Environmental Flow Components for Measuring Hydrologic Model Fit during Low Flow Events

Abstract

The indicators of hydrologic alteration (IHA) is a statistical flow methodology for characterizing ecologically important streamflows. Typically, IHA has been used to identify the extent of human effects on a stream’s hydrology and to set management goals to restore the stream ecology. In this work, the use of the seven “extreme low flow” statistics of the IHA is extended to the evaluation of the performance of a hydrologic simulation model under low flow conditions. Specifically, this work uses the IHA framework to evaluate the accuracy of the Chesapeake Bay Program Phase 5 (CBP5) watershed model during low flow events on a regional scale that is relevant to many water supply planners and managers. Because the CBP5 model’s primary focus is predicting the Bay’s water quality, the measures used to calibrate the CBP5 model focused primarily on the calibration of the entire hydrological record and had only a secondary emphasis on specific flow regimes, such as low flows and very low flows, although these flows are important for both stream ecologies and water supply planners. To provide a comparative performance benchmark, the performance of the simple drainage area ratio (DAR) method relative to the IHA low flow statistics is also determined. This paper demonstrates the use of IHA statistics for model evaluation in a case study, the Rivanna River watershed, a subcatchment within the Chesapeake Bay drainage. For rivers with a large proportion of unregulated flow contributions, the authors conclude that the computationally simple DAR model with appropriate surrogate watershed generally characterizes the extreme low flow conditions slightly more accurately than the CBP5 model. However, unlike the CBP5 model, the DAR model predicts future flows based solely on historical data; and thus, the DAR model cannot predict flow effects caused by hydrological alterations, thus limiting its use in water supply management. Nevertheless, this IHA analysis suggests that the incorporation of a low-flow—specific metric into the CBP5 calibration could improve its utility for water supply management and planning at a regional scale.