Trends in baseflows based on observed daily streamflow data are evaluated in this study at several sites in the least anthropogenically affected watersheds in the USA. Trends were determined for annual maximum, annual mean, and annual median baseflow. Baseflow values derived at 574 stations in the USA for the 44 years from 1970 through 2013 are analyzed using two nonparametric trend tests (Spearman’s rho (SR) test and Mann-Kendall (MK)). Results from the trend tests are compiled for 18 major regions to understand the spatial variability of changes in baseflows across the USA. Results from SR tests indicate that almost half of the stations show statistically significant trends in annual maximum baseflows. Trends in annual median baseflows show that 32.06% of the gauging stations have downward trends, and a total of 56.45% of sites show significant trends for annual mean baseflows. The Souris-Red-Rainy, Missouri, and California watershed regions have a larger number of sites with higher upward trends compared with those from other regions in the USA. The results from the SR test indicate that 262 sites have statistically significant trends in annual maximum baseflow compared with the 254 sites with similar trends noted from the MK test. Based on limited data, it can be concluded that baseflow and precipitation values accumulated for the same month are correlated in some regions. In general, the number of sites with decreasing trends for annual maximum, mean, and median baseflows is larger than the number of sites with increasing trends. Decreasing trends in baseflows are cause for concern and have serious implications on future planning for low flow management strategies for several streams in the USA.

在这项研究中，在美国受人为影响最小的流域的几个站点上，基于观察到的每日流量数据对基流趋势进行了评估。确定了年度最大值，年度平均值和年度中位数基准流的趋势。使用两个非参数趋势检验（Spearman的rho（SR）检验和Mann-Kendall（MK））分析了美国574个站点从1970年到2013年的基础流量值。趋势测试的结果针对18个主要地区进行了汇总，以了解整个美国基础流量变化的空间变异性。SR测试的结果表明，几乎一半的站点在年度最大基本流量方面显示出统计学上的显着趋势。年度中位数基准流量趋势显示，有32.06％的测量站呈下降趋势，共有56个。45％的站点显示出年平均基准流量的显着趋势。与美国其他地区相比，Souris-Red-Rainy，密苏里州和加利福尼亚州的分水岭地区有更多的地点，且上升趋势更高。SR测试的结果表明，与254个站点相比，有262个站点的年度最大基本流量具有统计上显着的趋势，而MK测试中注意到了类似的趋势。根据有限的数据，可以得出结论，在某些地区，当月累积的底流和降水值是相关的。一般而言，年度最大流量，平均流量和中位数基本流量的趋势减少的站点数量要大于趋势增加的站点的数量。