Specifications
Summary
Drought is a common climate feature caused by prolonged low precipitation, poor rainfall timing, or increased water loss due to high temperatures. Monitoring droughts involves analysing various indicators and indices that reflect different parts of the hydrological cycle, such as precipitation, soil moisture, and water levels. These indicators show statistical anomalies compared to long-term climate data, indicating the severity of a drought.
There are three main methods for monitoring droughts:
- Single indicator or index.
- Multiple indicators or indices.
- Composite or hybrid indicators.
In the implementation of the ANIN project and through consultation with the South African partners, the following indices and indicators were developed for drought early warning, monitoring and forecasting for South Africa.
The following sections describe the drought indices presented in this Handbook, as well as their key characteristics. For more information detailed information, please refer to the ATBD and product specification document of the ANIN project
Standardised Precipitation Index (SPI)
SPI was developed to quantify precipitation deficits over various timescales, ranging from 1 to 36 months. This flexibility allows it to reflect short-term soil moisture conditions as well as long-term groundwater and reservoir storage levels.
Standardised Precipitation-Evapotranspiration Index (SPEI)
SPEI extends the capabilities of SPI by incorporating both precipitation and potential evapotranspiration (PET). This inclusion allows the SPEI to account for the effects of temperature on drought conditions. SPEI is able to reflect the impact of increased temperatures on water demand, which is crucial for understanding drought severity under changing climate conditions. The SPEI can be calculated over various timescales, from 1 to 48 months, allowing it to capture both short-term and long-term drought conditions.
Vegetation Condition Index (VCI)
VCI is used to asses vegetation health by comparing current vegetation conditions to historical trends. It is particularly useful in monitoring drought conditions and their impact on vegetation. The VCI is calculated using the Normalized Difference Vegetation Index (NDVI), which normalises current NDVI values against historical minimum and maximum NDVI values for the same period, effectively separating weather-related impacts from ecological factors.
Soil Moisture Anomaly (SMA)
SMA is used to assess deviations in soil moisture levels from the long-term average. It helps in understanding the extent of dryness or wetness in the soil, which is crucial for drought monitoring, agricultural planning, and water resource management. SMA is calculated by comparing current soil moisture levels to historical averages. Positive anomalies indicate wetter-than-average conditions, while negative anomalies signify drier-than-average conditions.
Combined Drought Indicator (CDI)
CDI is a comprehensive tool used to detect and monitor agricultural drought. It integrates multiple drought indicators to provide a detailed assessment of drought conditions, namely SPI, SMA and FAPAR Anomaly.
