Analyses of used BWR fuel assay data with the integrated burnup code system SWAT4.0

Criticality safety of the fuel debris from the Fukushima Daiichi Nuclear Power Plant is one of the most important issues, and the adoption of burnup credit is desired for criticality safety evaluation. To adopt the burnup credit, validation of the burnup calculation codes is required. Assay data of the used nuclear fuel irradiated by the Fukushima Daini Nuclear Power Plant Unit 2 are evaluated to validate the SWAT4.0 code for solving the BWR fuel burnup problem. The calculation results revealed that the number densities of many heavy nuclides and fission products show good agreement with the experimental data, except for those of ²³⁷Np, ²³⁸Pu, and samarium isotopes. These differences were considered to originate from inappropriate assumption of void fraction. Our results implied overestimation of the (n, γ) cross-section of ²³⁷Np in JENDL-4.0. The Calculation/Experiment – 1 (C/E–1) value did not depend on the type of fuel rod (UO₂ or UO₂–Gd₂O₃), which was similar to the case of PWR fuel. The differences in the number densities of ²³⁵U, ²³⁹Pu, ²⁴⁰Pu, ²⁴¹Pu, ¹⁴⁹Sm, and ¹⁵¹Sm have a large impact on k_eff. However, the reactivity uncertainty related to the burnup analysis was less than 3%. These results indicate that SWAT4.0 appropriately analyzes the isotopic composition of BWR fuel, and it has sufficient accuracy to be adopted in the burnup credit evaluation of fuel debris.     

Comparison of variance-based and moment-independent global sensitivity analysis approaches by application to the SWAT model

Global Sensitivity Analysis (GSA) is an essential technique to support the calibration of environmental models by identifying the influential parameters (screening) and ranking them.In this paper, the widely-used variance-based method (Sobol') and the recently proposed moment-independent PAWN method for GSA are applied to the Soil and Water Assessment Tool (SWAT), and compared in terms of ranking and screening results of 26 SWAT parameters. In order to set a threshold for parameter screening, we propose the use of a “dummy parameter”, which has no influence on the model output. The sensitivity index of the dummy parameter is calculated from sampled data, without changing the model equations. We find that Sobol' and PAWN identify the same 12 influential parameters but rank them differently, and discuss how this result may be related to the limitations of the Sobol' method when the output distribution is asymmetric.     

Integrating statistical and hydrological models to identify implementation sites for agricultural conservation practices

Watershed models are scarcely used by watershed managers due to their complexity. This study facilitates information transfer by introducing simpler techniques related to easily obtained watershed characteristics, including distance to the watershed outlet and stream order. The Soil and Water Assessment Tool (SWAT) was calibrated for the Saginaw River Watershed, Michigan. Five agricultural best management practices (BMPs) were implemented in SWAT one at a time in each subbasin. Five statistical models were used to determine the pollution reduction at the watershed outlet using distance and BMP type, with results suggesting that a mixed effects model (model 5) was optimal. This model included subbasin as a random effect, while distance to watershed outlet and BMP type were fixed effects. Native grass and strip cropping were the most effective BMPs for reducing sediment and nutrient transport. Subbasins containing stream orders 1–3 were ideal for BMP implementation throughout the watershed.     

SWAT模型在我国的发展历程及其在寒地应用展望

SAWT模型是目前世界范围内广泛用于复杂流域水资源与水环境研究的优选模型。自21世纪初引入我国后,该模型取得了飞速发展与应用,研究区域涵盖水系发达、污染集中、水资源问题严峻的全国大部分地区,但在西部高原(山)和北方高纬度寒冷地区的应用相对较少。基础资料匮乏,降水年度内分布不均和独特的积融雪过程是限制该模型在当地发展的主要障碍。通过对模型在我国的发展历程和应用现状的回顾、总结与综合分析,结合寒冷地区的气候与水文特征,深入剖析SWAT模型的特征模块及算法功能,探讨其在该地区的应用前景与展望,以期为我国寒冷地区流域水文过程模拟和非点源污染控制系统研究提供一个新思路,为丰富全国流域水资源综合管理和水环境系统防治工作提供参考。     

Soil erosion dynamics response to landscape pattern

Simulating soil erosion variation with a temporal land use database reveals long-term fluctuations in landscape patterns, as well as priority needs for soil erosion conservation. The application of a multi-year land use database in support of a Soil Water Assessment Tool (SWAT) led to an accurate assessment, from 1977 to 2006, of erosion in the upper watershed of the Yellow River. At same time, the impacts of land use and landscape service features on soil erosion load were assessed. A series of supervised land use classifications of Landsat images characterized variations in land use and landscape patterns over three decades. The SWAT database was constructed with soil properties, climate and elevation data. Using water flow and sand density data as parameters, regional soil erosion load was simulated. A numerical statistical model was used to relate soil erosion to land use and landscape. The results indicated that decadal decrease of grassland areas did not pose a significant threat to soil erosion, while the continual increase of bare land, water area and farmland increased soil erosion. Regional landscape variation also had a strong relationship with erosion. Patch level landscape analyses demonstrated that larger water area led to more soil erosion. The patch correlation indicated that contagious grassland patches reduced soil erosion yield. The increased grassland patches led to more patch edges, in turn increasing the sediment transportation from the patch edges. The findings increase understanding of the temporal variation in soil erosion processes, which is the basis for preventing local pollution.     

Modeling effectiveness of agricultural BMPs to reduce sediment load and organophosphate pesticides in surface runoff

Quantifying effectiveness of agricultural BMPs at the watershed scale is a challenging issue, requiring robust algorithms to simulate not only the agricultural production system but also pollutant transport and fate. This research addresses the challenge to simulate performances of BMPs in reducing organophosphates (OPs) runoff at the watershed scale. The SWAT model is calibrated and validated following a sensitivity analysis combining Latin Hypercube sampling and One-factor-At-a-Time simulation. The calibrated model is then applied in the Orestimba Creek Watershed to simulate BMPs including buffer strips, sediment ponds, vegetated ditches, use reduction, and their combinations. BMP simulation suggested that sediment ponds trap 54-85% of sediment from field runoff, but less than 10% of dissolved diazinon and chlorpyrifos. Use reduction can reduce pesticide load in a close-to-linear fashion. Effectiveness of vegetated ditches and buffers depends on their physical dimension and vegetation cover. Combining individual BMPs provides enhanced mitigation effects. The combination of vegetated ditches, buffer strips and use reduction decreases diazinon and chlorpyrifos load by over 94%. This study has suggested that the SWAT model reasonably predicts BMP effectiveness at the watershed scale. Results will assist decision making in implementing BMPs to reduce pesticide loads in surface runoff.     

The modified SWAT model for predicting fecal coliforms in the Wachusett Reservoir Watershed, USA

This study assessed fecal coliform contamination in the Wachusett Reservoir Watershed in Massachusetts, USA using Soil and Water Assessment Tool (SWAT) because bacteria are one of the major water quality parameters of concern. The bacteria subroutine in SWAT, considering in-stream bacteria die-off only, was modified in this study to include solar radiation-associated die-off and the contribution of wildlife. The result of sensitivity analysis demonstrates that solar radiation is one of the most significant fate factors of fecal coliform. A water temperature-associated function to represent the contribution of beaver activity in the watershed to fecal contamination improved prediction accuracy. The modified SWAT model provides an improved estimate of bacteria from the watershed. Our approach will be useful for simulating bacterial concentrations to provide predictive and reliable information of fecal contamination thus facilitating the implementation of effective watershed management.     

基于SWAT模型的白莲河流域径流模拟研究

以湖北省白莲河流域为研究区,探讨了SWAT模型在中小流域尺度的径流模拟中的适用性,利用白莲河流域的水文气象数据、DEM数据、土壤类型和土地利用等资料建立了白莲河流域的SWAT模型,并选用1995~1999年汛期(4~10月)为模型参数的率定期、2002~2004年汛期(4~10月)为模型参数的检验期.模拟结果表明;SWAT模型基本能反映白莲河流域的日径流水文过程,具有一定的适用性,为研究水资源提供了科学依据.     

Modeling nitrogen loss from switchgrass agricultural systems

A water quality modeling study was conducted to determine and compare nitrogen loss from water flow to surface water and ground water from agricultural systems growing switchgrass and cotton in the southeastern USA. The Soil and Water Assessment Tool (SWAT) was used to replicate field conditions observed at switchgrass research plots at Pee Dee Research and Educational Center at Florence, South Carolina. The plot scale modeling suggested that in the early years of growth there is significant nitrogen loss from switchgrass to streamflow and groundwater but the loss is significantly less as the switchgrass matures. During the second year of growth the modeled nitrogen loss from switchgrass was approximately 50% that of cotton. The long term nitrogen loss over the lifetime of switchgrass was approximately 10% that of cotton. The results also indicate that the use of computational models to quantify nitrogen fluxes in Life Cycle Assessment (LCA) of agricultural systems growing switchgrass gives a more accurate representation of the system than using constant emission factors.     

土地利用/覆被变化对SWAT模型水循环模拟结果的影响研究——以海河流域为例

土地利用/覆被的变化(Land Use/Land Cover Change,LUCC)直接体现和反映了人类活动的影响程度,它对水循环过程的影响结果十分显著.本文利用SWAT模型定量解析土地利用/覆被变化对蒸发、径流等水循环要素的影响.研究表明,1995-2000年海河流域的土地利用覆被变化很大,土地利用/覆被变化对SWAT模型水循环模拟结果的影响也是很大.人类活动对于海河流域的影响总体上是流域蒸发量在增加、而地表与地下径流和土壤水量在减少.