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An overview of the hydrology of non‐perennial rivers and streams

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Abstract Non‐perennial rivers and streams are ubiquitous on our planet. Although several metrics have been used to statistically group or compare streamflow characteristics, there is currently no widely used definition of how many days or over what reach length surface flow must cease in order to classify a river as non‐perennial. At the same time, the breadth of climate and geographic settings for non‐perennial rivers leads to diversity in their flow regimes, such as how often or how quickly they go dry. These rivers have a rich and expanding body of literature addressing their ecologic and geomorphic features, but are often said to be ignored by hydrologists. Yet there is much we do know about their hydrology in terms of streamflow generation processes, water losses, and variability in flow. We also know that while they are prevalent in arid regions, they occur across all climate types and experience a diverse set of natural and anthropogenic controls on streamflow. Furthermore, measuring and modeling the hydrology of these rivers presents a distinct set of challenges, and there are many research directions, which still require further attention. Therefore, we present an overview of the current understanding, methodologic challenges, knowledge gaps, and research directions for hydrologic understanding of non‐perennial rivers; critical topics in light of both growing global water scarcity and ever‐changing laws and policies that dictate whether and how much environmental protection these rivers receive. This article is categorized under: Science of Water > Science of Water
Streamflow generation processes in (a) a stream that is hydraulically connected to the groundwater system (common in humid climates and perennial rivers) and (b) a stream underlain by the unsaturated zone (common in dryland river systems). Streamflow can be generated by [1] water falling directly on the stream channel or flowing down the stream channel from further up in the watershed [2] infiltration excess overland flow (due to the rainfall rate exceeding the infiltration rate of the soil) or saturation excess overland flow (once soil becomes fully saturated; panel a only), [3] interflow through unsaturated or partially‐saturated soils that may be perched above a low‐permeability layer (as in panel b), [4] interflow through saturated soils, and [5] groundwater outflow into the stream. See also Gutierrez et al. (2019)
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Topography, which is determined by a combination of geology and climate, plays a controlling role in the presence of streamflow within a catchment, especially in the persistence of seasonal flow within a catchment. Prancevic and Kirchner (2019) propose that a combination of drainage area, slope, and changes in transmissivity (i.e., the capacity to transmit flow through the subsurface) within the drainage area can be used to predict the expansion and contraction of the stream network seasonally. Figure generalized from the authors' original; 3D catchment base image courtesy of Jason C. Fisher, Integration and Application Network, University of Maryland Center for Environmental Science (ian.umces.edu/imagelibrary/)
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Non‐perennial stream networks are dynamic in space and time. They can (a) expand upstream from perennial reaches (e.g., alpine headwaters), (b) wet from upstream to downstream (e.g., ephemeral desert rivers), or (c) flow as a convergence of pools (e.g., in karst regions or where persistent pools line the river). At the catchment scale, a combination of these possibilities may exist in time and space, due to variability in geology, topography, and groundwater levels. See also Bhamjee and Lindsay (2011)
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