Managed aquifer recharge and changes in crop type or nutrient management on agricultural lands are promising approaches to address groundwater quality degradation by nitrate. Tools to assess nonpoint-source contaminant transport are needed to better understand the interaction between agricultural management practices and long-term nitrate dynamics in groundwater basins. This study investigates the impact of time-resolution upscaling of groundwater flow stresses (i.e., recharge, pumping, and evapotranspiration rates) on the long-term prediction of nitrate transport at the regional scale. A three-dimensional, monthly transient flow and nitrate-transport model using MODFLOW and MT3D is applied as the reference simulation. The reference model results are compared to temporally upscaled models with (1) upscaled annual-averaged flow and transport stresses and (2) steady-state flow stresses, across different management scenarios. Models with annual-averaged flow and nitrate-loading stresses were found to be the best alternative to the reference model. However, employing a steady-state flow field to parameterize transient transport models, using a time series of spatially variable annual total contaminant loading, provides a useful alternative to predict the trend and variability of nitrate-concentration breakthrough curves at wells across the regional scale and to differentiate the effects of various agricultural management scenarios, if the history of the source contaminant mass is known. The difference between concentrations resulting from steady-state-flow versus transient-flow models is less than 2 mgN/L for nearly 75% of shallow groundwater cells in the model. However, the steady-state-flow-model-based transport simulation does not capture short-term oscillations of nitrate concentrations in pumping wells at the local scale.

有管理的含水层补给以及农用土地上作物类型的变化或养分管理的变化，是解决硝酸盐对地下水质量下降的有前途的方法。需要使用评估非点源污染物迁移的工具来更好地了解农业管理实践与地下水盆地中长期硝酸盐动态之间的相互作用。这项研究调查了地下水流应力的时间分辨率提升（即补给，抽水和蒸散速率）对区域规模硝酸盐运移的长期预测的影响。使用MODFLOW和MT3D的三维月度瞬态流量和硝酸盐迁移模型被用作参考模拟。在不同的管理场景中，将参考模型的结果与时间上的扩展模型进行比较，该模型具有（1）扩展的年度平均流量和运输压力以及（2）稳态流量压力。发现具有年平均流量和硝酸盐负荷应力的模型是参考模型的最佳替代方案。但是，采用稳态流场来参数化瞬态传输模型，使用空间可变的年度总污染物负荷的时间序列，可提供有用的替代方法，以预测区域范围内各井的硝酸盐浓度突破曲线的趋势和变化。如果已知源污染物质量的历史记录，则可以区分各种农业管理方案的影响。对于模型中将近75％的浅层地下水单元，稳态流模型与瞬态流模型产生的浓度之间的差异小于2 mgN / L。但是，基于稳态流模型的运输模拟无法捕获局部规模的抽水井中硝酸盐浓度的短期波动。