Soil Moisture Anomaly (SMA)
Index name: Soil Moisture Anomaly (SMA).
Ease of use: Yellow.
Origins: Developed by Bergman et al. at the National Weather Service in the United States during the mid-1980s as a way to assess global drought conditions.
Characteristics: Can use weekly or monthly precipitation and potential evapotranspiration values in a simple water balance equation. It is intended to reflect the degree of dryness or saturation of the soil compared with normal conditions and to show how soil moisture stress influences crop production around the world.
Input parameters: Weekly or monthly temperature and precipitation data along with date and latitude. Values for soil moisture-holding capacity and site-specific data can be used, although defaults are included.
Applications: Developed and used extensively for monitoring drought impacts on agriculture and crop production around the world.
Strengths: By taking into account the effects of both temperature and precipitation, the water balance aspects that make PDSI so popular are included with the ability to change constants with site-specific data. It considers moisture at different layers of the soil and is more adaptable than PDSI to different locations.
Weaknesses: The data requirements make it challenging to calculate. Potential evapotranspiration estimates can vary quite substantially by region.
Resources: The inputs and calculations are described thoroughly in the literature. No program exists at this time to provide the calculations.
Reference: Bergman, K.H., P. Sabol and D. Miskus, 1988: Experimental Indices for Monitoring Global Drought Conditions. Proceedings of 13th Annual Climate Diagnostics Workshop, United States Department of Commerce, Cambridge, MA.