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农业面源污染(简称“面源污染”)具有影响范围广、作用时间长、隐蔽性强等特点。目前,全球30% ~50%的地表水体受到面源污染的威胁[1-5]。河流生态缓冲带(简称“缓冲带”)是位于天然水体与污染区域之间,对面源污染物具有拦截、渗透、吸附、固化、降解作用的屏障区域[6]。构建缓冲带可实现对面源污染的阻控,还具有较好的生态价值、美学价值,实现水土保持等功效。已有学者研究表明,缓冲带对于径流的拦截效率为40%~100%[7-10],因此,缓冲带被认为是阻控营养物和农药等面源污染的最佳管理措施之一[11] 。20世纪80年代,美国自然资源和保护局将缓冲带作为最佳管理措施以减少面源污染带来的影响[12],美国农业部也提出了国家保护缓冲带的倡议[13]。在我国,构建缓冲带逐渐称为缓解城市或面源污染的重要手段[14]。缓冲带坡度、土壤特性、降雨强度、入流污染物的浓度和性质、入流流量、植被属性、规模、土地利用等多种因素均会对污染物的阻控效率产生影响。其中,缓冲带宽度尤为重要。通常,缓冲带宽度适宜与否直接影响其阻控效率的高低。因此,合理确定缓冲带的宽度是有效控制面源污染的关键[15]。在缓冲带的设计过程中,不同因素对其污染物截留效率的影响是极其复杂的,数学模型是对其进行研究的重要工具。
土壤和水评价工具(soil and water assessment tool,SWAT)是常用于模拟面源污染的模型之一,因其模拟效果较好已被广泛应用[16-17]。但SWAT在模拟缓冲带截留效果方面有一定的缺陷,即不能模拟地表径流路径,仅考虑了缓冲带宽度对截留效率的影响[18]。植被缓冲带模型(vegetative filter strips modelling,VFSMOD)由MUNOZ-CARPENA等开发[19],该模型已在北美的沿海平原[20]和北卡罗莱纳州皮埃蒙特[21]得到了验证,作者使用当地的自然事件数据对模型进行了测试,取得了很好的模拟效果。GHARABAGHI等[22]的研究也表明,模型预测值与入渗量、流出量和泥沙捕集效率的实测值之间存在显著的线性关系。自模型发布以来,在国外已经得到了广泛的应用和功能扩展。DOSSKEY等[23]利用该模型探讨了缓冲带面积与水沙截留效率的关系,研究结果表明,对于阻控可溶性污染物,在更陡峭的坡度、更细的土壤质地和更高的通用土壤流失方程(USLE)中的覆盖和管理因子(C因子)条件下,需要更大的缓冲带面积。
虽然VFSMOD模型在国外较为成熟,但针对国内的立地条件,对其进行验证、应用的研究仍然很少。本研究采用VFSMOD模型,模拟不同因素对缓冲带截留效率的影响,主要从土壤质地、坡度等方面对缓冲带在特定条件下对径流和沉积物的拦截效果进行模拟,并与现有的缓冲带研究相比较,以期对基于面源污染阻控功能的缓冲带设计提供参考。
基于VFSMOD模型的不同情景下河流生态缓冲带的截留效果模拟
VFSMOD model-based simulation of interception by ecological buffer zone under different scenarios
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摘要: 采用植被缓冲带模型(VFSMOD)对不同情景下河流生态缓冲带(简称“缓冲带”)的截留效果进行了模拟,考察了土壤可蚀性、土壤质地、坡度、径流源区长度、缓冲带宽度等对缓冲带拦截效果的影响。结果表明: 随着土壤可蚀性因子(K因子)变大,土壤可蚀性增强,缓冲带泥沙输入量变大;坡度变大,拦截效率下降,在坡度分别为1%、3.5%、9%和30%的条件下,达到60%的径流截留效率(RIR),缓冲带的宽度分别需要43.9,50.6,52.9和68.8 m;当RIR达到60%,砂质黏壤土、粉质黏壤土、壤土和黏土所需缓冲带的宽度为109.2、114.6、50.6、128.7 m;径流源区长度越长,缓冲带拦截效率越低。上述模拟结果可为缓冲带的设计提供参考。Abstract: Non-point source pollution has become an important water pollution source all over the world because of its complex process and wide range of influence. As one of the best management measures (BMPs) for non-point source pollution, ecological buffer zone (EBZ) has become a widely acknowledged solution. In this paper, the VFSMOD model was used to evaluate the effect of ecological buffer zone under different scenarios. The influence of soil erodibility, soil texture, slope, length of runoff source area and width of buffer zone on the interception effect of ecological buffer zone was investigated. The results showed that the soil erodibility increased with the K factor, causing the increase of the silt input into the buffer zone. The greater the gradient, the lower the interception efficiency was. With the gradient of 1%, 3.5%, 9% and 30%, the width of EBZ was 43.9, 50.6, 52.9 and 68.8 m, respectively, for reaching RIR of 60%. At the same RIR, the width of EBZ was 109.2 m for sandy clay loam, 114.6 m for silty clay loam, 50.6 m for loam and 128.7 m for clay. The longer the length of runoff source area, the lower the interception efficiency of buffer zone was. The simulation results could provide reference for the design of ecological buffer zone.
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表 1 通用参数的相关内容
Table 1. General parameter determination
总降雨量/mm 降雨持续时间/h 曲线号 风暴类型 径流源区长度/m 径流源区坡度 径流源区面积/hm2 土壤侵蚀K因子/((kg·N−1)·(h·m−2)) 25 1 88 II 200 1~30 10 推荐值 表 2 地表径流参数
Table 2. Surface runoff parameters
宽度/m 长度/m 计算节点数 时间加权系数 算法选择标记 库朗数 迭代次数 输出要素标记 各段曼宁系数/(s·m−1/3) 各段坡度/% 3~170 500 57 0.5 3 0.8 350 1 0.4 1~30 注:库朗数为调节计算的稳定性与收敛性的参数;曼宁系数为反映管渠壁面粗糙情况对水流影响的参数。 表 3 土壤参数
Table 3. Soil parameters
土壤饱和导
水率/(m·s−1)湿润峰处平
均吸力/m土壤初始
含水率土壤饱和
含水率最大表面
贮水量/m推荐值 推荐值 推荐值 推荐值 0 表 4 缓冲带性能参数
Table 4. Ecological buffer zone parameters
过滤介
质茎杆
间距/cm过滤介质
修正糙率/
(S·cm−1/3)过滤介
质高度/
cm泥沙淤满过滤带
后裸露表面的
糙率/(S·cm−1/3)沉积楔坡度及
表面糙度变化
反馈标记1.35 0.016 15 0.02 1 表 5 泥沙过滤模型泥沙特性参数
Table 5. Sediment characteristic parameters of sediment filtration model
入流泥沙浓度/(g·cm−3) 入流泥沙颗粒分级数 入流泥沙的中值粒径/cm 沉积泥沙的孔隙率 入流泥沙粗沙百分数 泥沙密度/(g·cm−3) 推荐值 7 推荐值 推荐值 0.50% 推荐值 表 6 不同土壤质地径流截留效率的拟合方程参数
Table 6. Fitting results of runoff interception efficiency of different soil texture
土壤质地 a b 调整后R2 砂质黏壤土 110.359±6.808 7.36±3.57×10-4 0.976 3 粉质黏壤土 117.875±8.399 0.006 35±6.82×10-4 0.989 5 壤土 99.384±1.142 0.016 92±4.58×10-4 0.996 5 黏土 116.378±11.335 0.005 7±7.975×10-4 0.985 0 注:a和b为式(3)中的拟合参数。 表 7 不同坡度径流拦截效率拟合方程
Table 7. Effect of widths of and ecological buffer zones with different slopes on runoff delivery ratio
坡度 a b 调整后R2 1 104.475±1.082 0.197±5.826×10-4 0.993 3.5 100.928±0 0.016 5±3.9 0.996 9 91.130±0.863 0.193±4.829×10-4 0.995 30 77202±0.519 0.019 9±3.855×10-4 0.997 注:a和b为式(3)中的拟合参数,式(3)为 。$y = a(1-{\rm{e}}^{-bx})$ -
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