Agricultural Drought Indices - US Department of Agriculture

creation of a system of European supervision of droughts that attempts to predict drought and
develop guidelines to follow in case of drought.
Operational drought indices in France
Two drought indices are currently run by Météo-France in an operational context. The first index is
based on a simplified operational water balance with two reservoirs and a fixed soil depth (Figure
2). This index is applied all over France on fescue grass. In order to take into account soil
variability, four soil types are considered for the available water.
Figure 2. Water balance model with two reservoirs.
Input data are daily rainfall and daily Penman-Monteith PET and output data are remaining water
(value, percentage) and runoff. The main aim of this tool is to provide a synthetic view of the water
balance without interaction of crop and soil kinds. It is easy to calculate at station level, and it is
accurate. The main limitation of this model is that there is no interaction of crop and soil types, and
spatialisation is not easy. Charts are generated automatically at station level and on daily time
steps.
Drought assessment can be performed through a comparison of modeled water content with
different statistical thresholds based on percentiles in order to assess the relative frequency of the
event. This drought index is thus directly derived from model outputs (soil water content). The first
product refers to daily soil water content, which is defined as the ratio of soil moisture to soil
moisture storage capacity (Figure 3). A reference dry year is plotted on this chart (1976; black
line). Daily values (blue line) are compared to different statistical thresholds based on daily
percentiles in order to characterize soil moisture. The 10 th , 20 th , and 50 th percentiles are
considered. The 10th (20th, 50 th , etc.) percentile is the value below which 10 (20, 50, etc.) percent
of the daily observations may be found.
• drier than 10 th percentile: red
• drier than 20 th percentile: orange
• drier than 50 th percentile: yellow
• wetter than 50 th percentile: y
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