Abstract
Environmental flows, or the practice of allocating water in river systems for ecological purposes, is a leading strategy for conserving aquatic species and improving river health. However, consideration of surface-groundwater connectivity is seldom addressed in environmental flow development due to a lack of methodologies that account for groundwater contributions to instream flow. Groundwater-influenced streams have been identified as key refugia for native biota under a rapidly changing climate. These ecosystems are anticipated to be more resistant to climate change because groundwater input buffers the adverse effects of low flows and high temperatures, particularly in the dry season. Less understood, however, is the relative contribution of groundwater inputs to streamflow and how these surface-groundwater water interactions should be accounted for in environmental flow assessments and management actions. In order to assess ecological flow needs in groundwater-influenced streams, we applied the California Environmental Flows Framework (CEFF) in two river systems in California, United States. The Little Shasta River and the lower Cosumnes River are representative of many groundwater-influenced streams throughout the semi-arid western United States. Historically, perennial streamflow once sustained diverse native aquatic species in these ecosystems, but water withdrawals for irrigated agriculture has resulted in periodic stream dewatering. We found CEFF was useful in quantifying ecological flow needs for seasonal components of the flow regime that support ecosystem functionality. In particular, CEFF offered flexibility to incorporate information on the seasonal and spatial dimensions of groundwater influences in the development of ecological flow targets. The focus on ecosystem functions in CEFF, and ability to account for groundwater influences on those functions, creates opportunities for integrated surface-groundwater management strategies that support the recovery and protection of streamflows in groundwater-influenced streams.
Highlights
Development of river systems for human use is ubiquitous across the globe (Lehner et al, 2011; Grill et al, 2019; Cooley et al, 2021) and has resulted in drastic reductions in freshwater biodiversity and ecosystem services (Vorosmarty et al, 2010; Reid et al, 2019)
Comprehensive descriptions of the environmental flow assessments following California Environmental Flows Framework (CEFF) are provided in technical reports for the Little Shasta River (Yarnell et al, 2021) and for the Lower Cosumnes River (Yarnell and Obester, 2021)
We provide a summary of the outcomes from CEFF sections A and B at each study site, with a focus on how groundwater influences were accounted for in the determination of ecological flow needs that support stream functionality
Summary
Development of river systems for human use is ubiquitous across the globe (Lehner et al, 2011; Grill et al, 2019; Cooley et al, 2021) and has resulted in drastic reductions in freshwater biodiversity and ecosystem services (Vorosmarty et al, 2010; Reid et al, 2019). Environmental flows are often implemented in regulated rivers through re-operation of large dams, but less attention has been given to rivers where flow is affected by other water management activities, including diversions from surface waters, springs, and groundwater sources. Existing environmental flow programs and methodologies rarely account for the influence of groundwater withdrawals on river flows, despite well-recognized interactions between surface water and groundwater in many river systems (Rohde et al, 2017). Groundwater management programs are typically focused on urban or agricultural uses and rarely account for environmental water needs of groundwater dependent ecosystems (GDEs), defined as “terrestrial, aquatic, and coastal ecosystems that require access to, replenishment or benefit from, or otherwise rely on subsurface stores of water to function or persist” (Howard and Merrifield, 2010).
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