Evolution Of Stream-flow And Sediment Load With Its Response To Land Use/Cover Change In Yanhe River Catchment

Effects of global climate change and human activities on ecological environment especially on stream-flow and sedimen load and the temporal and spatial distribution of water resource on the earth surface are getting intensified since the 19 th century and have attacted extensive concerns around the world. Climate change and human activities can change the status of global hydrology cycle and lead to the redistribution of water resource, temporally and spatially, and finally have great impact on precipitation, evaporation, runoff and soil moisture. Owing to the rapid development of population and economy, the frequent human activities have greatly changed land use/cove situation in most area so that profoundly affected the natural process of water cycle. Human activities could change the underlying surface of water cycle, influce the spatial and temporal distribution of water resource and river network charactors. Additionally, human activities could influence catchment hydrological process and furtherly have direct impact on stream-flow and sediment load by changing surface physical process like land use/cover, tillage, urbanization and other ways.In this context, the discussion of factors climate, land use/cover and soil and water conservation on catchment stream-flow and sediment load variation, with analyses of the tendency of climate and hydrology, reavelling the relationship between climate change and human activities and evaluating the impact of human activities on stream-flow and sediment load is of great significant for the guidance and govern of catchment soil and water conservation, even the effectiveness increacement of catchment hydrologic management.The study area in this paper is Yanhe River catchment, which located in the middle reaches of Yellow River. Based on 50 years meteorological, hydrological data and 25 years normalized differential vegetation index(NDVI) data, we systematically analysed the temporal and spatial variation and tendency of precptation, temperature, stream-flow and sediment load by anomaly analyse, Mann-kendall tendency test Pettitt breakpoint test and slope estimation. Vegetation restoration situation from 1982 to 2006 especially since the Grain to Green Program(GTGP) in 1998 was checked based on the NDVI dataset to qualitatively analyse the relationship between vegetation recovery and precipitation as well as temperature. Meanwhile, the relation between revegetation and human activities related LUCC was also analysed in our study. We introduced index NDVI as parameter into the catchment common sediment curve to check the effectiveness of improved modes and behave the response of stream-flow and sediment load change to vegetation restoration. Based on the Budyko hypothesis and water balance equation, we evaluated quantitatively the influence of climate change and human activities leaded LUCC since 1990.We obtained the main results in this paper as follows:(1) The variation tendencies of precipitation(P) and temperature(T) through a year were similar which had severe fluctuation from 1961 to 2010. Annual, growing season and four seasons precipitation and temperature have some inter-annual variations with variation coefficient(CV) ranging from 0.24 to 0.75 and-0.39 and 0.09, respectively. Winter has the highest CV of P and T among all time series. There is no obvious flood and drought years when to compare the P in every year with annual rainfall. The climate tendency coefficient and Mann-Kendall tendency test both showed that different time series of annual P and T had a weak decrease and significant(P=0.01) increase trend in our study area, with speed-0.709 mm a-1 and 0.041℃ a-1 based on slope evaluation. The breakpoints of different time series of annual T all reached a significant level at P=0.05, among which annual, growing season and winter T reached highly significant at P=0.01.(2) A power function relationship is more suitable to describe the mathematic relationships in the R factor and mean annual precipitation MAP, with an R2 value of 0.75, compared with 0.73 for a linear fitting. The resulting R factors of the tested regression equations vary widely, ranging from the lowest at nearly 906.51 MJ mm ha-1 h-1 a-1(Eq. 4-4) to the highest at 5840.72 MJ mm ha-1 h-1 a-1(Eq. 4-7). When comparing our results to the evaluations from previous studies, the exponential regression equation based on the MFI proposed by Angulo-Martínez and Beguería(2009) was shown to be the most accurate(with mean absolute percentage error MAPE=3.62%) estimation of the R factor in our study area. As indices, the MFI and FF are better proxies of the MAP than F when approximating the R factor because their regression coefficients with the MAP are 0.85 and 0.77, respectively. The results of the SI(0.69-1.16) and PCI(14.51-30.61) analyses indicated a highly seasonal distribution of rainfall throughout the Yanhe River catchment. In addition to MAP, the extreme wave of monthly rainfall distributions weighted by the SI and PCI can also affect the temporal variability, particularly the annual variability of rainfall erosivity. The trend coefficient r was-0.12 for the long-term series of rainfall erosivity, which indicated a slight decrease in the Yanhe River catchment during the 1961-2010 period when the missing data were excluded. The multi-year variation of rainfall erosivity in Yanhe was weak; therefore, the influence of rainfall played less of a role in the changes of sediment in the Yanhe River catchment in recent decades.(3) The average annual and growng seasonal NDVI in Yanhe River catchment from 1982-2006 was 0.253 and 0.324 respectively, with certain spatial variation which decreased progressively from south to north and a weak increase trend temporally, while obviously significant increase since the GTGP. Based on the slope evaluation, the NDVI in this region remained stable, but large area of slight improvement since 1998. In the NDVI variation of four seasons, NDVI in summer dominated the annual value, which at the same time showed a non-significant decease tendency. On the contrary, NDVI of annual, growing season and others three seasons showed a non-significant increase tendency. All time series of NDVI did not have a significant breakpoint in our study period.(4) Temperature had more impact on NDVI inter-annual variation than precipitation. The largest partial correlation coefficient between P and NDVI showed at the first pre-half moth with rNDVI P·T 0.435, p=0.01. The largest partial correlation coefficient between T and NDVI showed at the first pre-half moth with rNDVI T·P 0.833, p=0.01. Vegetation responsed to P and T with lag phase of half month and one month, accumulative effect of one month and two month respectively. Annual, growing season and spring precipitation had a highly significant positive correlation while growing season and summer temperature had weak negative correlation with NDVI. For interannual variability, monthly NDVI also had some hysteretic and accumulative effect.(5) The land use/cover changed from 1982 to 2006. Land use types mainly cross-transformed among grassland, farmland and forest during our research decades, with the comprehensive dynamic degree of land use high upto 50.62%. The alteration of land use type can be refected by vegetation index NDVI and affect the style and area of vegetation cover. Meanwhile, land use/cover change can control hydrological process such as stream-low and sediment load by influencing soil infiltration and water interception(6)Stream-flow(Q) and sediment load(SL) in Yanhe River catchment mainly concentrated in flood season and accounted for 63.78% and 98.44% in a year. Different time series of both Q and SL had high CV, of which SL was higher. It suggested that the sediment load was more sensitive to erosion environment than stream-flow. Annual stream-flow and sediment load had remarkable inter-annual fluctuation and there existed a highly significant relationship between them. Annual stream-flow and sediment load in Ganguyi station had significant and highly significant decrease tendency by Mann-Kendall test but none significant breakpoint was tested in our study.(7)Stream-flow and sediment load in Yanhe River catchment both had positive relation whih precipitation and negative relationwith NDVI. Hydrological elements were more sensitive to climate because annual stream-flow and sediment load had stronger correlationship with precipitation than that with NDVI. The highest correlation value of both stream-low and sediment load with precipitation and NDVI appeared at current year and accumulation of current and the last year. Stream-flow and sediment load showed more hysteretic with NDVI than precipitation. When we introduced index NDVI as parameter into the catchment common sediment curve, it showed that the inprived Equation 6-2 and Equation 6-6 performed beter than common sediment curve to reveal and evaluate the respond of stream-flow and sediment load to land use/cover change.(8) The climate hydrology and vegetation varied somewhat and temporal trends in R, P, E0 and NDVI occurred in Yanhe River catchment from 1990 to 2010. Precipitation did not show a significant trend, whereas runoff decrease and potential evaporation increase to some degree(p=0.05). The GTGP has greatly changed the land use/cover, with NDVI significantly increase(P=0.01) since 1999. The increasements of grassland, shrubland and forest in GTGP would greatly alter the hydrologic cycle and water redistribution in our study area. Both climate fluctuation and LUCC have effect on the runoff decline. In S-1(6 years post GTGP), the climate fluctuations dominated such runoff decline with contribution proportion 65.8%, while in S-2(12 years post GTGP), the dominant force changed to the GTGP induced LUCC(presented by landscape parameter n), with accounting for the runoff decrease by 77.4%. The GTGP has played an increasing hydro-ecological significance in the controlling of regional soil and water conservation.