Despite rises in drought frequency and human water demands, streamflow regime shifts from perennial to non-perennial have not been evaluated in many arid/semi-arid regions. To document shifts, we created a methodology that classifies streams as naturally perennial or non-perennial. Our classification used historical, minimally disturbed-quality USGS streamflow gages (1950–2015) across California. The number of consecutive zero flow days (≥5 days) was used to classify 61% (96/158) and 39% (62/158) of gages as perennial and non-perennial, respectively. We developed a random forest model to predict flow regime class based on climate and watershed characteristics. To identify regime shifts, we compared the observed class of contemporary (1980–2023) minimally disturbed and disturbed gages with their modeled, natural class. For most minimally disturbed gages, the observed and natural predicted classes were the same, but 13% (7/52) of gages had a modeled perennial regime with an observed non-perennial class, indicating a drying trend. Among disturbed gages, 22% (64/290) shifted from perennial to non-perennial and 7% (21/290) from non-perennial to perennial. Trends in the minimum 7-day moving average and number of zero-flow days provided further evidence of drying at minimally disturbed streams, but no pattern at disturbed gages. Our results indicate that few minimally disturbed perennial streams have become non-perennial to date, but many streams have experienced drying from climate. Streams impacted by human activities had greater drying rates, but regulation has caused some non-perennial streams to become perennial. By quantifying expected natural streamflow regimes, this work can help monitor, manage, and conserve stream ecosystems.

中文翻译：

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美国加利福尼亚州的常年和非常年水流状况发生变化

尽管干旱频率和人类对水的需求增加，但在许多干旱/半干旱地区，水流形态从常年到非常年的转变尚未得到评估。为了记录变化，我们创建了一种方法，将溪流分类为自然常年流或非常年流。我们的分类使用了加利福尼亚州历史上干扰最小的 USGS 水流计（1950-2015 年）。连续零流量天数（≥5 天）用于分别将 61% (96/158) 和 39% (62/158) 的仪表分类为常年性和非常年性。我们开发了一个随机森林模型来根据气候和流域特征预测流态类别。为了识别政权转变，我们将观察到的当代（1980-2023）最小干扰和干扰量具的类别与其建模的自然类别进行了比较。对于大多数干扰最小的量具，观察到的和自然预测的类别是相同的，但 13% (7/52) 的量具具有模拟的多年生状态和观察到的非多年生类别，表明干燥趋势。在受到干扰的植物中，22% (64/290) 从多年生植物转变为非多年生植物，7% (21/290) 从非多年生植物转变为多年生植物。最小 7 天移动平均值和零流量天数的趋势提供了扰动最小的河流干燥的进一步证据，但扰动水位没有模式。我们的研究结果表明，迄今为止，很少有受到最小干扰的常年溪流变成了非常年溪流，但许多溪流都经历了气候造成的干燥。受人类活动影响的溪流干涸率更高，但监管已导致一些非常年溪流变成常年溪流。通过量化预期的自然水流状况，这项工作可以帮助监测、管理和保护河流生态系统。