Online Drought Indicators and Indices

Standardized Streamflow Index (SSFI)


Index name: Standardized Streamflow Index (SSFI).

Ease of use: Yellow.

Origins: Modarres introduced SSFI in 2007, and Telesca et al. investigated it further. In the original work, Modarres described how SSFI was similar to SPI in that SSFI for a given period was defined as the difference in streamflow from mean to standard deviation.

Characteristics: Developed using monthly streamflow values and the methods of normalization associated with SPI. Can be calculated for both observed and forecasted data, providing a perspective on high and low flow periods associated with drought and flood.

Input parameters: Streamflow data on a daily or monthly timescale.

Applications: Monitoring of hydrological conditions at multiple timescales.

Strengths: Easy to calculate using the SPI program. A single variable input that allows for missing data makes it easy to use.

Weaknesses: Only accounts for the streamflow in the context of monitoring drought, with no other influences being investigated.

Resources: It is described well in the literature, with mathematics and case studies available. The SPI program is available at


Modarres, R., 2007: Streamflow drought time series forecasting. Stochastic Environmental Research and Risk Assessment, 21: 223–233.

Telesca, L., M. Lovallo, I. Lopez-Moreno and S. Vicente-Serrano, 2012: Investigation of scaling properties in monthly streamflow and Standardized Streamflow Index time series in the Ebro basin (Spain). Physica A: Statistical Mechanics and its Applications, 391(4): 1662–1678.

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