Modeling Nutrients Exports by Rivers from Watersheds to River Mouth: Case Study of Beijiang River Basin

LI Lili1, LUAN Shengji2,3,†

1. Lee Kuan Yew School of Public Policy, National University of Singapore, Singapore 259774; 2. Key Laboratory for Urban Habitat Environmental Science and Technology of Peking University, Shenzhen 518055; 3. PKU-HKUST Shenzhen-hong Kong Institution, Key Laboratory of Environmental Simulation and Pollution Control, Shenzhen 518057; † Corresponding author, E-mail: luansj@pkusz.edu.cn

Abstract Global NEWS (Global Nutrient Export from Watersheds) is an international modeling effort with few Chinese application cases, and this essay applied the model with modifications to Beijiang River Basin, one of the three main sub-basins of the Pearl River, in order to estimate the river basin-level export of multiple nutrient elements and elemental forms from land sources within the river basin to river mouths. A reliable environmental database of Beijiang River Basin was established by literature review and statistics collection, and with the help of ARCGIS technology. Model calibration and verification showed that the Nash-sutcliffe model efficiency was 0.61 on DIN (Dissolved Inorganic Nitrogen) loads (t/a) exported at the basin mouth, indicating that the model performs reasonably well for DIN. Modelling results show that 1) in 2010, dissolved nitrogen exports (load) from Beijiang River Basin was 37.5 thousand t/a, which was 9.27% higher than that in 2000, with DIN accounting for 83.51% and DON (Dissolved Organic Nitrogen) accounting for 16.49%. 2) In 2010, dissolved phosphorus exports (load) from Beijiang River Basin was 46.3 thousand t/a, which was 30.05% higher than that in 2000, and contained 86.21% of DIP (dissolved inorganic phosphorus) and 13.79% of DOP (dissolved organic phosphorus). 3) Spatially, nutrients exports (load) from Sui River Basin, one of the downstream sub-basins, and nutrients exports (load) from Lian River Basin, one of the midstream sub-basins, were relatively higher than those from other sub-basins, indicating

the necessity of controlling nutrient pollution in the two sub-basins. 4) Atmospheric nitrogen deposition was the major source of DIN export load, followed by synthetic fertilizer and biological nitrogen fixation, while animal wastewater discharging was the major source of DIP export load, followed by synthetic fertilizer. The results also show that the NEWS model is applicable to China’s small-to-medium river basins. Key words Beijiang River Basin; nutrient exports; agricultural non-point pollution; Global NEWS model

近年来, 我国主要流域内农业技术、农业结构和农业对环境的影响等都在发生着转变。一方面,在农业生产中, 传统农业方式和现代农业生产要素并存; 另一方面, 农户依然维持小规模生产、低市场化的传统生产理念。农户在使用现代生产要素的过程中缺乏科学指导, 其生产方式与现代化生产要素的不匹配造成严重的农村内源性污染[1], 如化肥等生产资料不合理使用产生的污染。

20 世纪 80 年代以来, 珠江流域内大量营养盐随河流输送到河口及近海海域, 引起河口及近海海域富营养化, 造成赤潮频发、赤潮持续时间延长、

[2–5]水体缺氧等问题 。研究表明, 农业源污染是珠江水系营养盐污染的重要来源[6–7]。作为珠江的第二大支流, 北江的流域范围主要涉及粤北山区和广州市少部分地区, 是珠江流域的重要农业生产地区。本研究以北江流域为例, 研究流域内氮、磷等营养元素的循环过程, 应用国际上最新提出的流域营养盐输出模型 Global NEWS, 量化模拟流域内营养盐产生、汇集到水体及随河流输出的全过程, 为认识和控制流域营养盐污染提供理论依据和技术支持。

1 材料与方法1.1 研究区域概况

北江位于东经 111°55′—114°50′、北纬 23°10′— 25°25′之间, 流域总面积为 46710 km2, 其中广东省境内面积占总面积的 92%。本研究重点关注广东省境内污染问题。北江的主流浈江发源于江西省信丰县, 经江西大余县流入广东, 至韶关市的沙洲尾与武江汇流, 始称北江; 再自北向南流至三水河口,在思贤滘与西江汇合后进入珠江三角洲网河区, 其主流由虎门出海。流域内集水面积超过 1000 km2的支流共 13 条, 其中包含一级支流 9 条, 沿干流呈叶脉状排列, 从东西两侧汇入干流。流域内年平均降水量为 1800 mm, 变化范围在 1300~2400 mm 之间, 自南向北递减。流域内平均水面蒸发量为1000~1200 mm, 南北差异不大。北江流域多年平均径流量为 511 亿 m3, 多年平均流量为 1620 m3/s,

多年平均径流深 1092 mm, 多年平均径流模数为35 L/(s·km2)。

1.2 Global NEWS 模型介绍

Global NEWS 模型(Global Nutrient Export from Watersheds MODEL)是联合国教科文组织(UNESCO)开发的用于模拟流域营养盐输出过程的模型, 是机理模型、统计模型和经验模型的组合, 能够模拟多种元素的各形态营养盐在陆地系统上的循环过程及在河道中的迁移转化过程。本研究主要关注含氮(N)、磷(p)的溶解态营养盐。global NEWS 设计有溶解态无机氮子模型(NEWS-DIN[8])、溶解态无机磷子模型(NEWS-DIP[9])、溶解态有机氮子模型(NEWS-DON[10])、溶解态有机磷子模型 (NEWSDOP[10])。MAYORGA等[11]系统总结了各个子模型的概念框架和主要计算公式。溶解态营养盐子模型基于物质平衡的原理, 量化陆域上营养盐的各项输入、输出, 并且考虑营养盐由陆域输移到河道系统的过程以及在河道系统中的多项迁移转化过程。模型区分点源与非点源污染。点源的计算可参考 Van Drecht 等[12–13]的研究, 主要考虑污水处理厂点源。实际上, 在中国, 集约化养殖场由于通常有污水集中收集处理和排放设施, 排污量大且能够逐个统计, 因此也应视为重要的点源。由于中国畜禽散养模式所占比重逐渐减小, 且统计困难, 所以统计部门发布的畜禽养殖数量主要来自集约化养殖

[14]业 。因此, 本研究调整了模型的点源计算公式,增加了对养殖污水污染的计算。对于非点源, 模型主要考虑大气沉降源以及农业非点源数据, 可参考 Bouwman 等[15–16]的研究。陆域的汇主要考虑作物吸收。陆地表面盈余的非点源营养盐可以根据源和汇计算得到。由于这部分盈余的营养盐从污染源到受纳水体的输移过程中会受到阻隔, 所以只有一部分能够到达水体, 到达水体的量可根据经验公式计算[8,17–18]。对于点源营养盐, 根据污染物产生量和污水处理率计算到达受纳水体的污染物数量。此外, 模型还计算其他因淋溶或风化作用而进入水体的营养盐。在河道系统中, 溶解态营养盐可能因滞