The RVA method is a set of methods based on the IHA hydrological index system proposed by Richter et al. (2003) and Guo et al. (2018). The indicators selected by IHA can describe the changes in the hydrological situation in detail in five aspects: magnitude, timing, frequency, duration, and rate of change. In this study, an ecologically significant flow indicator system was established, including five groups of 32 indicators (Table 1). The following are the specific calculations:
(4)
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Table 1
Indexes of IHA
| IHA parameter group | Parameter index |
|---|---|
| Monthly median flow | Median data for monthly flow |
| Annual extreme flow | Annual average 1, 3, 7, 30, 90 days minimum and maximum flow, base flow indexa |
| Annual extreme flow occurrence time | The time of the largest and smallest daily pulses in the year |
| Frequency and duration of high and low pulses | The median of yearly high and low pulse countband pulse duration |
| Change rate and frequency of flow change | The mid-year value of increase (rate of increase) and decline (decline rate). The number of reversalsc for increase (rate of increase) and decline (decline rate) |
| IHA parameter group | Parameter index |
|---|---|
| Monthly median flow | Median data for monthly flow |
| Annual extreme flow | Annual average 1, 3, 7, 30, 90 days minimum and maximum flow, base flow indexa |
| Annual extreme flow occurrence time | The time of the largest and smallest daily pulses in the year |
| Frequency and duration of high and low pulses | The median of yearly high and low pulse countband pulse duration |
| Change rate and frequency of flow change | The mid-year value of increase (rate of increase) and decline (decline rate). The number of reversalsc for increase (rate of increase) and decline (decline rate) |
aBase flow Index: The ratio of the minimum annual flow of 7 consecutive days to the annual median value.
bLow pulse is defined as the median of the day lower than 25% of the frequency before the disturbance, and high pulse is defined as the median of the day higher than 75% of the frequency before the disturbance.
cThe number of reversals refers to the number of times the daily flow changes from increasing to decreasing or decreasing to increasing.