高原农业洱海流域农田氮素径流损失模拟研究

Nitrogen is one of the most active agricultural soil nutrient elements, as well as a factor of the increasingly serious environmental pollution. Nitrogen runoff from farmland is an important contributor to water eutrophication and water quality deterioration of lakes and rivers. So far, there is little comprehensive research on the effects of slope and nitrogen fertilizer application on the nitrogen runoff using agricultural non-point source pollution model (Soil & Water Assessment Tool, SWAT）. The study area located in the Erhai Lake basin in Yunnan province which is a representative highland farming region. Rice, rape and corn crops were chosen as representative crops. The methods including field observation, field experiment, experiment simulation in lab and model simulation were used to study the effects of different nitrogen fertilizer applications and slopes on the nitrogen runoff. Based on the parameters obtaining from experiment, the sensitivity analysis, calibration and validation of model parameters had been conducted. Nitrogen runoff and its spatial distribution characteristics were estimated at the watershed scale and the critical source area of nitrogen runoff in Erhai Lake basin was identified based on model simulation. By this study, it could provide a basis for increasing nitrogen use efficiency, decreasing non-point source pollution of farmland and improving water quality of Erhai Lake.

氮素是农田土壤中最活跃的营养元素之一，同时也是日趋严重的环境污染因子，农田氮素流失对我国湖泊河流水体富营养化及水质的恶化构成了严重的威胁。迄今为止，综合考虑坡度和施氮量等因素对氮素径流损失的影响，并结合模型进行流域氮素径流损失的模型模拟，还缺乏系统、深入的研究。本项目以云南洱海流域作为代表性的高原农业研究区，选取水稻、油菜、玉米等主要典型作物，采用野外监测、田间试验、室内模拟以及模型模拟相结合的方法，研究不同氮肥用量、不同坡度对农田径流氮素损失的影响规律，应用SWAT（Soil and Water Assessment Tool）模型，在获取模型相关参数的基础上，对模型进行敏感性分析、校准与验证。通过模型模拟，从流域尺度上估算农田氮素径流损失量及其空间分布特征，明确洱海流域氮素径流损失的关键源区，解析影响机理，旨在为提高氮肥利用率、减少农田面源污染、保护高原湖泊洱海水环境安全提供依据。

洱海属于初级富营养化湖泊，洱海流域的农田氮素流失对洱海湖泊水体富营养化及水质的恶化构成了严重的威胁。为了了解洱海流域农田氮素径流损失量及其空间分布特征，明确洱海流域氮素径流损失的关键源区，解析影响机理，本项目采用实地监测、人工模拟降雨和模型模拟等手段开展了相关研究。.通过实地监测和SWAT模型模拟，洱海流域农业源对氮贡献79.5%，农业源其中种植业源占24.5%，养殖业源占70.8%，农村生活源占4.7%。通过监测分析，5-6月泡田期及7-8月农事活动与强降雨事件重叠期，是氮素流失防控的关键期。通过模拟降雨试验，在不同降雨条件下，水稻土、红壤总氮、硝氮和氨氮在前20分钟的浓度呈急速下降的趋势，20分钟之后下降趋势减缓并趋于稳定。基于SWAT模型识别了流域氮素损失的关键区，氮素输出负荷来源主要分布在距流域出口较近的子流域，占流域17.7%的面积贡献了40.7%总氮输出负荷。基于人类活动净氮输入量（NANI）方法进行了洱海流域氮素输入的解析，氮素的输入来源主要为食品/饲料输入和肥料氮投入，分别占总输入量的65.94%、31.54%；总输入的氮素中约有22.08%通过气态损失，约31.54%通过径流、淋溶等途径损失，约11.77%作为产品输出，约14.30%遗留在土壤中。整体上来看，洱海流域净氮输入强度空间分布上存在明显的南北区域差异，在各组成上北部高于南部。在模型参数修正方面，通过收集了全国范围内55个站点的实测降雨径流数据等数据，计算得到了修正的CN参数，修订后的CN更适合中国大多数自然条件的径流模拟。在采样频率方面，该流域的最佳负荷估算方法和采样频率为两周一次下的全局平均法或线性插值法。.关键区、关键期以及氮素流失特征规律等研究为洱海流域农业面源污染防控提供依据，采样频率的研究将有利于降低采样成本，提高负荷估算的精度。模型参数CN值的研究将进一步推动制定我国的CN值查找表。