芦苇占优势农田溪流营养盐滞留的水文和生物贡献分析

Contribution of hydrological and biological processes to nutrient retention in an agricultural headwater stream predominated by Phragmites australis

为解析大型水生植物大量生长情形的源头溪流营养盐滞留的水文、生物作用贡献,于2014年9月—2015年6月在南淝河流域某一芦苇占显著优势的农田源头溪流段,开展8次野外示踪实验,计算渠段雷诺数Re、弗劳德数Fr、曼宁糙率系数n等水动力学参数,估算NH4^＋、PO4^3-的总滞留率以及水文、生物过程的实际滞留贡献率和相对滞留贡献率。结果表明,整个研究期间溪流水体表现出显著的紊流特征,水流流态属于缓流类型,渠道曼宁糙率系数n变化范围为0.066-0.112,平均值为0.089;NH4＋总滞留损失率变化范围为9.17%-28.27%,平均值为14.68%,水文作用和生物过程对NH4^＋实际滞留贡献率平均值分别为10.12%和4.57%,相对贡献率平均值分别为72.51%和27.49%,表明NH4＋滞留损失主要来自水文过程的影响;PO4^3-总滞留损失率变化范围为5.75%-17.79%,平均值为12.53%,水文作用和生物过程对PO43-实际滞留贡献率平均值分别为10.12%和2.41%,相对贡献率平均值分别为81.42%和18.58%,表明PO43-滞留损失也主要来自水文因素的影响;n与Re、Fr和Q均呈显著的幂函数关系,而实际滞留率ηNH4、ηPO4与n、Re之间均没有表现出明显的相关关系。

To examine the contribution of hydrological and biological processes to nutrient retention in aheadwater stream with growing lots of aquatic macrophytes,eight field tracer experiments were conducted atan agricultural headwater stream reach, predominated by Phragmites australis, in the Nanfeihe River fromSeptember 2014 to June 2015. Such three typical hydrodynamic parameters as Reynolds numbers（Re）,Froude numbers（Fr）,and Manning roughness coefficient（n） of stream flows were calculated firstly accord-ing to the hydraulic data. Subsequently,both the practical and relative contributions of hydrological and bio-logical processes to NH4＋and PO43-retention as well as the total retention ratios of the two nutrients wereestimated quantitatively. Study results show that the stream reach displayed striking turbulence characteris-tics over the eight tracer experiments and its flow status belonged to subcritical flow. The values of n ranged from 0.066 to 0.112,with a mean value of 0.089. The proportional NH4＋and PO4^3-retention rangedfrom 9.17% to 28.27% and 5.75% to 17.79%,with the averages of 14.68% and 12.53%,respectively. Thepractical contribution rates of hydrological and biological factors to NH4＋retention were 10.12% and 4.57%,respectively, and 10.12% and 2.41% for PO4^3-, respectively. The relative contributions of hydrological andbiological processes to NH4＋retention were 72.51% and 27.49%,respectively,and 81.42% and 18.58% forPO4^3-,respectively. The findings mentioned above indicate that hydrological process has a greater impact onthe retention of NH4^＋and PO4^3-for the studied stream reach. Moreover, relationship between the hydrody-namic parameters（i.e. Q, Re and Fr） and the Manning roughness coefficient（n） could be expressed byusing power functions, but no obvious relationship has been found between the practical retention ratio（i.e. ηNH4,ηPO4） and the following hydrodynamic parameters as n and Re.