APPENDIX: INDICES APPLIED

1.  Indices of Hydrologic Alteration (IHA) and Environmental Flow Components (EFC)

The IHA software provides 33 IHA and 34 EFC parameters (TNC 2011, based on Richter et al. 1996, 1997, 1998). In this study the following parameters were calculated for each hydrological year following a nonparametric approach:

IHA parameter group / Hydrologic parameters / Units
1. Magnitude of monthly flow conditions / Median flow for each calendar month / m3/s
2. Magnitude and duration of annual extreme flow conditions / 1-day minimum flow
3-day minimum flow a
7-day minimum flow a
30-day minimum flow a
90-day minimum flow a
1-day maximum flow
3-day maximum flow a
7-day maximum flow a
30-day maximum flow a
90-day maximum flow a
Base flow index: 7-day minimum flow/annual mean of daily flows / m3/s
m3/s
m3/s
m3/s
m3/s
m3/s
m3/s
m3/s
m3/s
m3/s
3. Timing of annual extreme flow conditions / Julian date of 1-day maximum
Julian date of 1-day minimum
4. Frequency and duration of high and low pulses b / Number of low pulses
Median duration of low pulses
Number of high pulses
Median duration of high pulses / /y
d
/y
d
5. Rate and frequency of flow condition changes / Rise rates: median of all positive flow differences between consecutive daily values
Fall rates: median of all negative flow differences between consecutive daily values
Number of hydrologic reversals c / m3/s/d
m3/s/d
/y

a 3-, 7-, 30-, and 90-day minima and maxima are moving averages calculated for every possible complete period within the hydrologic year

b Hydrologic low (high) pulses: periods within a hydrologic year during which the flow magnitude drops below (rise above) the 25th (75th) percentile of all daily values for the time period considered (for comparison of two periods pulse thresholds are calculated using only data in the pre-impact period)

c After dividing the hydrologic year into "rising" (positive daily flow changes) and "falling" (negative daily flow changes) periods, the number of reversals is the number of times that flow switches from one type of period to another.

EFC type / Hydrologic parameters / Units
Extreme low flows / Frequency of extreme low flows
Median duration of extreme-low-flow events
Peak flow (minimum)
Julian date of the peak flow / /y
d
m3/s
High flow pulses
Small floods
Large floods / Frequency of events
Median duration of events
Peak flow (maximum) of events
Julian date of the event peak flow
Rise rates of events
Fall rates of events / /y
d
m3/s
m3/s/d
m3/s/d

The software calculates the median of each parameter for each period and the degree of change between medians of the two compared periods, as the percentage of difference of the subsequent period relative to the preceding period.

2.  Flow Duration Curves (FDC)

For the estimation of Flow Duration Curves daily streamflow data are ranked from highest to lowest and then, the probability P that a given flow be equaled or exceeded (% of time) is calculated for each period according to: P= 100 M / (n + 1), where M = the ranked position and n = the number of days for the period. Results are shown in a graph of flow values in function of their corresponding exceedance probability (TNC 2011)

3.  Concentration Degree Index during the year (CDI)

Whether the monthly flow is taken as a vector, the number of days in a year (365) can be regarded as a circle (360º). The position of the flow vector in January is zero, and the angle increases with an equal angle difference of 30º with each month. Then, the CDI may be calculated for monthly flows according to the following expression (Ling et al. 2014):

where Ry and Rx are:

and i is the month, R(i) is the mean flow of each month in a year, and q is the angle corresponding to the month.