Agricultural Drought Indices - US Department of Agriculture

Soil Water Index maps are published and disseminated through the website of Météo-France on a
monthly basis (Figure 6). Ratio to mean maps comparing the current year with climatology are
also provided. These maps are disseminated every ten days throught the Météo-France website,
and are free of charge. This index is useful in assessing impacts on agriculture.
Figure 6. Soil Water Index anomaly over France (December 2007). Negative values indicate drier
conditions as compared to the normal, while positive values represent wetter than average
conditions.
This land surface scheme allows state-of-the-art hydrological modelling with realistic soil and
vegetation parameterization and explicit snowpack modelling through the computation of water and
energy budgets with soil and vegetation databases. Outputs are mainly soil moisture (SWI) but
also actual evapotranspiration, snow cover, drainage, runoff, etc. Products are available on a
regular 8 km grid over France. However, this approach has some inherent limitations: it is a
complex hydrometeorological suite to run and it is available only at national level. Nevertheless,
this surface scheme approach is undoubtedly the way to go in the future and is nearly mandatory
in mountainous areas.
Experimental Drought Indices in France
The SIM operational suite has been used to derive a 50-year hydrometeorological reanalysis,
running from 1958 to 2008. More details about the atmospheric part of the reanalysis performed
by the Direction of Climatology of Météo-France can be found in Vidal et al. (2009a). This long
dataset has been used to build experimental drought indices based on multilevel drought
reanalysis in the framework of the ClimSec project (Vidal et al. 2010). The ClimSec project is a 2-
year project dealing with the impact of climate change on drought and soil moisture in France. It
was motivated by the extensive damage to buildings caused by the shrinking and swelling of clay
soils following the 2003 drought.
Method
The method employed is inspired by the SPI computation, and it has been applied to the Soil
Water Index instead of rainfall amounts in order to derive a Standardized Soil Wetness Index. The
monthly variable is summed or averaged over n months and its distribution is projected onto a
normal distribution. The computation here is done with reference to the 50-year local climate, so
that we have a correspondence between the index value and a non-exceedance probability (Vidal
2010). That also ensures the spatial consistency of the index, and different time scales can be
considered from 1 to 24 months. The Standardized Soil Water Index is considered an
experimental index but is now automatically produced at a monthly time step by the Climatology
Section of Météo-France. Four time scales (i.e., 1, 3, 6, and 12 months) are considered (Figure 7).
A panel of standardized indices from 1958 to 2008 is also available for different time scales.
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