Xitiaoxi Catchment Research Articles

Bioavailability and Risk Variations of Heavy Metal(loid)s in Paddy Soils Developed from Eluvial and Alluvial Parent Materials in the Xitiaoxi Catchment, China

Bioavailability and Risk Variations of Heavy Metal(loid)s in Paddy Soils Developed from Eluvial and Alluvial Parent Materials in the Xitiaoxi Catchment, China

Response of flood events to extreme precipitation: two case studies in Taihu Basin, China

Abstract Flood events are typically triggered by extreme precipitation in rain-dominant basins. In this study, to better understand the genetic mechanisms and characteristics of floods, copula functions are used to analyze the response of flood events to extreme precipitation. The coincidence probabilities of the typical extreme flood and precipitation events are calculated; different return periods of their arbitrary combinations are calculated, whereas the dangerous domains for flood control under different return periods are identified; furthermore, flood risk analysis under different extreme precipitation scenarios is performed via their conditional exceedance probabilities. The Xitiaoxi catchment (XC) and Dongtiaoxi catchment (DC) in the Zhexi Region of the Taihu Basin are selected as the study area. The results show that in four scenarios with precipitation frequencies of 80%, 90%, 93.33%, and 95%, the probabilities of the dangerous flood are 9.72%, 10.57%, 10.86%, and 11.01% in the XC, respectively, and 5.91%, 6.31%, 6.44%, and 6.51% in the DC, respectively. This study provides a practical basis and guidance for the computation of rainstorm designs, management of flood control safety, and water resource scheduling in the Taihu Basin.

Parameter uncertainty assessment of a flood forecasting model using multiple objectives

The calibration of hydrological models used for flood forecasting is a difficult task that has received widespread attention. Assessing the uncertainty of flood forecasting models is fundamental to properly support decision‐making activities. Because of the demand of the different decision‐making objectives of a flood management system, we assess the uncertainty of flood forecasting models from a multiobjective perspective. Multicriteria decision analysis (MCDA) is adopted to evaluate the flood likelihood within a generalised likelihood uncertainty estimation (GLUE) framework. By combining four criteria representing different flood characteristics, the acceptability of the simulations is fully examined. The method is applied to an uncertainty analysis of a variable infiltration capacity model for the Xitiaoxi catchment. The results of 10,000 Monte Carlo simulations show that no single criterion can describe the characteristics of floods. Compared with two single‐criterion methods using the Nash–Sutcliffe efficiency, the multicriteria method shows advantages not only in assessing the prediction bounds but also in providing median GLUE estimates. The results indicate that a single criterion may be inadequate in parameter identification within the GLUE framework and that MCDA is an alternative approach to reduce the prediction uncertainty in flood forecasting.

Hydrological behaviour and water balance analysis for Xitiaoxi catchment of Taihu Basin

With the rapid social and economic development of the Taihu region, Taihu Lake now faces an increasingly severe eutrophication problem. Pollution from surrounding catchments contributes greatly to the eutrophication of water bodies in the region. Investigation of surface flow and associated mass transport for the Xitiaoxi catchment is of a significant degree of importance as the Xitiaoxi catchment is one of the major catchments within the Taihu region. A SWAT-based distributed hydrological model was established for the Xitiaoxi catchment. The model was calibrated and verified using hydrometeorological data from 1988 to 2001. The results indicate that the modeled daily and annual stream flow match the observed data both in the calibration period and the verification period, with a linear regression coefficient R2 and a coefficient e for modeled daily stream flow greater than 0.8 at Hengtangcun and Fanjiacun gauge stations. The results show that the runoff process in the Xitiaoxi catchment is affected both by rainfall and human activities (e.g., reservoirs and polder areas). Moreover, the human activities weaken flood peaks more noticeably during rainstorms. The water balance analysis reveals the percentages of precipitation made up by surface flow, evapotranspiration, groundwater recharge and the change of soil storage, all of which are considered useful to the further understanding of the hydrological processes in the Xitiaoxi catchment. This study provides a good base for further studies in mass transport modeling and comparison of modeling results from similar hydrological models.

Comparison of two different methods for determining flow direction in catchment hydrological modeling

Digital elevation models (DEMs) are widely used to define the flow direction in distributed hydrological models for simulation of streamflow. In recent decades, numerous methods for flow direction determination have been applied successfully to mountainous regions. Nevertheless, some problems still exist when those methods are used for flat or gently sloped areas. The present study reviews the conventional methods of determining flow direction for such landscapes and analyzes the problems of these methods. Two different methods of determining flow direction are discussed and were applied to the Xitiaoxi Catchment, located in the Taihu Basin in southern China, which has both mountainous and flat terrain. Both the agree method and the shortest path method use drainage networks derived from a remote sensing image to determine the correct location of the stream. The results indicate that the agree method provides a better fit with the DEM for the hilly region than the shortest path method. For the flat region where the flow has been diverted and rerouted by land managers, both methods require observation of the drainage network to determine the flow direction. In order to clarify the applicability of the two methods, both are employed in catchment hydrological models conceptually based on the Xinanjiang model and implemented with PCRaster. The simulation results show that both methods can be successfully applied in hydrological modeling. There are no evident differences in the modeled discharge when using the two methods at different spatial scales.

Scale-dependence effects of landscape on seasonal water quality in Xitiaoxi catchment of Taihu Basin, China

Further understanding the mechanisms of landscape-water interactions is of great importance to water quality management in the Xitiaoxi catchment. Pearson's correlation analysis, stepwise multiple regression and redundancy analysis were adopted in this study to investigate the relation between water quality and landscape at the sub-catchment and 200 m riparian zone scales during dry and wet seasons. Landscape was characterized by natural environmental factors, land use patterns and four selected landscape configuration metrics. The obtained results indicated that land use categories of urban and forest were dominant landscape attributes, which influenced water quality. Natural environment and landscape configuration were overwhelmed due to land management activities and hydrologic conditions. In general, the landscape of the 200 m riparian zone appeared to have slightly greater influence on water than did the sub-catchment, and water quality was slightly better explained by all landscape attributes in the wet season than in the dry season. The results suggested that management efforts aimed at maintaining and restoring river water quality should currently focus on the protection of riparian zones and the development of an updated long-term continuous data set and higher resolution digital maps to discuss the minimum width of the riparian zone necessary to protect water quality.

Integration of monthly water balance modeling and nutrient load estimation in an agricultural catchment

Non-point sources pollution has become a serious environmental problem in the aquatic systems throughout the world. The Xitiaoxi catchment is located in the southwest of Taihu Basin, contributing large amounts of runoff and associated nutrients to Taihu Lake. Thus, identifying critical non-point sources pollution in this catchment is urgent and essential to control water pollution, improve the water quality, and reduce the pollutants drained into water bodies. The present study integrated a monthly water balance model with the export coefficient model for total nitrogen and total phosphorus loads estimation in the Xitiaoxi catchment in southeastern China. The simulated monthly runoff are in good agreement with the observed streamflow at both Hengtangcun and Fanjiancun stations with Nash–Sutcliffe coefficients higher than 0.80. The predictions showed reasonable ranges from 1687 to 2046 t/y (2002–2005) for total nitrogen loads, and from 106 to 157 t/y for total phosphorus loads (1999–2007), respectively, which are consistent with the observed values at Hengtangcun. Overall, the monthly export coefficient model coupling monthly water balance simulation to export coefficient model presented both the seasonal dynamics and magnitude for streamflow and nutrients loads, which generally match well with the observations. These findings demonstrate that the proposed model can provide encouraging results and can be used as an efficient tool to identify the pollution sources for planning and management of large-scale agricultural catchment.

Estimating Nitrogen Yield for a Catchment of Lake Taihu Based on Distributed Hydrological Modelling

Nitrogen and phosphorous pollution is common in China and worldwide. The pollution deteriorates water quality and even causes significant change of the freshwater ecosystem. Management must be strengthened in order to reduce the pollution to secure water supply safety and to maintain healthy ecosystems. This paper investigated the yield of total nitrogen (TN) for different land use types, exemplified in the Xitiaoxi catchment of Lake Taihu basin, where catchment-generated nitrogen pollution is severe. Extensive water quality sampling was performed for different river levels and various land use types. A distributed hydrological model was also employed to simulate the river discharges at locations where flow observation is not available, and the simulated river discharges were used for the calculation of nitrogen yield. It is found that generally the catchment’s nitrogen export is still as high as 2.6 t/km2/year. The yield for forest is 0.8-1.0 t/km2/year, while the yields for arable land and for the river sections immediately downstream of urban area are higher than the average ranging 2.8-3.3 t/km2/year. It is also concluded that the hydrological modelling using a spatially distributed model is helpful and essential in obtaining flows at any locations, which can subsequently be used jointly with the water quality data to identify the critical source areas with high nitrogen yields.

Development and application of a nitrogen simulation model in a data scarce catchment in South China

Abstract The Xitiaoxi catchment is one of the most important catchments in the Taihu Basin in southeastern China. It contributes a significant amount of surface runoff and nutrient to Taihu Lake. Understanding the nutrient cycling and identification of critical non-point source pollution in this catchment are therefore of primary importance. In this paper, the Xinanjiang-Nitrogen (XAJ-N) model, a conceptual model of nutrient mobilization and transport is developed by integration of the Xinanjiang rainfall–runoff model, the Integrated Nitrogen CAtchment (INCA) model and the Modified Universal Soil Loss Equation (MUSLE). It is implemented with the environmental modelling language PCRaster and estimates the water fluxes and nutrient loadings on a cell-by-cell basis in daily time steps. The model includes the nitrogen cycling processes of mineralization, leaching, fixation, volatilization, nitrification, denitrification and plant uptake. Nitrogen is assumed to be mobilized by surface runoff and groundwater. The model performance was verified by comparing simulated and measured daily discharge and nutrient loadings. The results showed a fairly good relationship between predicted and observed values. Due to the scarce observed data, the simulation results were also validated using an internal mass balance method and values from the literature. It showed that the modelling approach can be used as a tool to estimate the export of nutrient with a daily resolution at a catchment scale.

Application of a Simple Raster-Based Hydrological Model for Streamflow Prediction in a Humid Catchment with Polder Systems

The hydrological processes are controlled by many factors such as topography, soil, climate and land management practices. These factors have been included in most hydrological models. This study develops a raster-based distributed hydrological model for catchment runoff simulation integrating flood polders regulation. The overland flow and channel flow are calculated by kinematic wave equations. A simple bucket method is used for outflow estimation of polders. The model was applied to Xitiaoxi catchment of Taihu Lake Basin. The accuracy of the model was satisfactory with Nash–Sutcliffe efficiencies of 0.82 during calibration period and 0.85 for validation at Hengtangcun station. The results at Fanjiacun station are slightly worse due to the tidal influence of Taihu Lake with high values of root mean square errors. A model sensitivity analysis has shown that the ratio of potential evapotranspiration to pan evaporation (K), the outflow coefficients of the freewater storage to groundwater (KG) and interflow (KSS) and the areal mean tension water capacity (WM) were the most sensitive parameters. The simulation results indicate that the polder systems could reduce the flood peaks. Additionally, it was confirmed that the proposed polders operation method improved the accuracy of discharge simulation slightly.

Impact of climate change on streamflow in the Xitiaoxi catchment, Taihu Basin

The impact of climate change on streamflow in the Xitiaoxi catchment is assessed by using VIC (variable infiltration capacity) model coupled with PRECIS (providing regional climate for impacts studies). Results show that the VIC model is adaptable for the study area. Both deterministic coefficient and Nash-Suttcliffe efficiency coefficient are greater than 0.75, with a good agreement between observed and simulated discharge. The runoff will increase in the future, especially during flood seasons. The magnitude of floods in the future (2021-2050) under A2 and B2 scenarios will be greater than that during the baseline period (1961-1990), but it may not exceed that during the 1990s.

Impacts of spatial data resolution on simulated discharge, a case study of Xitiaoxi catchment in South China

Abstract. In this paper we analyse the effects of different spatial input data resolution on water balance simulation using a simple distributed hydrological model: PCR-XAJ model. A data set consisting of land use and digital elevation model at 25 m resolution of Xitiaoxi catchment in South China is used for investigation. The model was first calibrated and validated at 50 m cell size, thereafter an aggregation of the digital elevation model (DEM) and land use maps at 100 m, 200 m, 300 m, 500 m and 1 km are applied to evaluate the effects of spatial data resolution on simulated discharge. The simulation results at a grid size of 50 m show a good correlation between measured and simulated daily flows at Hengtangcun station with Nash-Suttcliffe efficiency larger than 0.75 for both calibration and validation periods. In contrast, the model performs slightly worse at Fanjiacun station. The increasing grid size affects the characteristics of the slope and land use aggregation and causes important information loss. The aggregation of input data does not lead to significant errors up to a grid of 1 km. Model efficiencies decrease slightly with cell size increasing, and more significantly up to the grid size of 1 km.

Development and application of an integrated surface runoff and groundwater flow model for a catchment of Lake Taihu watershed, China

Severe pollution in Lake Taihu in the lower reach of the Yangtze River has led to the need to evaluate transport pathways and the overall water balance of the catchment. This paper describes the development of a grid-based spatially distributed hydrological model for integrated simulations of surface runoff and groundwater flow at catchment scales. Surface water flow is modeled based on rainfall-runoff transformation and stream flow routing, while the saturated groundwater is modeled by MODFLOW-2005. Surface water flow is coupled to the groundwater flow regime through the soil layer. Relatively simple equations are used to describe the soil storage and soil water percolation. These equations are computationally inexpensive and overcome some of the limitations in existing models for coupled simulation of surface and subsurface domains, such as either multiple soil layer data are needed in the vertical direction or numerical difficulties are encountered in dealing with the high non-linearity of the equations representing the variably-saturated subsurface. The model was verified against the V-catchment benchmark problem and was compared to existing models to demonstrate its accuracy and capability. The model was applied to Xitiaoxi catchment of Lake Taihu watershed. The accuracy of the model was satisfactory with daily and monthly efficiencies of 0.80 and 0.92, respectively for stream discharge comparison, and an absolute error of 0.144 m for groundwater level comparison. The model indicated that for Xitiaoxi catchment, a percentage of 37.4% of rainfall transferred to surface overland flow, and a percentage of 16% infiltrated as groundwater recharge, of which 81% returns to rivers as base flow and the rest is lost to the system via evapotranspiration. The model can be used as an alternative to more complex methods to assess impacts of land use or/and climate changes on water resources availability, and the interactions between surface and subsurface flow domains.

Modeling of Surface Runoff in Xitiaoxi Catchment, China

Xitiaoxi catchment is one of the most important catchments in the Taihu basin in China. Due to its significant contribution of surface runoff and associated nutrients input to Lake Taihu, understanding of the processes of surface runoff in this catchment is, therefore, of primary importance in quantifying water and nutrient balances for Lake Taihu. The generation of surface runoff in the catchment is mainly controlled by rainfall and land cover, so the variety of surface runoff in Xitiaoxi catchment is seasonal. Moreover, the annual change of surface runoff is distinct. Because of the diversity of land use and variety of hydrological characteristics, numerical simulation of the generation of surface runoff over this catchment is not straightforward. In this paper, attempts were made in applying the Large Scale Catchment Model (LASCAM) to Xitiaoxi catchment. The Xitiaoxi catchment is divided into 47 subcatchments connected via a river network based on topology. The model was first run in an optimization mode to calibrate the parameters against the observed runoff for the period of 1968-1977, and then was run in a prediction mode to try to reproduce the runoff for the next 10 years from 1978 to 1987. The model indicates that saturation excess runoff is probably the dominant process for the catchment. The modeling results indicate that water storage in shallow soils near the stream has a high correlation with daily rainfall, while the water storage changes in deep aquifers demonstrate an annual change trend, showing a rising level for wet seasons and a declining level for dry seasons. The success in modeling surface runoff leads to confidence in modeling nutrients transport as the next step of modeling work.

Surface runoff modeling for Xitiaoxi catchment, Taihu Basin

Investigation on surface runoff and associated mass transport for Xitiaoxi catchment is of significant importance as it is one of the major catchments of Lake Taihu basin. Outcomes from this investigation help understand the dynamic changes of hydrology and water quality of lake Taihu, and are also useful in carrying out similar research work on other catchments of lake Taihu basin. As the first step, this paper presents the construction of a surface runoff model for Xitiaoxi catchment using a conceptual model, LASCAM. The model is well calibrated using daily runoff data from 1968 -1988 from 2 monitoring stations within the catchment. The modeled daily and annual runoffs match well with the observed ones. The model indicates that saturation excess runoff is probably the dominant process for the catchment. Water storage in near-stream shallow saturated soil has high correlation with daily rainfall, while water storage changes in deep aquifers demonstrate an annual change period, showing a rising level for wet seasons and a declining level for dry seasons. The findings are helpful for further refining the runoff model and constructing nutrients transport model as the next step.

Impacts of Non-point Source Pollution on the Water Environment of Xitiaoxi Watershed, Upper Taihu Basin

水资源短缺是全球性关注的问题,水质恶化更加剧了这一问题的严重性,定量研究水质变化及其影响因素可为治理水环境提供基础依据.与实测数据及综合污染指数的对比表明,水质指数能够合理反映水质的变化程度和时空变化趋势.利用西苕溪流域1996-2000年水质监测数据的研究结果如下:西苕溪流域水质的空间变化趋势是自上游至下游逐渐恶化,时间变化的总体趋势是逐年转好;点污染源得到有效控制年份(1999年)的水质指数比以前年份(1996-1998年)有所提高但幅度不大,说明非点源污染是影响西苕溪流域水质的重要因素;流域水污染的主要形式是氮、磷污染,其主要非点源是农田、经济竹林和城镇径流及居民生活污水等.;The scare of fresh water has been becoming a focus issue in the world, while water quality deterioration enhances the seriousness of the problem. To assess the water quality and its impact factors can provide basic information for water resource management. This paper verified the usefulness of water quality indices (WQI) to assess the water quality from multiple measured parameters, and to evaluate the spatial and temporal change. The WQI which takes into consideration all the measured parameters not only shows a similar trend to the results of analyzing separate parameters and the index of compositive pollution, but also display the overall water quality and can predict precisely the change trend of water quality. As a case study, we use the WQI to evaluate spatial and temporal changes in the water quality from Xitiaoxi Catchment, upper reaches of Taihu Basin, and assess the degree of factors influencing water quality. The nonpoint source pollution caused by land use produces a serious and negative effect on the water quality because only controlling and management point sources only increase 6.5% of water quality. The water quality gradually deteriorated along the river from upper reaches to lower reaches mainly due to the land use change. So, in the future, the highest priority measure to improve water quality of Xitiaoxi Catchment should be controlling non-point pollution resources.