Streamflow Drought Index (SDI)
Index name: Streamflow Drought Index (SDI).
Ease of use: Yellow.
Origins: Developed by Nalbantis and Tsakiris using the methodology and calculations of SPI as the basis for their work.
Characteristics: Uses monthly streamflow values and the methods of normalization associated with SPI for developing a drought index based upon streamflow data. With an output similar to that of SPI, both wet and dry periods can be investigated, as well as the severity of these occurrences.
Input parameters: Monthly streamflow values and a historical time series for the streamflow gauge.
Applications: Used to monitor and identify drought events with reference to a particular gauge, which may or may not represent larger basins.
Strengths: The program is widely available and easy to use. Missing data are allowed, and the longer the streamflow record, the more accurate the results. As with SPI, various timescales can be examined.
Weaknesses: A single input (streamflow) does not take into account management decisions, and periods of no flow can skew the results.
Resources: It is described in the literature with mathematical examples provided. The SPI code is available at http://drought.unl.edu/MonitoringTools/DownloadableSPIProgram.aspx. See http://drinc.ewra.net/ for information on SDI.
References: Nalbantis, I. and G. Tsakiris, 2008: Assessment of hydrological drought revisited. Water Resources Management, 23(5): 881–897.