A comparison of desktop hydrologic methods for determining environmental flows

摘要

Determining environmental flows can be a daunting task because fluvial systems are physically and biologically complex, and there can be difficult trade-offs between instream and out-of-stream water uses. Desktop methods are primarily office-based exercises that often use more readily available hydrologic information to assess a proposed diversion of stream flows. Two-tiered water allocation decision processes, in which desktop methods represent the first tier or filter, and detailed methods the second tier, are common in many jurisdictions. A primary objective of desktop and detailed methods is to protect environmental resources, but the required information, time, and cost may be substantially different. This paper reviews six desktop hydrologic methods commonly used in Canada. Seventeen performance measures (PMs), for ecosystem health (hydrology, habitat, geomorphology, connectivity, and water quality) and out-of- stream water use, are used to compare and contrast the predicted outcome of the desktop methods to three mid-size rivers with different hydrologic regimes. Results of the PM calculations demonstrate that absolute and relative performance of the six desktop methods is not consistent across the three streams. As a group, the methods clearly underscored the trade-off between water use and ecosystem health; however, the methods had widely divergent performance for water use PMs and variable performance on the different ecosystem health PMs. All desktop methods performed well on the fish habitat PMs, but performance on the other ecosystem health PMs was variable and not consistent among methods. The results emphasize the need for clearly articulated objectives as an integral component of any desktop method and the benefit of transparent trade-off considerations during any water allocation decision.