Agriculturally-driven land use change and hydrologic modifications have influenced solute transport in midwestern U.S. streams. A clear understanding of the mechanisms driving nutrient export from agricultural watersheds will be critical in mitigating diffuse nutrient pollution, given anticipated shifts in hydrology associated with a changing climate. Specifically, more frequent, intense precipitation and altered snow patterns are predicted for the upper Midwest. We used four years of high-frequency nitrate (NO₃⁻–N) sensor data from two tile-drained, agricultural watersheds in Indiana to explore NO₃⁻–N export for 200 storms. We used concentration-discharge (C–Q) relationships and two indices, the hysteresis index (HI) and flushing index (FI), to understand physicochemical controls of NO₃⁻–N export across time scales. On both annual and seasonal time scales, we found NO₃⁻–N concentrations were largely chemostatic; however, patterns in FI suggested C–Q relationships for individual storms were highly variable, which may influence estimates of watershed-scale NO₃⁻–N export. We also found storm NO₃⁻–N export was strongly driven by mobilization of distal sources, given the predominance of counterclockwise hysteresis. In both watersheds, HI and FI values varied seasonally and with storm size, and patterns were linked to changes in hydrologic connectivity related to variation in seasonal tile drain flow. Variation in storm-specific NO₃⁻–N yields was driven by event runoff, storm duration, and antecedent basin moisture, rather than antecedent precipitation. Overall, we found that high-frequency NO₃⁻–N data accurately documented the magnitude of the ecological challenge presented by storm-driven nutrient export in agricultural watersheds.

农业驱动的土地利用变化和水文变化影响了美国中西部河流中的溶质运输。鉴于与气候变化相关的水文预期变化，清楚地了解推动农业流域养分输出的机制对于减轻弥散养分污染至关重要。具体而言，预计中西部上游地区将出现更频繁、更强烈的降水和改变的降雪模式。我们使用了来自印第安纳州两个瓷砖排水农业流域的四年高频硝酸盐 (NO ₃ ⁻ –N) 传感器数据来探索 NO ₃ ⁻-N 导出 200 个风暴。我们使用浓度-排放 (C-Q) 关系和两个指数，滞后指数 (HI) 和冲洗指数 (FI)，来了解 NO ₃ ^- -N 跨时间尺度的物理化学控制。在年度和季节性时间尺度上，我们发现 NO ₃ ^- -N 浓度在很大程度上是趋化的；然而，FI 模式表明单个风暴的 C-Q 关系变化很大，这可能会影响流域规模 NO ₃ ^- -N 输出的估计。我们还发现了风暴 NO ₃ ^-考虑到逆时针滞后的优势，-N 输出受到远端源的动员的强烈驱动。在这两个流域中，HI 和 FI 值随季节和风暴大小而变化，并且模式与与季节性瓷砖排水流量变化相关的水文连通性变化有关。特定风暴 NO ₃ ^- -N 产量的变化是由事件径流、风暴持续时间和先行盆地湿度驱动的，而不是先行降水。总体而言，我们发现高频 NO ₃ ^- -N 数据准确地记录了农业流域中风暴驱动的养分输出所带来的生态挑战的严重程度。