多级拦水堰坝调控农田溪流营养盐滞留能力的仿真模拟

为揭示多级拦水堰坝对于低等级小河流营养盐滞留能力的影响,以巢湖流域某一典型的农田源头溪流为对象,在野外示踪实验和计算机模拟的基础上,针对构建的多级简易水坝,采用暂态存储和营养螺旋指标,仿真模拟多级拦水堰坝对于氮磷营养盐滞留能力的调控效果。结果表明:多级拦水堰坝Darcy-Weisbach阻力系数明显超过无堰坝情形,但其弗劳德数Fr和雷诺数Re则较无堰坝情景低些;多级拦水堰坝相应的交换长度Ls值较无堰坝情景低1~2个数量级,水力持留因子Rh则较无堰坝情景明显增大,表明多级拦水堰坝使溪流的暂态存储能力得到很大提升;在多级拦水堰坝情景下,NH4+和PO43-的吸收长度Sw均有大幅度的下降,其中NH4+削减幅度达70.27%~89.47%,PO43-为75.59%~81.92%;不仅如此,在多级堰坝情景下,NH4+和PO43-的物质传输系数Vf、吸收速率U均显著增大,表明多级拦水堰坝可以有效提高农田溪流氮磷营养盐的滞留潜力。

Experimental manipulations of efficiency of nutrients retention by multilevel low-grade weirs in an agricultural headwater stream

To investigate the effects of multilevel low-grade weirs on nutrient retention of low-order streams, an agricultural headwater stream on the Nanfeihe River in Chaohu Lake basin was selected to describe the regulation of nitrogen and phosphorus retention efficiency in response to multilevel mimic weirs by using transient storage and nutrient spiraling metrics, based on three groups of tracer addition experiments and mathematical simulations. Study results show that the Darcy-Weisbach resistance coefficients in multilevel low-grade weirs situations are markedly higher than that of no weir, while the Froude numbers, Fr, and the Reynolds numbers, Re, are just opposite to it. The values of exchange length L_s, corresponding to multilevel low-grade weirs scenarios,are lower to two orders of magnitude than that of no weir,but its hydraulic retention factors are significantly higher than the later, suggesting that multilevel weirs could markedly improve the transient storage efficiency of agricultural headwater streams. Under the situations of multilevel low-grade weirs, the values of uptake length S_w corresponding to NH₄⁺ and PO₄^3- all greatly decrease, and its reductions are 70.27%~89.47% and 75.59%~81.92%, respectively. Moreover, the values of mass transfer coefficient, V_f, and uptake rate, U, corresponding to NH₄⁺ and PO₄^3- all increase significantly in the scenarios of multilevel weirs,indicating that multilevel low-grade weirs could effectively improve the retention efficiency of nitrogen and phosphorus in agricultural headwater streams.