Agricultural subsurface drainage can be an important conduit of nitrate from agricultural fields to streams. This study focused on understanding the variability in nitrate concentrations and loads, exported by subsurface drains, into a small, north-central Iowa stream. Ninety-three subsurface drains in this watershed were sampled up to 5 times between 2006 and 2008. Additionally, 2 subsurface drains and the stream draining the study area (South Fork Iowa River near Blairsburg, IA, USA) were sampled frequently during the growing seasons in 2007 and 2008. Spatial variability analysis revealed no distinct spatial pattern in nitrate concentrations. The median nitrate concentrations were not significantly different when the drain outlets were characterized by diameter (17–23 cm, 27–48 cm, 60–108 cm). The eight large subsurface drains (part of the public drainage network) had less variability in nitrate concentration than the smaller drain sizes and generally contributed 70–87% of the total water and nitrate loads exported by subsurface drains to the stream. During high-discharge events, the medium-sized (27–48 cm) subsurface drains discharging to the stream became more important by contributing a higher discharge and nitrate load. The temporal variability examined in this study found that discharge and nitrate loads were influenced by the amount of precipitation that had occurred over the previous months. This paper demonstrates the spatial and within-season homogeneity of nitrate delivery to a stream from an intensely agricultural landscape that has subsurface drainage.

农业地下排水可能是硝酸盐从农田到河流的重要管道。这项研究的重点是了解地下排水管输出到爱荷华州中北部的一条小溪中硝酸盐浓度和负荷的变化。在2006年至2008年之间，对该流域的93个地下排水渠进行了多达5次采样。此外，在生长季节中，还频繁采样了2个地下排水渠和流向研究区域的溪流（美国爱荷华州布莱尔斯堡附近的南福克爱荷华河）。在2007年和2008年。空间变异性分析显示硝酸盐浓度没有明显的空间格局。当排水口的直径为直径（17–23 cm，27–48 cm，60–108 cm）时，硝酸盐的中值浓度无显着差异。与较小的排水沟相比，八个大型地下排水沟（公共排水管网的一部分）硝酸盐浓度的变化较小，通常占地下排水沟向河道排放的水和硝酸盐总量的70-87％。在高排放事件中，通过贡献更高的排放量和硝酸盐负荷，向河流排放的中型（27–48 cm）地下排水变得更加重要。在这项研究中检查的时间变异性发现，排放量和硝酸盐负荷受前几个月降水量的影响。本文证明了硝酸盐在具有地下排水的密集农业景观中向河流输送的空间和季节内的均匀性。