The Standardized Streamflow Index (SSI) is a hydrological drought index used to assess and monitor drought conditions based on streamflow data. It is conceptually similar to the Standardized Precipitation Index (SPI) but focuses on river discharge rather than precipitation.

Key Features of SSI:

1. Statistical Standardization - SSI is derived by fitting historical streamflow records to a probability distribution (commonly gamma or lognormal), which is then transformed into a standard normal distribution.
2. Temporal Scales - Like SPI, SSI can be calculated over different timescales (e.g., 1-month, 3-month, 6-month, 12-month) to analyze short-term or long-term hydrological conditions.
3. Drought Classification - The SSI values are categorized into different drought intensities:
   • 0 to -0.99 $\rightarrow$ Mild drought
   • 1.00 to -1.49 $\rightarrow$ Moderate drought
   • 1.50 to -1.99 $\rightarrow$ Severe drought
   • $\leq-2.00 \rightarrow$ Extreme drought
4. Application in Hydrology - SSI is particularly useful for water resource management, flood and drought forecasting, and assessing climate variability impacts on river basins.

Calculation of the Standardized Streamflow Index (SSI)

The SSI is computed in a way similar to the Standardized Precipitation Index (SPI) but using streamflow data. The process involves the following steps:

1. Data Collection

• Obtain historical monthly streamflow records from gauging stations in a given river basin.
• Ensure that the data is continuous and of sufficient length (usually at least 30 years).

2. Probability Distribution Fitting

• Transform the observed streamflow values into a probability distribution.
• The most commonly used distributions for streamflow data are:
  • Gamma distribution
  • Log-normal distribution
  • Generalized extreme value distribution
  • Pearson Type III distribution
• The choice of distribution depends on the characteristics of the specific river basin.

3. Normalization and Standardization

• Convert the fitted probability distribution into a standard normal distribution (mean = 0, standard deviation = 1).
• This step allows for the comparison of streamflow conditions across different regions and time periods.
• The standardized values form the SSI time series.

4. Time Scale Selection

• SSI is calculated for different time scales (e.g., 1-month, 3-month, 6-month, 12-month).