Aridity Anomaly Index (AAI)
Index name: Aridity Anomaly Index (AAI).
Ease of use: Green.
Origins: Developed in India by the India Meteorological Department.
Characteristics: A real-time drought index in which water balance is considered. The Aridity Index (AI) is computed for weekly or two-weekly periods. For each period, the actual aridity for the period is compared to the normal aridity for that period. Negative values indicate a surplus of moisture while positive values indicate moisture stress.
Input parameters: Actual evapotranspiration and calculated potential evapotranspiration, which require temperature, wind and solar radiation values.
Applications: Impacts of drought in agriculture, especially in the tropics where defined wet and dry seasons are part of the climate regime. Both winter and summer cropping seasons can be assessed using this method.
Strengths: Specific to agriculture, calculations are simple, and descriptions of drought (mild, moderate or severe) are based on departure from normal. Responds quickly with a weekly time step.
Weaknesses: Not applicable to long-term or multiseasonal events.
Resource: India Meteorological Department (IMD) methodology of drought monitoring.
References: World Meteorological Organization, 2012: Chapter 6, Agrometeorological Forecasting. In: Guide to Agricultural Meteorological Practices (GAMP), WMO-No. 134, World Meteorological Organization, Geneva, Switzerland.
Currently used by: India.