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Standard Precipitation Index

Standard Precipitation Index (SPI)

SPI is used to characterize meteorological drought on a range of timescales. On short timescales, the SPI is closely related to soil moisture, while at longer timescales, the SPI can be related to groundwater and reservoir storage. The SPI can be compared across regions with markedly different climates. It quantifies observed precipitation as a standardized departure from a selected probability distribution function that models the raw precipitation data. The raw precipitation data are typically fitted to a gamma or a Pearson Type III distribution, and then transformed into a normal distribution

Key Strength of SPI 

 • Uses precipitation only: it can characterize drought or abnormal wetness at different times scale which corresponds to the time availability of different water resources (eg: soil moisture, snowpack, river discharge, reservoir storage) 

 • Less complex to calculate than PDSI

Key Limitation of SPI 

 • As a measure of water supply only, the SPI does not account doe evapotranspiration and limits its ability to capture the effects of increasing temperature (Associated with climate change) on moisture demand availability. 

• Sensitive to the Quantity and reliability of data used to fit the distribution,30-50 years recommended

 • Dose not consider the intensity if precipitation and impacts on runoff, streamflow, and water availability within the system of interest.

More about SPI Index 

A 1-month SPI typically compares well to the percent of normal precipitation for the month. However, as with other SPI time scales, it is actually considered to be a more accurate representation of monthly precipitation for a given location because the long-term precipitation record (over 30 years or more) is fitted to a probability distribution. It is then transformed into a normal distribution so that the median SPI for the location and period is zero; that is, half of the historical precipitation amounts are below the median and half are above the median. Positive SPI values indicate greater than median precipitation (i.e. wet conditions), and negative values indicate less than median precipitation (i.e. dry conditions). The 1-month SPI is a short-term value and during the growing season can be important for the correlation of soil moisture and crop stress.

The 3-month SPI provides a comparison of the precipitation over a specific 3-month period with the precipitation totals from the same 3-month period for all the years included in the historical record. In other words, a 3-month SPI at the end of February compares the December-JanuaryFebruary precipitation total in that particular year with the December-February precipitation totals of all the years. A 3-month SPI reflects short- and medium-term moisture conditions and provides a seasonal estimation of precipitation. It is important to compare the 3-month SPI with longer time scales. A relatively normal 3-month period could occur in the middle of a longer-term drought that would only be visible at longer time scales. Looking at longer time scales would prevent a misinterpretation that any "drought" might be over.

The 3-month SPI provides a comparison of the precipitation over a specific 3-month period with the precipitation totals from the same 3-month period for all the years included in the historical record. In other words, a 3-month SPI at the end of February compares the December-JanuaryFebruary precipitation total in that particular year with the December-February precipitation totals of all the years. A 3-month SPI reflects short- and medium-term moisture conditions and provides a seasonal estimation of precipitation. It is important to compare the 3-month SPI with longer time scales. A relatively normal 3-month period could occur in the middle of a longer-term drought that would only be visible at longer time scales. Looking at longer time scales would prevent a misinterpretation that any "drought" might be over.

The 9-month SPI provides an indication of precipitation patterns over a medium time scale. Droughts usually take a season or more to develop. SPI values below -1.5 for these time scales are usually a good indication that fairly significant impacts are occurring in agriculture and may be showing up in other sectors as well. Some regions of the country may find that the pattern displayed by the map of the Palmer Index closely relates to the 9-month SPI maps. For other areas, the Palmer Index is more closely related to the 12-month SPI. The Palmer Index maps are updated each week, although the patterns usually do not change significantly. The SPI maps are updated at the end of each month.

A 12-month SPI is a comparison of the precipitation for 12 consecutive months with the same 12 consecutive months during all the previous years of available data. The SPI at these time scales reflects long-term precipitation patterns. Because these time scales are the cumulative result of shorter periods that may be above or below normal, the longer SPIs tend toward zero unless a specific trend is taking place. SPIs of these time scales are probably tied to streamflows, reservoir levels, and even groundwater levels at the longer time scales. In some locations of the country, the 12-month SPI is most closely related to the Palmer Index, and the two indices should reflect similar conditions.

Result of a Case study: 




Code for the above calculation attached 

GitHub: https://github.com/Barbhuiya12/Hydrology-/blob/main/Standard_Precipitation_Index.ipynb



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