Reconstructing the natural hydrology of the San Francisco Bay–Delta watershed

P Fox,A J Draper,P H Hutton,L Sears,D J Howes

doi:10.5194/hess-19-4257-2015

P Fox, A J Draper + Show 3 more

Open Access

https://doi.org/10.5194/hess-19-4257-2015

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Abstract

Abstract. We evaluated the impact of landscape changes on the amount of delta outflow reaching San Francisco Bay. The natural landscape was reconstructed and water balances were used to estimate the long-term annual average delta outflow that would have occurred under natural landscape conditions if the climate from 1922 to 2009 were to repeat itself. These outflows are referred to as natural delta outflows and are the first published estimate of natural delta outflow. These natural delta outflows were then compared with current delta outflows for the same climate and existing landscape, including its re-engineered system of reservoirs, canals, aqueducts, and pumping plants. This analysis shows that the long-term, annual average delta outflow under current conditions is consistent with outflow under natural landscape conditions. The amount of water currently used by farms, cities, and others is about equal to the amount of water formerly used by native vegetation. Development of water resources in California's Central Valley transferred water formerly used by native vegetation to new beneficial uses without substantially reducing the long-term annual average supply to the San Francisco Bay–Delta estuary. Based on this finding, it is unlikely that observed declines in native freshwater aquatic species are the result of annual average delta outflow reductions.

Highlights

The San Francisco Estuary, composed of San Francisco Bay and the Sacramento–San Joaquin River delta, is the largest estuary along the Pacific coast of the USA and the home to a rich ecosystem
A long-term annual average was computed from individual yearly results and compared with California Department of Water Resources (CDWR)’s (2007, 2012) estimates of long-term annual average delta outflow under current conditions and unimpaired conditions for a similar period of record
This study shows that long-term annual average current and natural outflows fall within the same range, when controlled for climatic conditions

Summary

Introduction

The San Francisco Estuary, composed of San Francisco Bay and the Sacramento–San Joaquin River delta, is the largest estuary along the Pacific coast of the USA and the home to a rich ecosystem. This valley is entirely surrounded by mountains except for a narrow gap on its western edge through which the combined Sacramento and San Joaquin rivers flow to the Pacific Ocean through San Francisco Bay (Fig. 1). This valley is the agricultural heartland of the USA, producing over 360 products and more than half of the country’s vegetables, fruits and nuts. The Sacramento River from the north and the San Joaquin River from the south flow toward each other, joining in the delta

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