How Much Water Does a River Need? The Range of Variability Approach (Richter et al. 1997)

Key companion paper to the 1996 IHA methodology introducing the Range of Variability Approach (RVA) for setting streamflow-based river ecosystem management targets. Defines a three-category system based on the 25th and 75th percentiles of the pre-impact hydrological record: Low (0-33rd percentile), Middle (34th-67th percentile), and High (68th-100th percentile). Under natural conditions, each IHA parameter value would fall in each category approximately 33% of the time. The Hydrological Alteration (HA) factor quantifies deviation from expected frequencies: HA = (Observed - Expected) / Expected. A value of 0 indicates no change, +1 indicates doubled frequency, and -1 indicates elimination. The RVA provides a practical framework for translating IHA analysis into management recommendations. Published in Freshwater Biology 37: 231-249.