Ecosystem Viltality Water Quantity (WQT) Deviation from Natural Flow Regime(DvNF)

Deviation from natural flow regime measures the degree to which current flow conditions have shifted from historic natural flows. The greater the deviation from natural flow indicates a higher risk of damage to the freshwater ecosystem (Poff and Zimmerman 2010). This measure can be derived from a wide range of variables, including deviation in annual mean, minimum and maximum discharge in the basin, proportion of the year that annual mean discharge was exceeded, etc.

Calculation in FHI Toolbox:

Option 1- Amended Annual Proportion of Flow Deviation (Gehrke et al. 1995, Gippel et al 2011):

$$ AAPFD = \ \sum_{j = 1}^{p}\frac{\sqrt[2]{\sum_{i = 1}^{12}\left\lbrack \frac{m_{i} - n_{i}}{\overline{n_{i}}} \right\rbrack^{2}}}{p} $$

Where, mi is monthly flow data accruing to current condition, ni is modeled natural flow for the same period. p is the number of years and $$\bar{n_{i}}$$ is mean reference flow for month i across p years (Note: in ephemeral streams, this should be changed to incorporate annual average flow to avoid extremely large values). Values are normalized as follows using thresholds reported in Gehrke et al. 1995 and Gippel et al 2011:

$$ DvNF = \ \left\{ \begin{matrix} 100 - 100 \times AAPFD\ \ \mathrm{\text{for}}\ 0 \leq AAPFD < 0.3 \\ 85 - 50 \times AAPFD\ \ \mathrm{\text{for}}\ 0.3 \leq AAPFD < 0.5 \\ 70 - 20 \times AAPFD\ \mathrm{\text{for}}\ \ 0.5\ \leq AAPFD < 2 \\ 50 - 10 \times AAPFD\ \ \mathrm{\text{for}}\ 2 \leq AAPFD < 5 \\ 0\ \ \mathrm{\text{for}}\ AAPFD\ \geq 5 \ \end{matrix} \right.\ $$

In case of lakes, monthly flow data can be replaced with ‘level’ data (See Liang et al. 2015 as an example).

Option 2- Global Monthly Index of Hydrological Regime Alteration (Pumo et al., 2018)

A recent addition to monthly flow indicators to describe flow regime alteration, this method uses a multi-step process and is similar to the indicators of hydrologic alteration (Richter et al. 1996, 1997). Under it, 22 interannual statistics of the flow series (at the monthly scale) from pre- and post- impact period are calculated and then combined into a single score. This method may be useful when observed monthly time series of flow that can approximate current and natural period (>20 years, each) is available or can be modelled.

The 22 interannual statistics can be grouped under 5 categories:

1. Magnitude of monthly water conditions
2. Magnitude and Duration of annual extremes of seasonal
3. Timing of annual extreme water conditions
4. Frequency of high and low pulses
5. Rate and Frequency of change in water conditions

Alteration indicators scores for each 22 interannual statistics are calculated by comparing the current period with the percentiles (25th and 75th, respectively) derived from the natural period. Next, these scores are combined to obtain the monthly index for hydrological regime for each of the 5 group, and then finally, scores from the 5 groups are combined to give the global score (GMI). The values are normalized using ranges referred to by the authors as:

$$ DvNF =\ \ 100 * \left( 1 - 3*GMI \right)\ $$