Hydrologic indicator statistics for streamflow sites throughout Minnesota

Hydrologic indicator statistics were computed for 82 selected surface water sites located throughout Minnesota using daily streamflow data from the U.S. Geological Survey (USGS) National Water Information System (NWIS). The 187 hydrologic indicator statistics were computed in RStudio version 3.5.0 using the EflowStats version 5.0.0 (Mills and Blodgett, 2017) and NWCCompare version 5.0 (Blodgett, 2017) packages. The computed hydrologic indicator statistics encompass the five components of hydrologic conditions: magnitude, frequency, duration, timing, and rate of change. Magnitude is the amount of water moving past a fixed location in a given unit of time. Frequency refers to how often streamflows above a given magnitude recur over a specified time interval. Duration is the period of time associated with a specific streamflow condition. Timing refers to the regularity with which streamflows of a given magnitude occur, and rate of change refers to how quickly the magnitude of streamflow changes (Poff and others, 1997). Site selection was based on sites previously selected in three other studies evaluating long-term streamflow records for trends (Novatny and Stefan, 2007; Peterson, Nieber, and Kanivetsky, 2011; Ziegeweid and others, 2015). Nontrend sites were shown to not have trends in streamflow that were not related to precipitation. Hydrologic indicator statistics were computed for two periods: 1) the pre-effect period from 10-1-1944 through 9-30-1979 and 2) the post-effect period from 10-1-1980 through 9-30-2015. Exact dates of the start of trends varied among sites, but 1980 was the selected cutoff period based on an approximation of the largest cluster and on other anecdotal evidence of changes in land use practices. Both categories also had at least 10 water years with complete streamflow data. Blodgett, D., 2017, NWCCompare: Returns NWC comparison stats for two daily data sets version 5.0, https://github.com/USGS-R/NWCCompare. Mills, J., and Blodgett, D., 2017, EflowStats: Hydrologic Indicator and Alterations Stats version 5.0.0, https://github.com/USGS-R/EflowStats. Novotny, E.V., and Stefan, H.G., 2007, Stream flow in Minnesota: Indicator of climate change, Journal of Hydrology 334: 319-333. Peterson, H.M., Nieber, J.L., and Kanivetsky, R., 2011, Hydrologic regionalization to assess anthropogenic changes, Journal of Hydrology 408: 212-225. Ziegeweid, J.R., Lorenz, D.L., Sanocki, C.A., and Czuba, C.R., 2015, Methods for estimating flow-duration curve and low-flow frequency statistics for ungaged locations on small streams in Minnesota: U.S. Geological Survey Scientific Investigations Report 2015 - 5170, 23 p., http://dx.doi.org/10.3133/sir20155170.

本研究针对美国明尼苏达州全境甄选的82个地表水监测站点，以美国地质调查局（U.S. Geological Survey, USGS）国家水信息系统（National Water Information System, NWIS）的日径流数据为基础，计算得到水文指标统计量。共计187项水文指标统计量通过RStudio 3.5.0版本，借助EflowStats 5.0.0版本（Mills与Blodgett，2017）及NWCCompare 5.0版本（Blodgett，2017）R包完成计算。所计算的水文指标统计量涵盖水文情势的五大核心组分：流量量级、发生频率、持续时长、出现时机以及变化速率。其中，流量量级指特定时段内流经固定点位的水量；发生频率指给定量级的径流在指定时间区间内重现的频次；持续时长对应特定径流条件维持的时间周期；出现时机指特定量级径流发生的规律性；变化速率则指径流量级的变化快慢（Poff等，1997）。本次站点遴选参考了此前三项针对长期径流序列趋势分析的研究（Novatny与Stefan，2007；Peterson、Nieber与Kanivetsky，2011；Ziegeweid等，2015）所采用的站点。经筛选，非趋势站点的径流序列不存在与降水无关的趋势性变化。本研究针对两个时段计算水文指标统计量：其一为影响前期，即1944年10月1日至1979年9月30日；其二为影响后期，即1980年10月1日至2015年9月30日。尽管各站点的趋势起始时间存在差异，但基于最大聚类的近似结果及土地利用方式改变的相关佐证证据，最终选取1980年作为两个时段的分界点。此外，两个时段均包含至少10个完整水文年的径流数据。布洛杰特（D. Blodgett），2017年，NWCCompare：双日数据集NWC比较统计量工具（版本5.0），https://github.com/USGS-R/NWCCompare。米尔斯（J. Mills）与布洛杰特（D. Blodgett），2017年，EflowStats：水文指标与改变量统计工具（版本5.0.0），https://github.com/USGS-R/EflowStats。诺沃特尼（E.V. Novotny）与斯特凡（H.G. Stefan），2007年，《明尼苏达州径流：气候变化的指示指标》，《水文杂志》（Journal of Hydrology）334卷：319-333页。彼得森（H.M. Peterson）、尼伯（J.L. Nieber）与卡尼韦茨基（R. Kanivetsky），2011年，《用于评估人为改变的水文区域化方法》，《水文杂志》408卷：212-225页。齐格韦德（J.R. Ziegeweid）、洛伦茨（D.L. Lorenz）、萨诺基（C.A. Sanocki）与祖巴（C.R. Czuba），2015年，《明尼苏达州小型溪流无监测点位的流量历时曲线与低流频率统计量估算方法》，美国地质调查局科学调查报告2015-5170，共23页，http://dx.doi.org/10.3133/sir20155170。