Agricultural Drought Indices - US Department of Agriculture

efers to deficiencies in surface and subsurface water supplies, leading to lack of water for meeting
normal and specific water demands. Agricultural drought is a deficiency in water availability for
specific agricultural needs, such as a deficiency in soil moisture, which is one of the most critical
factors in defining crop production potential.
Standardized Precipitation Index (SPI)
Drought indices assimilate thousands of bits of data on rainfall, streamflow, and other water supply
indicators into a comprehensible “big picture.” A drought index value is typically a single number,
far more useful than raw data for decision making. Several indices measure how precipitation for a
given period of time has deviated from historically established norms.
The Standardized Precipitation Index (SPI) is an index based on the probability of precipitation for
any time scale. It can be computed for different time scales to provide early warning of drought
and help assess drought severity in any given locality. The SPI is designed to quantify the
precipitation deficit for multiple time scales. These time scales reflect the impact of drought on the
availability of the different water resources. Soil moisture conditions respond to precipitation
anomalies on a relatively short scale. Groundwater, streamflow, and reservoir storage reflect the
longer-term precipitation anomalies. McKee et al. (1993) originally calculated the SPI for 3-, 6-,
12-, 24-, and 48-month time scales.
The SPI calculation for the various agro-ecological zones in the GHA sub-region is based on the
long-term precipitation record of more than thirty years. This long-term record is fitted to a
probability distribution, which is then transformed into a normal distribution so that the mean SPI
for the zone and desired period is zero (Edwards and McKee 1997). Positive SPI values indicate
greater than median precipitation, and negative values indicate less than median precipitation.
Because the SPI is normalized, wetter and drier climates can be represented in the same way, and
wet periods can also be monitored using the SPI. The spatial patterns of the observed rainfall and
rainfall stress severity index on 10-day, monthly, and seasonal timescales are used in monitoring
drought (Figure 3).
(a) Rainfall distribution
(b) Drought severity index
Figure 3. Map showing (a) distribution of rainfall and (b) calculated drought severity index for
February 2010.
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