Rainfall, Runoff, And Sediment Delivery Relationships Associated With Soil Conservation Measures At The Small Watershed Level

Soil erosion is a hazard traditionally associated with agriculture that is known to have long-term effects on soil productivity and sustainable agriculture. Soil erosion also leads to environmental damage through sedimentation, pollution and increased flooding. Soil erosion and its associated problems have already deteriorated land and water resources of China. Soil erosion affects an area of3.6×106km2in China, or about37%of its land area. Not surprisingly soil erosion has become important topics on the agenda of local and national policy makers. This has led to an increasing demand for proper understanding of the watershed and the hydrological processes to delineate target zones in which soil and water conservation measures are likely to be the most effective.The Three Gorges area (TGA) is characterized by hilly topography, high mountains, steep slope soil of poor structure and low organic matter content. Following construction of the Three Gorges Dam, many farmers resettled in surrounding mountain areas and cultivated marginal lands, which are mostly on steep slopes with soil of poor structure. The soil erosion problem is obvious. Small watersheds are a convenient scale for soil conservation planning because they are easily identified on maps and on the ground, and they allow suitable descriptions of some ecosystem processes and capabilities. The objectives of this study is:(1) to investigate rainfall-runoff-sediment transport relationships; and (2) to assess the hydrological and sedimentary response of the Wangjiaqiao watershed, which is a representative watershed within the TGA. Soil erosion characteristics and main controlling factors were also discussed in the present small watershed. Based on observation data analysis and field survey, a erosion model was used to evaluate the impacts of integrated small watershed management in the sutdy area. The main results are as follows:(1) The initial abstraction ratio (Ia/S) in SCS-CN equation was determined using rainfall-runoff event analysis from an agricultural watershed in the TGA of China. The results indicated that the Ia/S values varied from0.010to0.154, with a median of0.048. The average initial abstraction ratio of the watershed was equal to0.052. This is mainly attributed to the landscape and geological characteristics in the study watershed. A comparison between standard and modified Ia/S values showed that modified Ia/S value improved the agreement between measured and predicted direct runoff to a high degree. The standard SCS-CN method underestimates large runoff events, yielded a slope of the regression line of0.559and an intercept of0.301. The modified Ia/S valuewas about0.05that better predicted runoff depths with an R2of0.804and a linear regression slope of0.834. It also improved model efficiency coefficient (E) to0.768compared with0.482for traditional Ia/S value. Improvements in SCS-CN predictive ability may be expected with a0.05Ia/S value in the TGA of China.(2) The relationships among rainfall, runoff, and sediment transport were analysised in the Wangjiaqiao watershed. Strong seasonal and monthly variability in sediment load was found. Sediment was strongly transported during summer months, a period when frequent flood events of high magnitude and intensity occurred. Analysis of the relationships between precipitation, discharge and sediment transport at an individual event scale showed significant correlations between total precipitation, peak discharge, total water yield, maximum30min rainfall intensity, and sediment-related variables. Stepwise multiple regression analysis revealed that rainfall amount is the major cause of runoff, while events producing a large discharge in a short time play an important role in inducing severe soil erosion. During40flood events, three different types of hysteretic loops were observed:clockwise (28events,70%), figure-eight (5events,12.5%), and complex (7events,17.5%). The results of this study confirm the complex and heterogeneous nature of sediment response in the Wangjiaqiao watershed.(3) Based on10years of rainfall measurements and K-means clustering,152rainfall events were classified into3rainfall regimes. Rainfall Regime I is the aggregation of the rainfall events of medium amounts (31.8mm) and medium duration (1371min). Rainfall Regime II is an aggregation of the rainfall events with high amounts (54.0mm), long duration (2548min), and an infrequent occurrence. Rainfall Regime III is the aggregation of the rainfall events of low depth (22.2mm), short duration (494min) and high frequency. In each rainfall regime, there are differing levels of runoff and erosion. Rainfall Regime I causes the greatest proportion of accumulated discharge (368.7mm)and soil loss (4283t). In the different rainfall regimes, the values of the mean runoff coefficient (RC) and the mean sediment load were as follows:Rainfall Regime II> Rainfall Regime I> Rainfall Regime III. These results suggest that greater attention should be paid to Rainfall Regimes I and II because they had the most erosive effect. In the Wangjiaqiao watershed, the changes in land use primarily affected the paddy fields, where the cropland decreased significantly, while the forest and orchards increased by9.9%and7.7%, respectively, during the study years1995-2004. These land use changes caused significant decreasing trends in the runoff coefficients and sediment loads. In order, the most sensitive response of runoff and erosion to land uses was exhibited by Rainfall Regime II, Rainfall Regime I, and then Rainfall Regime III. We conclude that rainfall characteristics are decisive for the relative importance of different storm-runoff generation mechanisms and that the land use changes have considerably decreased runoff and soil loss in the study watershed.(4) Integrated small watershed management (ISWM) for soil conservation in the TGA was rapidly developed. The impact of ISWM on soil erosion and sediment delivery in the Wangjiaqiao watershed was investigated. The WATEM/SEDEM distributed erosion and sediment transport model were used to evaluate the effectiveness of the ISWM project. The model was calibrated against long-term measured suspended sediments at the watershed outlet. Land use and conservation measures were mapped and analyzed for1995and2005, paying particular attention to quantification of changes in soil erosion and sediment delivery due to ISWM. The results showed that a combination of decreased soil loss (from18.5tha-1y-1in1995 to13.2t ha-1y-1in2005) and increased sediment deposition (from7.7to12.4t ha-1y-1) has led to a strong decrease in sediment yield (from8.4to3.9t ha-1y-1) and the sediment delivery ratio (from0.454to0.295). The results of scenario analysis showed that soil conservation measures taken in fields effectively reduce on-site soil loss and sediment yield. However, off-site sediment control measures appear to be much less effective at reducing sediment yield. This diachronic comparison of soil erosion and sediment delivery revealed that ISWM is quite effective and efficient; therefore, it is the appropriate method to combat soil erosion in the TGA and similar areas.