基于SWAT模型的东北水稻灌区水文及面源污染过程模拟

灌区水文过程对于面源污染物的迁移、转化起到了重要的驱动作用。为揭示东北水稻灌区的水文及面源污染过程,该文在多年试验的基础上,运用修正的SWAT模型对其开展了模拟研究。2009-2011年在吉林省前郭灌区针对水稻生育期及冻融期内的灌区水文过程和农田面源污染物迁移、转化过程开展了系统的监测与试验。水稻生育期内,各级排水系统表现出不同的水文过程:末级排水沟中,由田间通过表层渗流进入排水沟的高浓度的水被灌溉退水所稀释,汇流排水沟的槽蓄量则在很大程度上影响了排水过程及污染物的对流和掺混过程。根据水稻灌区水文特性,以汇流排水沟为子流域,分别采用非稳定渗流公式和马斯京根法描述子流域中稻田向排水沟的渗流排水过程以及向子流域出口的排水汇流过程。试验和模拟结果表明:铵氮(NH4+)、硝氮(NO3-)和化学需氧量(COD)的浓度变化主要取决于排水过程,表层渗流和深层渗流过程决定了排水沟中NH4+和NO3-浓度过程,而排水沟中COD浓度还受到灌溉退水的影响。采用溴(Br-)作为示踪剂,通过测定土壤含水率、温度及示踪剂浓度变化,研究了冻结期的水文过程和面源污染物迁移过程,示踪试验结果显示,冻融期土壤中水流运动受到土壤基质势、温度势及重力势的影响,冻土中平衡状态下基质势为土壤温度的函数,土壤中污染物渗出通量与水分渗出通量表现出线性关系。基于水稻灌区下垫面产汇流特性和冻融期土壤对于灌区水文过程以及面源污染物迁移的影响机理,在SWAT模型模块修订的基础上,模拟了东北地区水稻灌区面源污染迁移流失过程,模拟流量、NH4+、NO3-、COD浓度与实测值符合较好,表明改进的模型能够用于东北地区水稻灌区的水文及面源污染过程模拟。

Simulation of drainage and agricultural non-point source pollutions transport processes in paddy irrigation district in North-East China using SWAT

Abstract: Hydrological processes are the driving force of transportation, transformation, and accumulation of agricultural non-point-source pollutants in a paddy irrigation district. The aim of this study was to simulate hydrological processes and non-point source pollution transport processes in a complex irrigation and drainage system in north-east China. Field experiments were conducted to measure quality and quantity of leakage and irrigation return water in field and drainage system in the Qianguo irrigation district (Jilin, China) during the period from 2009 to 2011. The hydrological processes in the various drains were different. In the field canals, the subsurface flow and leakage with high N concentrations were diluted by irrigation return water. The storage process in the lateral drains impacted the drainage process and the convection and mixing processes of the non-point pollutants significantly. In a lateral irrigation canal controlled region, the subsurface flow and the direct seepage flow from paddy fields through the side walls of field drains to the drains were simulated using unsteady flow equation. The water and contaminants in the field drains converged to lateral drains, and finally reached the main drain. A modified Muskingum method was proposed to calculate the processes of water flow and chemical transport in the drainage system. The results from the field experiments and simulations indicated that the fate of ammonium (NH4+), nitrate (NO3-), and chemical oxygen demand (COD) in the system were primarily controlled by the drainage processes. The NH4+and NO3- transport processes were mainly affected by the surface leakage and the deep leakage process, respectively. Besides from irrigation return water, the COD mass discharged from paddy fields to drains was through both subsurface flow and deep leakage. The results clearly showed the contributions of different drainage processes to non-point source pollution in a complex drainage system during the rice-growing period. Bromide (Br-) was used as tracer to investigate water flow and solution transport in the soil during the freezing and thawing periods. The redistributions of soil water, temperature, and Br- tracer were monitored. In the frozen soil, water movement was caused by the temperature gradient, matric potential gradient and gravity. The matric potential of unfrozen water in frozen soil at an equilibrium state was estimated using a temperature-based function. A linear relationship was observed between solution flux and soil water flux. The SWAT was modified and applied successfully to simulate the drainage and contaminants (NH4+, NO3- and COD) transport processes. The methods and results from this study should be useful to characterize non-point source pollutions in paddy irrigation district of north-east China.