Agricultural Drought Indices - US Department of Agriculture

The second index is extracted from the SAFRAN ISBA MODCOU hydrometeorological model, which is
now used to provide consistent computation of variables within the hydrological cycle. The SIM
hydrometeorological model (Figure 5) consists of three independent modules: the SAFRAN
atmospherical analysis, the ISBA land surface model, and the MODCOU hydrogeological model. It
deals with state-of-the-art hydrological modelling with a spatial resolution of 8 km. This
hydrometeorological suite is described in Habets et al. (2008), Noilhan and Mahfouf (1996), and
Quintana-Seguí et al. (2008).
Figure 5. The SIM hydrometeorological model run by Météo-France consists of three independent
modules: the SAFRAN atmospherical analysis, the ISBA land surface model, and the MODCOU
hydrological model.
The first model, SAFRAN, is an atmospheric analysis system that combines ground observations
with large-scale fields from a global reanalysis for producing gridded outputs of several variables
with an 8-km resolution at an hourly time scale. These variables are used to force the ISBA land
surface scheme, which computes water and energy budgets at the soil-vegetation-atmosphere
interface. One output particularly relevant to this study is the Soil Wetness Index. The drainage
and runoff outputs are then used by the MODCOU hydrogeological model to compute river flows at
more than 900 hydrometric stations in France. The SAFRAN model deals with meteorological
drought and the ISBA model deals with agricultural drought. The key output of this model is the
Soil Water Index, which is defined as a normalized soil water content index ranging from 0 (water
content at wilting point) to 1 (water content at field capacity).
w − w
SWI =
w − w
fc
wilt
wilt
Where
w = water content
wwilt = wilting point
wfc = field capacity
90