Tools for managing hydrologic alteration on a regional scale: Estimating changes in flow characteristics at ungauged sites

Abstract

Hydrologic alteration is a predominant stressor for biological resources in streams. This stress is further aggravated by competing human and ecological demands for limited water resources. Understanding flow–ecology relationships and establishing relevant and implementable flow targets are essential to protect biological communities. Estimating degree of ecologically relevant hydrologic alteration depends on the availability of long‐term flow data at sites with biological information. However, measured flow data are seldom available at sufficient density to support large‐scale analyses of the biological effects of hydrologic alterations. The ability to accurately simulate flows and estimate flow metrics at many ungauged locations across a broad geographical area remains a fundamental challenge. We address this challenge by applying a novel technique to simulate flow regimes at any stream reach of interest by first developing an ensemble of regionally calibrated and validated hydrological models, and then using a selection tool to match the “best‐fit” model to ungauged stream reaches. An ensemble of 26 HEC‐HMS rainfall–runoff models were calibrated to represent the range of catchment conditions in the southern California region. We developed current and historical flow regimes and a suite of flow metrics at 572 ungauged sites in southern California with bioassessment monitoring data. The flow metrics represent hydrograph characteristics of magnitude, timing, frequency, duration and variability. The flow metrics were estimated under three precipitation conditions—dry, wet and average. In addition, we estimated aggregated flow metrics for (dry + wet + average) condition. Hydrologic alteration was estimated as the deviation between the modelled current and historical flow metrics. Approximately 79% of the region shows some degree of hydrologic alteration, and approximately 40% of the sites are estimated to be severely altered. Magnitude metrics tend to increase in response to urban and agricultural land uses, whereas the timing and duration metrics are mostly unchanged. This mechanistic modelling approach demonstrates the feasibility of estimating flow alterations for ungauged catchments with relative ease of transferability over a broad geographical region. The continuous granular flow data allow for computation and consideration of metrics that may be applicable to a variety of ecological endpoints and consideration of a range of management trade‐offs.

Tools for managing hydrologic alteration on a regional scale: Setting targets to protect stream health

“…any change in flow metric values from reference conditions) was evident at 79% of sites in the region; magnitude metrics exhibited more widespread and severe alteration than other metrics (for details, see Sengupta et al. in review).…”

“…The ensemble of 26 models allows estimation of hydrologic alteration at any ungauged site, provided that adequate precipitation data are available (Sengupta et al. in review). Expanding the ensemble with additional models would likely improve performance at stream types that are currently underrepresented (e.g.…”

“…Hydrologic alteration was estimated at ungauged bioassessment sites using an ensemble of 26 HEC‐HMS models developed as part of the regional flow‐ecology analysis (Sengupta et al. in review). conditions.…”

“…Sengupta et al. (in review) created a set of simple Hydrologic Engineering Center‐Hydrologic Modeling System (HEC‐HMS) models that can be matched to diverse catchments throughout coastal southern California based on catchment similarities between ungauged sites and calibration gauges. This set of models, or ensemble, allows estimation of hydrologic conditions (under present‐day and historical land use) for a large number of ungauged sites in the region.…”

“…We applied an ensemble of simple rainfall–runoff models from Sengupta et al. (in review) to hundreds of ungauged bioassessment sites in southern California to (1) evaluate the relationships between hydrologic alteration and biological condition, (2) recommend flow metric targets associated with healthy streams (i.e. ecological limits of hydrological alteration, Poff et al.…”