Table 1 Summary of hydrologic parameters used in IHA and their ecological influences.

Adapted from IHA Manual V7 (28).

IHA statistics groupRegime characteristicsEcosystem influences
Group 1: Magnitude
of monthly water
conditions (12 indices)
Mean or median value for
each calendar month
Habitat availability for aquatic organisms
Soil moisture availability for plants
Availability of water for terrestrial animals
Availability of food/cover for furbearing mammals
Reliability of water supplies for terrestrial animals
Access by predators to nesting sites
Water temperature, oxygen levels, and
photosynthesis in water column
Group 2: Magnitude and
duration of annual
extreme water conditions
(12 indices)
Annual minima, 1-day means
Annual minima, 3-day means
Annual minima, 7-day means
Annual minima, 30-day means
Annual minima, 90-day means
Annual maxima, 1-day means
Annual maxima, 3-day means
Annual maxima, 7-day means
Annual maxima, 30-day means
Annual maxima, 90-day means
Number of zero flow days
Base flow index
Balance of competitive, ruderal, and stress-tolerant organisms
Creation of sites for plant colonization
Structuring of aquatic ecosystems by abiotic versus biotic factors
Structuring of river channel morphology and physical habitat conditions
Soil moisture stress in plants
Dehydration in animals
Anaerobic stress in plants
Volume of nutrient exchanges between rivers and floodplains
Duration of stressful conditions such as low oxygen and
concentrated chemicals in aquatic environments
Distribution of plant communities in lakes, ponds, and floodplains
Duration of high flows for waste disposal and aeration of
spawning beds in channel sediments
Group 3: Timing of
annual extreme
water conditions
(2 indices)
Julian date of each annual,
1-day maximum
Julian date of each annual,
1-day minimum
Compatibility with life cycles of organisms
Predictability/avoidability of stress for organisms
Access to special habitats during reproduction or to avoid predation
Spawning cues for migratory fish
Evolution of life history strategies and behavioral mechanisms
Group 4: Frequency and duration
of high and low pulses
(4 indices)
Number of low pulses within each water year
Mean or median duration of low pulses (days)
Number of high pulses within each water year
Mean or median duration of high pulses (days)
Frequency and magnitude of soil moisture stress for plants
Frequency and duration of anaerobic stress for plants
Availability of floodplain habitats for aquatic organisms
Nutrient and organic matter exchanges between river and floodplain
Soil mineral availability
Access for waterbirds to feeding, resting, and reproduction sites
Bed load transport, channel sediment textures, and
duration of substrate disturbance (high pulses)
Group 5: Rate and
frequency of water
condition changes
(3 indices)
Rise rates: Mean or median of all positive
differences between consecutive
daily values
Fall rates: Mean or median of all negative
differences between consecutive
daily values
Number of hydrologic reversals
Drought stress on plants (falling levels)
Entrapment of organisms on islands and floodplains
(rising levels)
Desiccation stress on low-mobility stream
edge organisms