Trend Of Streamflow, Sediment Load And Their Dynamic Relation At Watershed In The Middle Rea Ches Of Yellow River During The Past Five Decades

In this study, the data from 5 catchments with the different vegetation coverage in the Loess Plateau were analyzed to investigate the responses of streamflow, sediment load and the dynamic relation to the soil conservation measures and the ecological construction project. 3 catchments located in the transition zone between sandy area and the loess hilly-gully region was covered with low forest coverage less than 10%. 2 catchments in the rocky mountain region were with high forest coverage greater than 55%. The data were standardized with annual/monthly precipitation and the area of catchment. The nonparametric Mann-Kendall test and the Pettitt test were used to identify trends and change points in the streamflow and sediment load records. It is important to understand the impacts of these measures and project on streamflow, sediment load and their dynamic relation in the catchment, especially the differences in catchments under the different vegetation coverage. The results showed that:(1) Changes in streamflow and sediment load characteristics: 5 catchments, except for Yanhe River, had significant negative trend in annual streamflow and sediment load, two catchments with 10% vegetation had higher trend with the average rate -3.242m³/km².mm^-1.a^-1 than two catchments with 55% vegetation with -0.67m³/km².mm^-1.a^-1. Same with the streamflow, the average change rate of sediment load for two 10% vegetation catchments was -0.556t/km².mm^-1.a^-1 and much higher than two 55% vegetation catchments with -0.05t/km².mm.a. The change points in sediment load occurred in catchments were reasonably consistent with those of streamflow from 1977 to 1994. The change point detected in two 10% vegetation catchments located in transition zone was earlier than that in two 55% vegetation catchments located in rocky mountain region.Using the cumulative departure methed tested annual streamflow and sediment for 5 catchments, the results shows that: 5 catchments, except for Yanhe River, streamflow and sediment load had a significant reduction between 70s-80s of last century, the significant reduction year was same as the change points.(2) The data in each catchment were divided into three periods using the detected change point and the year 1999. The relative changes in top five percent of flows /sediment load (5%, i.e. flows or sediment load which are exceeded 5% of the time), in median flow /sediment load (50%, i.e. flows or sediment load which are exceeded 50% of the time) and in low flow/sediment load (95%, i.e. flows or sediment load which are exceeded 95% of the time) was compared through flow/sediment duration curves. It showed that, except for Yanhe river, the streamflow and sediment load were gradually reduced as the time changed. It showed that, the relative changes in sediment load was much greater than that of streamflow with the periods altered in most of catchments, and an increased number of zero-sediment load days was observed. In general, the annual average reduction in both streamflow and sediment load in the low-vegetation-coverage catchments was greater than that of the high-vegetation-cover age catchments.(3) Changes in sediment concentration characteristics:Annual sediment concentration for 5 catchments was tested using Mann-kendall and Pettitt method, it showed that sediment concentration experienced strong downward trend during the past fifty years. Tueihe and Jialuhe had higher trend with the average rate -4.312kg/m³.a^-1 than Yunyanhe and shiwangchuan with -2.026 kg/m³.a^-1, the average change rate of Yanhe was -4.547kg/m³.a^-1, the change points were between 1971 and 1982.The data in each catchment were divided into two periods using the detected change point. The relative changes in top five percent of sediment concentration (5%,i.e. sediment concentratio which are exceeded 5% of the time), in median sediment concentration (50%, i.e. sediment concentratio which are exceeded 50% of the time) and in low sediment concentratio (95%, i.e. sediment concentratio which are exceeded 95% of the time) was compared through sediment concentration duration curves. It showed that, except for Yanhe river, the sediment concentration were gradually reduced as the time changed. The annual average reduction in in sediment concentration in the high-vegetation-coverage catchments was greater than that of the low-vegetation-coverage catchments.Dividing the sediment concentration into four levels,≤20kg/m³,20-80kg/ m³,80-300 kg/m³,≥300kg/m³, analysis the return period of different daily average sediment concentration in the five study catchments, the return period increased of the same sediment concentration level as the time changed, in the same time, higher sediment concentration return period less than lower sediment concentration.(4) Trend of streamflow, sediment load dynamic relation: The scattered distribution for monthly streamflow and sediment load from May to October in 5 catchments indicated that, the three 10% vegetation catchments have wider domain by one order of magnitude than that of the two 55% vegetation catchments. The sediment load was simply regressed with the streamflow and the regression coefficient was considered as"sediment production coefficient"to illustrate the sediment production ability in the catchment. During the past 50 years, the 5 catchments, whatever the background of vegetation coverage and geomorphology types, the sediment production coefficient was observed to decrease by periods. Soil and water conservation measures implemented in 1970s-1980s, and"Grain for Green"project application from 1999 resulted in an average reduction of the sediment production coefficient with 22.4% and 63.4%, respectively. It was also observed that, the absolute value of the constant in the linear regression, which implies the sediment storage situation for each period, was much higher in the low vegetation catchments located in transition zone from sandy area to the loess hilly-gully region, and also showed the much higher decreasing trend (96.7%) than the high vegetation catchments in rocky mountain region. The streamflow volume when the catchments reach the scour and silting balance situation turned out a decreasing trend by periods especially in the low vegetation catchment, for example Jialu river, compared to the 1^st period, the reduction for the later two periods was reached to 45.8% and 72.6%, respectively.The impacts of soil and water conservation implementation and"Grain for Green"policy application on streamflow, sediment load and their dynamic relation was further illustrated.