Comparative evaluation of phosphorus losses from subsurface and naturally drained agricultural fields in the Pike River watershed of Quebec, Canada

Phosphorus (P) is the limiting nutrient responsible for the development of algal blooms in freshwater bodies, adversely impacting the water quality of downstream lakes and rivers. Since agriculture is a major non-point source of P in southern Quebec, this study was carried out to investigate P transport under subsurface and naturally drained agricultural fields with two common soil types (clay loam and sandy loam). Monitoring stations were installed at four sites (A, B, C and D) in the Pike River watershed of southern Quebec. Sites A-B had subsurface drainage whereas sites C-D were naturally drained. In addition, sites A-C had clay loam soils whereas sites B-D had sandy loam soils. Analysis of data acquired over two hydrologic years (2004-2006) revealed that site A discharged 1.8 times more water than site B, 4 times more than site C and 3 times more than site D. The presence of subsurface drainage in sandy loam soils had a significant beneficial effect in minimizing surface runoff and total phosphorus (TP) losses from the field, but the contrary was observed in clay loam soils. This was attributed to the finding that P speciation as particulate phosphorus (PP) and dissolved phosphorus (DP) remained relatively independent of the hydrologic transport pathway, and was a strong function of soil texture. While 80% of TP occurred as PP at both clay loam sites, only 20% occurred as PP at both sandy loam sites. Moreover, P transport pathways in artificially drained soils were greatly influenced by the prevailing preferential and macropore flow conditions.     

基于SWAT模型的日本农业流域养分模拟（摘选）

在山田河流域利用SWAT模型对循环灌溉稻田和畜牧生产系统的流域特征进行调查。在茨城县地区土地利用呈现典型的景观镶嵌式。谷地中的稻田循环灌溉系统沿河岸在低处修建,森林、菜地和畜禽舍则建在周围地势较高处。灌期与非灌期土壤径流特征存在明显差异,通过设定参数可以利用SWAT模型对径流进行预测。灌期效率系数(ENS)、系数测定(r2)和相对误差(RE)分别为0.86、0.87和2.0%,非灌期分别为0.67、0.68和2.6%。但是对于养分浓度,模型没法给出满意的预测。对菜田及牲畜废弃物养分的模拟应对SWAT进行修正,对稻田系统的模拟应对算法进行修订。     

Applicability of targeting vegetative filter strips to abate fecal bacteria and sediment yield using SWAT

Quantifying and evaluating effects of best management practices (BMPs) on water quality is necessary to maximize the effectiveness of BMPs for minimizing pollutants. Watershed-scale evaluation of effects of BMP implementation on fecal bacteria and sediment yield can be estimated using a watershed water quality model, and strategies for identifying critical areas in a watershed can be pollutant specific. The soil and water assessment tool (SWAT) model was used in the Upper Wakarusa watershed (950 km²) in northeast Kansas to explore effectiveness of vegetative filter strip (VFS) lengths applied at the edge of fields to reduce non-point source pollution. The Upper Wakarusa watershed is a high priority total maximum daily load (TMDL) designation watershed for fecal bacteria in Kansas. This study characterizes fecal bacteria sources (human, livestock, and wildlife) and targets VFS to abate sediment and fecal bacteria pollution from the Upper Wakarusa watershed.Objectives of this study were to test the effectiveness of VFS lengths (0, 10, 15 and 20 m) for removing overland process sediment and fecal bacteria concentration, rank sub-watersheds after determining sediment and fecal bacteria contribution of each sub-watershed, and demonstrate the SWAT model's ability to evaluate effectiveness of VFS application to abate sediment and fecal bacteria using targeted and random approaches to select 10, 25 and 50% of the sub-watersheds. The 15-m VFS reasonably reduced fecal bacteria concentration in the watershed. The greatest difference between the target and random approaches to fecal bacteria reduction was at 50% VFS adoption; the target approach removed about 60% of fecal bacteria, and the random approach removed about 42%. For sediment yield, the greatest reduction was at 25% VFS adoption; the target approach removed about 63% of sediment yield, and the random approach removed about 33%. A targeted watershed modeling approach using SWAT was effective at reducing both fecal bacteria concentration and sediment yield.     

Development and application of a modeling approach for surface water and groundwater interaction

Investigation of the interaction of surface water (SW) and groundwater (GW) is critical in order to determine the effects of best management practices (BMPs) on the entire system of water resources. The objective of this research was to develop a modeling system for considering SW–GW interactions and to demonstrate the applicability of the developed system. A linked modeling approach was selected to consider SW–GW interaction. The dual-simulation scheme was developed to consider different time scales between a newly developed surface model: Dynamic Agricultural Non-point Source Assessment Tool (DANSAT), and existing groundwater models; a three-dimensional finite-difference groundwater flow model (MODFLOW) and a modular three-dimensional transport model (MT3D). A distributed and physically based DANSAT predicts the movement of water and pesticides in runoff and in leachate at a watershed scale. MODFLOW and MT3D simulate groundwater and pesticide movement in the saturated zone. Only the hydrology component of the linked system was evaluated on the QN2 subwatershed in the Nomini Creek watershed located in the Coastal Plain of Virginia mainly due to lack of observed data for MT3D calibration. The same spatial scale was used for both surface and groundwater models while different time scales were used because surface runoff occurs more quickly than groundwater flow. DANSAT and MODFLOW were separately calibrated using the integrated GW approach which uses own lumped baseflow components in DANSAT, and using the steady-state mode in MODFLOW, respectively. Then the linked system was applied to QN2 based on the parameters selected for DANSAT and MODFLOW to simulate time-dependent interactions on the entire system. The linked approach was better than the integrated approach for predicting the temporal trends of monthly runoff by improving the monthly Nash–Sutcliffe efficiency index from 0.53 to 0.60. The proposed linked approach will be useful for evaluating the impacts of agricultural BMPs on the entire SW–GW system by providing spatial distribution and temporal changes in groundwater table elevation and enhancing the reliability of calibrated parameter sets.     

A framework for priority non-point source area identification and load estimation integrated with APPI and PLOAD model in Fujiang Watershed, China

The non-point source (NPS) pollution is difficult to manage and control due to its complicated generation and formation. In large scale watersheds, the priority sources areas (PSAs) identification is an important and necessary process for efficient aquatic environmental management. Here, a framework for the PSAs identification and pollution load estimation in PSAs screened for Best Management Practices (BMPs) is proposed. Fujiang Watershed, a branch of Jialingjiang, the upper reach of Yangtze River, was chosen for evaluation of the method proposed here. The entire Fujiang Watershed was divided into 21 subbasins, after which the Agricultural Pollution Potential Index (APPI) was used to identify the PSAs, and a modified runoff coefficient was introduced to mitigate the impact of the rainfall heterogeneity in the process. Next, the identified PSAs were further divided into 34 subbasins, after which quantification of the pollution load was conducted using the Pollution Load (PLOAD) model. The results indicated that there are five subbasins have much higher NPS pollution load intensities, with an average value of 6.05 t/km²/year for TN and 0.31 t/km²/year for TP. According to the cluster analysis on land use structure, these five subbasins were featured by higher proportion of agricultural land, suggesting a need for better fertilizer application management. The method developed here provided a helpful framework for conducting NPS pollution management in a large watershed.     

海河流域农田氮磷面源污染的空间分布特征及关键源区识别

[目的]分析海河流域农田氮磷面源污染及对水质影响的空间分布特征,并识别关键源区,以期为该流域面源污染治理提供参考依据.[方法]基于InVEST营养物传输率模型和产水量模型,估算了海河流域农田氮磷入河负荷、河流断面氮磷入河通量和潜在氮磷径流质量浓度,结合GIS空间热点分析、水文网络分析识别面源污染关键源区.[结果]海河流...     

Effects of snowmelt on phosphorus and sediment losses from agricultural watersheds in Eastern Canada

Snowmelt is the most important hydrological event in cold climates. However, snowmelt effects on suspended sediment (SS) and phosphorus (P) loss are poorly documented in Canada. Using two agricultural watersheds in Eastern Canada, this study aimed to quantify SS and P loss during the snowmelt period and to investigate how snowmelt contributes SS and P loss. Water samples were collected from the outlets of the Bras d’Henri watershed (BHW, 2007-2009) and Black Brook watershed (BBW, 2008-2009) and measured for SS and P concentrations. Hydrological parameters (precipitation, snow water equivalent, and runoff discharge), soil frozen status and soil temperature were also measured. Results revealed inter-annual variation of snowmelt conditions and SS and P losses in each watershed. The 2008 snowmelt in BHW and BBW mainly occurred on unfrozen soils, while the 2007 and 2009 snowmelts in BHW and 2009 snowmelt in BBW mainly on frozen soils. In BHW, 2008 snowmelt caused much higher median concentrations of SS, total P (TP), dissolved P (DP) and particulate P (PP) in stream water than 2007 and 2009; ratios of PP fractions in TP were variable with events but the median values were similar, suggesting both DP and PP important contrubutors to TP loss. In BBW, the median concentration of dissolved reactive phosphorus (DRP) in stream water was greater in 2008 snowmelt than in 2009 snowmelt; PP dominated TP loss. This study also suggests that soil state (i.e. frozen status) and rainfall were the most important factors influencing SS and P losses during snowmelt. Furthermore, snowmelt P export represented more than 20% of the total annual P export in BHW, and more than 12% of the annual DRP export in BBW. Thus, we strongly recommend adopting Best Management Practices (BMPs) that specifically target sediment and P loss during snowmelt.     

Simulation of land use impacts on groundwater levels and streamflow in a Virginia watershed

Increase in withdrawal and decrease in recharge of groundwater due to urbanization influences subsurface flow regimes. The overall objective of this study was to determine the impact of land development activities on the subsurface flow regime in the Upper Roanoke River Watershed (URRW). A regional groundwater model of the URRW was constructed using Modular Three-Dimensional Ground-Water Flow Model (MODFLOW) and calibrated for steady-state conditions. Then, eight land use management scenarios were simulated on the Back Creek (BC) subwatershed to assess the impacts of residential density, residential location, and land-cover on hydraulic head of groundwater and streamflow. The average recharge output from the Hydrological Simulation Program, FORTRAN (HSPF) simulation was used as the direct input to MODFLOW to take changes in land use into account in the BC watershed. Development of agriculture and forest areas with low-population density on larger area (low-density scenario), near the middle of the watershed (mid-section scenario), and with changes all open space to lawn (lawn scenario) had greatest overall impact on the BC watershed for both hydraulic head and streamflow among density, location, and land-cover scenarios, respectively. The simulated scenarios indicated that decreases in both hydraulic head and streamflow coincided with the increases in impervious land. The reductions in hydraulic head and streamflow were restricted to the subwatershed where land use changes occurred. The urbanization impacts on both surface and subsurface regimes were very local with 20.8 cm of maximum difference in local hydraulic head and 0.532% of maximum percent difference in local streamflow at lawn scenario while average corresponding values through BC watershed was 4.3 cm and 0.153%, respectively. Use of a fully distributed surface model in a dynamic manner was recommended to solve the inconsistencies in the spatial and temporal scale of surface and groundwater models. However, the proposed approach can be used as a management and planning tool for evaluating the local and overall impacts of land use management on the surface and subsurface flow regimes.     

Hydrochemical and stable isotopic assessment of nitrate contamination in an alluvial aquifer underneath a riverside agricultural field

Major ions and stable isotopic (δD_water, δ¹⁸O_water, δ¹⁵N_nitrate, δ¹⁸O_nitrate) measurements in concert with hydrochemical modeling were used in order to elucidate the sources and geochemical processes controlling nitrate contamination of shallow alluvial groundwater underneath a riverside agricultural field in the Buyeo area, Korea. Beneath vegetable fields in the sandy soil, the mean nitrate concentration of groundwater was 148.6 mg/L, which is significantly higher than in groundwater (mean 28.8 mg/L) beneath silty soils underneath rice paddy fields. Nitrogen isotope data indicate that synthetic fertilizers are the predominant source of groundwater nitrate in the study area. Denitrification during recharge through rice paddy soils appears to be responsible for the lower nitrate concentrations in groundwater beneath the silty soil zone. The relationship between nitrogen and oxygen isotope data of nitrate also suggests mixing of two different groundwater bodies with nitrates from the silt zone and the sand zone. Geochemical mass balance modeling on hydrochemical data indicates that various agricultural chemicals such as urea, lime, magnesium sulfate and potassium chloride dissolve in vegetable fields of the sandy zone, resulting in significant enrichment of various solutes such as K⁺, Ca²⁺, Mg²⁺, NO₃⁻, SO₄²⁻ and Cl⁻. As a consequence of over-utilization of synthetic nitrogen fertilizers, the sand zone is characterized by very high nitrate concentrations in the groundwater. This study suggests that a reduction of over-fertilization especially on vegetable fields in the riverside sand zone is required to minimize the nitrate contamination of groundwater. This study also shows that combination of geochemical and isotopic techniques with simple mass balance modeling provides information about the causes and processes of nitrate contamination of groundwater underneath a riverside agricultural field. The study also provides sustainable measures to optimize fertilization rate as an important basis of eco-friendly agriculture.     

深圳水库污水截排工程规模的选择

在如何选定污水截排工程的截排规模上，分析了套用市政排水截流倍数或选取某频率洪峰流量来确定截排流量的方法的不足，结合水库水质以面源污染为主的特征，采用综合选择方法——即推求典型年降雨所形成的全年逐时径流过程和全年径流量，计算各截排流量下的截排水量，通过水库水质模型，根据各截排流量对应未截排水量所携带的入库污染负荷量，建立截排流量与水库水质的关系，最终选定符合水库水质标准的截排流量为工程规模。     

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

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.     

Evaluation of fertigation scheduling for sugarcane using a vadose zone flow and transport model

Micro-irrigation has become an optimal means for providing water and nutrients to crops. There is an ample space for improving fertilizer use efficiency with micro-irrigation, if the movement and reactions of fertilizers in the soil are well understood. However, the rhizosphere dynamics of nutrients is very complex, depending on many factors such as soil temperature, pH, water content, and soil and plant characteristics. Many factors cannot be easily accurately quantified. However, using state-of-the-art modelling techniques, useful and reliable information can be derived.An attempt was made to evaluate the reactive transport of urea in the root zone of a sugarcane crop under drip irrigation, and to quantify the fluxes of urea, ammonium, and nitrate into the crop roots, volatilization fluxes, and deep drainage using a numerical model. This quantification helped in designing an optimal fertigation schedule. Various parameters used in the model were taken from either the literature or the field study. A typical scenario, based on the recommended total quantity of urea for sugar cane crop under drip irrigation in India, was tested using HYDRUS-2D. The total amount of urea was divided into fortnightly doses, depending on the stage of crop growth. For this scenario, the modelled crop uptake was found to be 30% higher than the crop demand. Consequently, an optimal fertigation schedule was developed that reduced the use of urea by 30% while at the same time providing enough N for its assimilation at all stages of crop growth. This type of modelling study should be used before planning field experiments for designing optimal fertigation schedules.     

Using a hydro-dynamic flow model to plan maintenance activities and improve irrigation water distribution: application to the Fordwah distributary in Punjab, Pakistan

In secondary canals in Pakistans Punjab, the waterdistribution depends on the hydraulic characteristicsof channels, cross-structures and tertiary outlets.Maintenance of channels and structures plays a crucialrole in upholding equitable distribution of water tothe tertiary units. In the past, maintenance has beenundertaken by irrigation managers based on experienceand observations. In the present study a hydro-dynamicmodel (SIC – Simulation of Irrigation Canals) is usedto assess a priori the impact of maintenance measureson water distribution. Maintenance measures can thusbe selected that remove existing bottlenecks in thewater distribution in the most cost-effective way. The methodology is applied to a secondary canal insouth-east Punjab. The simulation results show thatthe main cause of present inequity in waterdistribution are deviations in dimensions of tertiaryoutlets, which, if restored, would make the mosteffective contribution to improve the waterdistribution. Maintenance of the channel is necessaryif its capacity diminishes.     

Modification of the SPUR rangeland model to simulate species composition and pasture productivity in humid temperate regions

Plant, water, and soil components of the Simulation of Production and Utilization of Rangelands model (SPUR 2.4) were incorporated into the Integrated Farm System Model (IFSM 1.2) to represent the growth and competition of multiple plant species in pastures and their effects on pasture productivity and botanical composition in temperate climates. Developed for semi-arid rangelands, SPUR required major adjustment to represent temperate pastures adequately. In particular, the effects of soil moisture on root and shoot mortality and photosynthetic rates were adjusted to represent greater susceptibility of temperate plants to drought. Sensitivity analysis showed that predicted total shoot dry matter appeared most sensitive to photosynthesis and growth parameters in the spring, soil moisture parameters in the summer, and senescence parameters in autumn. Across all seasons, shoot dry matter appeared most sensitive to optimum photosynthetic temperatures, specific leaf area, start and end dates of senescence, maximum nitrogen concentration in live shoots, and a maximum shoot specific growth rate. The revised pasture model incorporated into IFSM was calibrated with 2002 field data from experimental pastures in central Pennsylvania, USA containing primarily orchardgrass (Dactylis glomerata) and white clover (Trifolium repens). Predictive accuracy of the model was then further evaluated by comparing 2003 data from the same pastures to simulated production. The integrated submodel predicted soil water content and dry matter production relatively well. It did not achieve a desired degree of accuracy in predicting the dynamics of botanical composition; however, adjustment of SPUR subroutines to allow variable maximum root:shoot ratios and competition for light and water may improve predictions. Further development and use of this integrated model can help researchers improve their understanding of temperate pasture systems, identify gaps in knowledge, and prioritize future research needs. Ultimately, the integrated model could provide more accurate assessment of the influence of management strategies on pasture productivity, animal production, and economics at the whole-farm scale.     

Development and credibility assessment of a metamodel relating water table depth to agricultural production

Phreatic groundwater pumping is affecting water availability for crops in areas with a shallow water table. This can reduce crop growth and so affect farm income. There is a need for a generic and transparent method to assess the agricultural damage caused by water table drawdown. This paper proposes such a method that consists of ‘damage tables’ relating agricultural production losses to the groundwater regime for different soil/crop combinations found in Northern Belgium. The damage tables are constructed based on numerous simulations with the agrohydrological model SWAP, in which the bottom boundary conditions are gradually changed to reflect different groundwater regimes. The credibility of the resulting metamodel is assessed in three ways: using (1) field data, (2) an existing local expert system for land suitability assessment and (3) literature applying to a wider region. Field data of actual transpiration for two grasslands do not systematically deviate from the model predictions. This provides some credibility to the claim that the model captures the processes determining evapotranspiration and agricultural production. The local expert system allows us to evaluate the range of groundwater regimes where optimal growth is expected for maize and grassland across different soil types. Diverging predictions of the optimal groundwater regime between the metamodel and the local expert system can be explained in terms of differences in assumptions underlying both models. One notable limitation of the damage tables is that only direct physiological stress is reckoned while indirect effects of wet conditions (decreased accessibility of the terrain, soil structural damage) may also limit growth on soils with a water table near the surface. Further comparison with literature data focused on two issues: the contribution of groundwater to evapotranspiration and the extinction depth, i.e., the depth at which groundwater no longer contributes to evapotranspiration. This comparison revealed that damage tables developed for our area of interest are only valid under similar climatic conditions for the following two reasons: they assume a relatively small groundwater contribution to evapotranspiration, which is typical for humid climates, and they take into account temporal variations in plant characteristics such as root depth, which is also climate dependent.     

A dynamic bio-economic model to simulate optimal adjustments of suckler cow farm management to production and market shocks in France

Tactical adjustments to seasonal weather conditions and beef price may generate additional income or avoid losses in French beef cattle farms. Due to the length of the suckler cow production cycle, adjustment decisions may impact not only on current production and profit but also on future farm outcomes. To better understand the consequences of shocks and subsequent production adjustments on the evolution of farm earnings and production over time, we built a dynamic recursive bio-economic farm model. Our model introduced simultaneously the possibility of adjusting herd size and herd composition, diet composition and diet energy content, as well as crop rotation, haymaking and feed stocks, taking into account both their short- and long-term consequences. An application is provided to test impacts of crop yield and beef price shocks of different intensities. Main simulated adjustments to face unfavourable weather shocks are (1) purchased feed in order to maintain animal production objectives, and (2) area of pasture harvested for haymaking. Very severe beef price shocks induce forced sales. Weather shocks affect farm net profit not only of the current year but also of the following years. Profit losses caused by unfavourable weather conditions are not compensated by gains in favourable ones and this differential is amplified when intensity of shocks rises.     

Assessment of groundwater quality and its variations in the capture zone of a pumping well in an agricultural area

Agricultural activities are frequently associated with water contamination. The spreading and storage of fertilizers, for instance, may result in groundwater contamination due to pollutants leaching into an aquifer. Nitrates and fecal bacteria are two important contaminants associated with agriculture. Thus, the development of efficient strategies for groundwater protection in agricultural areas requires an assessment of these two contaminants. Given this perspective, groundwater quality monitoring was carried out over the whole capture zone of a municipal well located in an agricultural area in the St.-Lawrence Lowlands in Québec. Thirty-eight piezometers were installed within the roughly 2 km² capture area of the well to measure physico-chemical parameters such as major ions, field measured parameters (pH, electrical conductivity, dissolved oxygen, water level, temperature), and isotopic ratios, bacteriological parameters (Heterotrophic Plate Count—HPC, enterococci, total coliforms, Escherichia coli) and their variations in space and time. Groundwater was sampled from the pumping well and the piezometers during 25 field campaigns in 2005, 2006 and 2007. The results demonstrate the impact of agricultural activities on nitrate contamination. They indicate high spatial and temporal variations in nitrate concentrations, from 6 to 125 mgNO₃⁻/L within the capture area, with 40% of the samples exceeding the Québec drinking water limit of 45 mgNO₃⁻/L. Nitrate pollution in the municipal well exceeded 45 mgNO₃⁻/L during 2005, but no bacteriological contamination was observed. The results also show a high variability of nitrate concentration with depth within the capture zone. Electrical conductivity appears as a good indicator of the presence of nitrate and calcium ions in this capture zone. Correlations between nitrate, calcium and chloride suggest that these ions come from the same source of fertilizer. Nitrate isotopic composition suggests that nitrate in groundwater originates from both chemical and organic fertilizers. The bacteriological results show that the extracted volume of water during sampling of a piezometer has a significant impact on the bacteria count. The variability of bacteriological pollution is important in space and time, showing a higher contamination during summer. Only 2% of the raw water samples exhibit contamination exceeding the drinking water standard for treated water. Total coliforms seem to be a good precursor of E. coli or enterococci contamination. Globally, the physico-chemical and bacteriological groundwater quality within the studied capture area and the pumping well shows contamination by nitrates, but low contamination levels by fecal bacteria.     

撒施肥料一维畦灌地表水流与溶质运移模型构建与验证

模拟撒施肥料下的一维畦灌地表水流与溶质运移过程可为采用先进的畦灌液体施肥方式提供对比依据。该文基于湍流理论垂向流速线性与对数分布规律及不可压缩流体力学连续方程,构造沿畦长及任意垂向断面的非均布流速场和溶质浓度场,建立起撒施肥料下的一维畦灌地表水流与溶质运移模型,并利用典型畦灌施肥试验结果,检验该模型的模拟效果。结果表明,建立的模型不仅具有在撒施肥料状况下较好模拟地表水流运动和溶质浓度时间变化过程的能力,还具备较佳的水量和溶质质量守恒性,从而为评价撒施肥料下的畦灌施肥系统性能及与其它施肥方式下的畦灌施肥系统性能对比,提供了实用的数值模拟工具。     

水稻灌区农业面源污染物迁移转化规律模拟研究

模拟水稻灌区中非自然水文过程及其驱动下的面源污染物迁移转化过程对了解水稻灌区面源污染形成机理以及污染控制具有重要的意义。将水稻灌区水文过程分为陆面水文过程和排水沟道水文过程,采用均衡法模拟陆面水文过程中稻田深层渗漏水量以及氨氮(NH4~+)、硝氮(NO_3~-)和磷酸根(PO_4~(3-))的渗漏通量,在此基础上,基于非稳定饱和渗流方程计算稻田深层渗漏过程所形成的侧向排水过程,作为排水沟道水文过程的输入项,基于动力波方程和一阶动力学方程描述了水稻灌区排水网络下的面源污染物运移和转化过程,提出了水稻灌区农业面源污染物迁移转化模型。根据前郭灌区2008—2009年的监测数据对模型渗流过程、主要面源污染物迁移转化过程参数进行率定,采用2010年的监测资料对模拟结果进行了验证。结果表明,基于所提出的方法模拟了稻田和排水沟道2个水文过程中NH4~+,NO_3~-和PO_4~(3-)的质量浓度峰值过程的差异,实现了水稻灌区陆面水文和排水沟水文过程的耦合。稻田水层中模拟NH4~+,NO_3~-和PO_4~(3-)的Nash-Sutcliffe系数分别为0.772、0.758和0.709,相对均方根误差(RMSE)分别为0.042、0.050和0.071,排水系统中模拟NH4~+、NO_3~-和PO_4~(3-)污染物的Nash-Sutcliffe系数分别为0.645、0.704和0.854,相对均方根误差(r RMSE)分别为0.072、0.060和0.031。所提出的方法可有效地模拟稻田渗流过程和排水过程中的面源污染物迁移转化过程。