Agricultural Drought Indices - US Department of Agriculture

Figure 8 shows the combination of mean area, mean duration, and mean severity. Similar mean
severities have been reached for different events, but with different combinations of mean area
and mean duration. The 1988-1990 drought stands out for this specific drought index and
threshold. Figure 9 shows the evolution of the area affected by droughts, defined here by a
threshold of 5% on 3-month indices. Different drought periods like the 1976 drought, autumn 1978
drought, multiyear 1989-1990 drought, and recent 2003 drought can be identified in this chart.
There are differences between the three types of droughts. For example, the soil moisture deficits
were much more severe in 2003 than the precipitation deficits. Standardized drought indices are
undoubtedly powerful analytical tools for characterizing events in space and time. Drought
reanalysis offers identification and description of past drought events at both the local and national
scale in France (Vidal et al. 2010).
Figure 9. Evolution of the area affected by droughts, defined here by a threshold of 5% on 3-month
indices (from Vidal et al. 2009).
Conclusions
There is no universal drought index and there is no uniform approach for drought monitoring in
Europe. The complexity of drought indices depends on the number of parameters taken into
account, and different levels of complexity can be considered. However, a focus on drought
indices that offer a consistent estimation of soil water content through a realistic calculation of
evapotranspiration is recommended. Rainfall-based indices are unable to estimate severe soil
moisture deficits when precipitation deficit is not the only driver (2003 drought, for example).
The land surface scheme approach allows explicit snowpack modeling through the computation of
water and energy budgets with soil and vegetation databases. This approach is undoubtedly the
way forward and is nearly mandatory in mountainous areas where snowpack modeling is a key
issue. Lastly, we showed that the SPI normalization approach is not restricted to rainfall and can
be applied to soil moisture. Standardized drought indices are undoubtedly powerful analytical tools
for characterizing events in space and time. Drought reanalysis offers identification and
description of past drought events at both the local and national scale in France.
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