Standardized Precipitation Index (SPI)

"The Standardized Precipitation Index (SPI) is a widely used index 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. [...]

SPI = (P - P*) /  σp

where P = precipitation

p* = mean precipitation

σp = standard deviation of precipitation" (Keyantash, John & National Center for Atmospheric Research Staff, 2018)


"The Climate Data Guide: Standardized Precipitation Index (SPI)". Keyantash, John & National Center for Atmospheric Research Staff (Eds). Last modified 07 Aug 2018.….
Accessed Mar 1, 2019.

Related Content


Real-time drought monitoring from Climate Hazards group Infrared Precipitation with Stations (CHIRPS)

Different parts of world are experiencing extreme hydrological hazards such as droughts, flooding and other related events. Droughts are associated with absence of rainfall occurrence over an extended period. According to the United Nations (2022), the frequency and intensity of drought events in the last two decades has increased by 29%. These figures are expected to increase further in the coming years due to climate change (Gunathilake et al., 2020). 

Space-based Solution

Collaborating actors (stakeholders, professionals, young professionals or Indigenous voices)
Suggested solution

Why is the approach suggested

Rainwater harvesting is a crucial solution for water scarcity in semi-arid countries like Kenya. Kenya’s arid and semi-arid lands (ASALs) cover 80% of its territory, making rainwater harvesting essential. There are various reasons why this approach can be beneficial in Samburu County.  

  • Water Scarcity Mitigation: Semi-arid regions face unpredictable rainfall and frequent droughts, exacerbated by climate change. Rainwater harvesting captures the little rainfall received, providing a reliable water source. 
  • Sustainable Water Supply: Rainwater harvesting techniques include small planting basins, trenches, stone bunds, and grass strips. These structures redirect runoff towards crops and pastures. By capturing rainwater, communities can sustain livestock, crop production, and domestic needs. 
  • Environmental Resilience: Droughts in Kenya are becoming more frequent due to environmental degradation and climate change. Rainwater harvesting helps mitigate the impact of these droughts.  
  • Cost-Effective and Low-Tech: Rainwater harvesting doesn’t require complex infrastructure. It utilizes existing resources effectively. 


  • Site Selection: Identify suitable locations for rainwater harvesting. Consider factors such as rainfall patterns, topography, and proximity to communities. Choose areas with consistent rainfall during specific seasons. 
  • Catchment Area: Determine the catchment area where rainwater will be collected. Common catchment surfaces include rooftops, roads, or open fields. Ensure that the catchment area is clean and free from contaminants. 
  • Conveyance System: Design an efficient system to channel rainwater from the catchment area to storage facilities. Components include gutters, downspouts, pipes, and first-flush diverters. Proper sizing and maintenance are crucial. 
  • Storage Tanks or Reservoirs: Select appropriate storage options based on community needs. Common choices include: 
  • Roof catchment tanks: Placed near buildings to store rainwater from rooftops. 
  • Ground-level tanks: Buried or partially buried to store larger volumes. 
  • Rock catchments: Natural depressions or excavated pits lined with impermeable materials. 
  • Consider tank capacity, material durability, and accessibility for maintenance. 
  • Water Quality and Treatment: Rainwater may contain impurities. Implement filtration systems to improve water quality. Use first-flush diverters to discard initial runoff (which may contain debris). 
  • Climate Resilience: Adapt the project to changing climate conditions. Monitor rainfall patterns and adjust storage capacity accordingly. 

Data requirements 

  • Rainy season identification 
    Decadal Precipitation in Kenya
    Figure 1: Decadal Precipitation in Kenya. Precipitation information during 21-31 December 2023. (Source: Dekadal Rainfall (


    • Precipitation data from at least the last three years: CHRIPS  
    Decadal rainfall data
    Figure 2: Decadal rainfall in mm from July 2020 to July 2023. (Source: Dekadal Rainfall (


    • Digital elevation Model (DEM)
    DEM in the Samburu County
    Figure 3: Elevation Map (DEM) made with QGIS. Version 3.32.3 / Version 3.28.11 LTR 

    Future steps

    • Map available rainfall with CHIRPS with Kenya Space Agency data ​
    • Determine if enough water can be stored during the rainy season to last the dry season
    • Combine with meteorological data (SPI values)
    • NDVI and DEM to get seasonality of the water cycle
    Relevant publications
    Related space-based solutions
    Keywords (for the solution)
    Climate Zone (addressed by the solution)
    Habitat (addressed by the solution)
    Region/Country (the solution was designed for, if any)
    Relevant SDGs