Despite the economic stimulus provided by many dams historically, the global experience with dam building warns that traditional approaches to water infrastructure development in a rapidly changing world carry severe risks of economic and environmental failure. First, large water projects are very capital-intensive and long-lived, costing billions of dollars to plan, build, and maintain. Yet, they are vulnerable to biased economic analyses 3 , cost overruns and construction delays, and changing environmental, economic and social conditions that can diminish projected benefits 4,5 . Under a variable and changing climate, large water infrastructure even risk becoming stranded assets 6 . Second, the principles of economic efficiency inherent in cost-benefit analysis dominate project design and performance assessment, and integrating social and environmental benefits and costs into a comprehensive economic evaluation presents significant challenges 7,8 .These costs can be substantial, as evidenced by human displacement 5,9 , local species extinctions 10 , and the loss of ecosystem services such as floodplain fisheries and other amenities 11,12 .As unanticipated economic, social and environmental costs accumulate with aging water infrastructure, society is investing in restoration projects to undo some of the long-term environmental degradation, including modifying flow releases from dams 13,14 and in some cases dam removal 15 . As the global impairment of aquatic ecosystem function becomes increasingly documented and articulated 16,17 , a broader conception of sustainable water resources management that formulates environmental health as a necessary ingredient for water security and the social wellbeing it supports is urgently needed [18][19][20] . Notably, new national directives are emerging to develop and manage river ecosystems in less environmentally harmful and more sustainable ways, including in the US 21 , Europe 22,23 , and Australia 24 . Towards a more sustainable water resources management paradigmHere we ask if a more sustainable water management philosophy can be forged to guide investment in, and design of, water infrastructure while avoiding adverse, sometimes irreversible, social and

The science and practice of environmental flows have advanced significantly over the last several decades. Most environmental flow approaches require quantifying the relationships between hydrologic change and biologic response, but this can be challenging to determine and implement due to high data requirements, limited transferability, and the abundance of hydrologic metrics available for evaluation. We sug-

Abstract:Variability in flow as a result of seasonal precipitation patterns is a defining element of streams and rivers in Mediterranean-climate regions of the world and strongly influences the biota of these unique systems. Mediterranean-climate areas include the Mediterranean Basin and parts of Australia, California, Chile, and South Africa. Mediterranean streams and rivers can experience wet winters and consequent floods to severe droughts, when intermittency in otherwise perennial systems can occur. Inter-annual variation in precipitation can include multi-year droughts or consecutive wet years. Spatial variation in patterns of precipitation (rain vs. snow) combined with topographic variability lead to spatial variability in hydrologic patterns that influence populations and communities. Mediterranean streams and rivers are global biodiversity hotspots and are particularly vulnerable to human impacts. Biomonitoring, conservation efforts, and management responses to climate change require approaches that account for spatial and temporal variability (including both intra-and inter-annual). The importance of long-term data sets for understanding and managing these systems highlights the need for sustained and coordinated research efforts in Mediterranean-climate streams and rivers.

SUMMARY1. In Mediterranean and other water-stressed climates, water management is critical to the conservation of freshwater ecosystems. To secure and maintain water allocations for the environment, integrated water management approaches are needed that consider ecosystem flow requirements, patterns of human water demands and the temporal and spatial dynamics of water availability. 2. Human settlements in Mediterranean climates have constructed water storage and conveyance projects at a scale and level of complexity far exceeding those in other, less seasonal climates. As a result, multiple ecological stressors associated with natural periods of flooding and drying are compounded by anthropogenic impacts resulting from water infrastructure development. 3. Despite substantial investments in freshwater ecosystem conservation, particularly in California, U.S.A., success has been limited because the scales at which river management and restoration are implemented are often discordant with the temporal and spatial scales at which ecosystem processes operate. Often, there is also strong social and political resistance to restricting water allocation to existing consumptive uses for environmental protection purposes. Furthermore, institutions rarely have the capacity to develop and implement integrated management programmes needed for freshwater ecosystem conservation. 4. We propose an integrated framework for streamflow management that explicitly considers the temporal and spatial dynamics of water supply and needs of both human and natural systems. This approach makes it possible to assess the effects of alternative management strategies to human water security and ecosystem conditions and facilitates integrated decision-making by water management institutions. 5. We illustrate the framework by applying a GIS-based hydrologic model in a Mediterranean-climate watershed in Sonoma County, California, U.S.A. The model is designed to assess the hydrologic impacts of multiple water users distributed throughout a stream network. We analyse the effects of vineyard water management on environmental flows to (i) evaluate streamflow impacts from small storage ponds designed to meet human water demands and reduce summer diversions, (ii) prioritise the placement of storage ponds to meet human water needs while optimising environmental flow benefits and (iii) examine the environmental and social consequences of flow management policies designed to regulate the timing of diversions to protect ecosystem functions. 6. Thematic implications: spatially explicit models that represent anthropogenic stressors (e.g. water diversions) and environmental flow needs are required to address persistent and growing threats to freshwater biodiversity. A coupled human-natural system approach to water management is particularly useful in Mediterranean climates, characterised by severe competition for water resources and high spatial and temporal variability in flow regimes. However, lessons learned from our analyses are applicable to other highly se...

For 100 years, California's State Water Resources Control Board and its predecessors have been responsible for allocating available water supplies to beneficial uses, but inaccurate and incomplete accounting of water rights has made the state ill-equipped to satisfy growing societal demands for water supply reliability and healthy ecosystems. Here, we present the first comprehensive evaluation of appropriative water rights to identify where, and to what extent, water has been dedicated to human uses relative to natural supplies. The results show that water right allocations total 400 billion cubic meters, approximately five times the state's mean annual runoff. In the state's major river basins, water rights account for up to 1000% of natural surface water supplies, with the greatest degree of appropriation observed in tributaries to the Sacramento and San Joaquin Rivers and in coastal streams in southern California. Comparisons with water supplies and estimates of actual use indicate substantial uncertainty in how water rights are exercised. In arid regions such as California, over-allocation of surface water coupled with trends of decreasing supply suggest that new water demands will be met by re-allocation from existing uses. Without improvements to the water rights system, growing human and environmental demands portend an intensification of regional water scarcity and social conflict. California's legal framework for managing its water resources is largely compatible with needed reforms, but additional public investment is required to enhance the capacity of the state's water management institutions to effectively track and regulate water rights.