Agricultural Drought Indices - US Department of Agriculture
Agricultural Drought Indices - US Department of Agriculture
Agricultural Drought Indices - US Department of Agriculture
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<strong>Agricultural</strong> <strong>Drought</strong> <strong>Indices</strong> in France and Europe:<br />
Strengths, Weaknesses, and Limitations<br />
Emmanuel Cloppet<br />
Météo-France, Toulouse Cedex (France)<br />
Abstract<br />
<strong>Drought</strong> is a recurrent event in Europe and part <strong>of</strong> natural climate variability. It may occur in all<br />
European climatic zones. Different levels <strong>of</strong> complexity <strong>of</strong> droughts can be considered, and as a<br />
consequence, a huge diversity <strong>of</strong> drought indices are used in Europe, from rainfall-based indices to<br />
indices based on soil water content estimation and numerous input parameters like rainfall,<br />
temperature, relative humidity, solar radiation, wind speed, soil moisture, or soil characteristics. At<br />
the moment there is no uniform approach for drought monitoring in Europe. An overview <strong>of</strong> some<br />
agricultural drought indices used in Europe is presented with suitable examples. Two drought<br />
indices currently being used by Météo-France in an operational context are described.<br />
Experimental drought indices based on multilevel drought reanalysis in the framework <strong>of</strong> the<br />
Climsec project in France are presented. <strong>Drought</strong> reanalysis <strong>of</strong>fers identification and description <strong>of</strong><br />
past drought events at both local and national scale in France.<br />
Introduction<br />
<strong>Drought</strong> is considered as an abnormal water deficit in at least one part <strong>of</strong> the land surface<br />
hydrological cycle. It can also be described in three dimensions: intensity, duration, and the area it<br />
covers. <strong>Drought</strong> is a recurrent event in Europe and part <strong>of</strong> natural climate variability. It may occur<br />
in all European climatic zones. It differs from aridity, which is restricted to low rainfall regions like<br />
Mediterranean area and which is a permanent feature <strong>of</strong> climate. Meteorological datasets show an<br />
increase in mean summer water deficit during the past 30 years, and climate change scenarios<br />
also predict significant decreases in summer precipitation in southern Europe, which could lead to<br />
an increase in drought frequency. Recent droughts in Europe, like the one in 2003, are a good<br />
illustration <strong>of</strong> the potential impact <strong>of</strong> increasing drought frequency. Different types <strong>of</strong> drought can<br />
be considered: meteorological, hydrological, agricultural, and socio-economic. <strong>Agricultural</strong> drought<br />
deals with agricultural impacts such as extensive damage to crops and loss <strong>of</strong> yield, focusing on<br />
precipitation shortages, evapotranspiration, and soil water deficits. This paper focuses on<br />
agricultural drought and drought indices used in Europe.<br />
<strong>Drought</strong> <strong>Indices</strong> at the European Level<br />
WMO defines a drought index as an index that is related to some <strong>of</strong> the cumulative effects <strong>of</strong> a<br />
prolonged and abnormal moisture deficiency. A drought index value is typically a single number,<br />
which is more useful than raw data for decision making. The choice <strong>of</strong> relevant drought indices<br />
depends on the socio-economic activity domain. There is no universal drought index. <strong>Drought</strong><br />
monitoring can also be based on a synthesis <strong>of</strong> multiple drought indices. The complexity <strong>of</strong><br />
drought indices depends on the number <strong>of</strong> parameters taken into account. Different levels <strong>of</strong><br />
complexity can be considered and as a consequence a huge diversity <strong>of</strong> drought indices are used<br />
in Europe, from rainfall-based indices to indices that are based on soil water content estimation<br />
and numerous input parameters like rainfall, temperature, relative humidity, solar radiation, wind<br />
speed, soil moisture, or soil characteristics. Evapotranspiration estimation is usually a key issue.<br />
Data availability should allow potential evapotranspiration (PET) estimation with the Penman<br />
approach or Penman-Monteith formula. This approach is more accurate and is consistent with<br />
WMO recommendations. It is worth emphasizing the role that evapotranspiration plays in the<br />
development and therefore also the definition <strong>of</strong> a drought. It is therefore important to define<br />
evapotranspiration as overall water loss through evaporation and transpiration from plants. The<br />
PET depends on the meteorological factors listed above and follows the hypothesis that there is<br />
enough water in the soil for vegetation at any time. The difference between actual and potential<br />
evapotranspiration depends on soil moisture. Following this approach, both crop and soil water<br />
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