration of the slow flow is longer. Therefore, the SFDC is flatter and its α is larger.
4. For total flow and fast flow, the base flow index (BFI ) can be considered as the main influencing factor on the shape of normalized FDCs (TFDC and SFDC),Pmax*αp influences the shape of FFDC curves. CIM indirectly affects the change of fast flow through influencing precipitation. However, because the precipitation process is relatively complex, the impact of CIM on the shape of PDC is greater than that on FFDC.