Agricultural Drought Indices - US Department of Agriculture

In Germany, the Deutsche Wetterdienst (DWD) uses the Martonne index, Standardized
Precipitation Index (SPI), and output from the water balance model in order to assess the intensity
of droughts. The agrometeorological department publishes maps of the soil moisture and the
current agromet conditions (soil moisture calculated for a region). The model AMBAV, part of a
complex agrometeorological model toolbox of the German Weather Service, simulates the water
balance in the crop-soil system using the Penman-Monteith formula on an hourly basis. The
model separately calculates soil evaporation, transpiration, and interception for up to 13 different
crop covers considering the relevant processes of heat, water, and vapor transport in the soil-cropatmosphere
interface, including water losses during irrigation. Crops considered are winter wheat,
spring wheat, winter barley, rye, oats, maize, sugar beets, potatoes, oilseed rape, grassland, fruit
trees, and coniferous and deciduous forest. The model is used to produce recommendations for
irrigation amounts and scheduling, which are disseminated by the DWD by fax.
Other European countries have carried out research studies on drought indices, but have not used
this research for forecasting. In Great Britain, research has been done on the Meteorological
Drought Severity Index (DSI); in Greece, on the Reconnaissance Drought Index (RDI); and in
Portugal, on the Palmer Drought Severity Index (PDSI). PDSI is a good candidate as it performs a
parameterized computation of the soil water balance and compares the estimated soil moisture
content with its climatological mean. Drought patterns are presented in monthly PDSI maps that
show the spatial distribution of drought in Portugal. These maps are used to monitor spatial and
temporal variations in drought across mainland Portugal, which is helpful in delineating potential
disaster areas for agriculture and other sectors, allowing for improved on-farm decisions to reduce
impacts (WMO 2006). Instituto de Meteorologia of Portugal uses a Geographical Information
System (GIS) to map the PDSI to monitor drought over the country. These maps are critical to
determining drought-prone areas. The maps are available on the meteorological institute’s
website, and are updated monthly. Drought indices and coefficients are also used as applications
for operational agrometeorology in Bulgaria. This agrometeorological monitoring is performed by
the division of Agrometeorology of the National Institute of Meteorology and Hydrology of Bulgaria.
In this area, crop production is mainly limited by water stress. Different parameters and indicators
can produce useful information for water stress monitoring as cumulative rainfall amounts or soil
moisture monitoring. The Balance of Atmosphere Moisturizing (BAM) model is run by the division
of Agrometeorology in Bulgaria. This model is defined as the difference between cumulative
rainfall and cumulative PET. The drought index is defined in Bulgaria as the ratio between total
cumulative rainfall amount and cumulative PET. This P/PET index can be used in operational
context or for climatological purpose.
Several international projects at the European level aim to standardize the definitions of drought
and develop plans and actions to be taken in case of drought. One of these research projects is
under the lead of the Joint Research Center (JRC) of the European Commission: the Natural
Hazard of the Action Institute for Environment and Sustainability (NAHA-IES). This work was
initiated after the severe droughts in recent years and is based on the experience gained with the
implementation of the system of flood prevention (European Flood Alert System—EFAS). The aim
is to establish a system for monitoring, detecting, and forecasting droughts at the European level.
Precipitation anomalies, soil moisture, and soil moisture anomalies are available freely on the JRC
website. Precipitation anomalies are represented by monthly SPI. SPI values reflect short-term
changes in precipitation as compared to the long-term average of the respective month, and can
be compared well to the top soil moisture as produced by LISFLOOD simulations. Positive SPI
values indicate greater-than-median precipitation, and negative values indicate less-than-median
precipitation. Values of SPI are commonly classified as McKee et al. 1993. In the forecasting
mode, the European Flood Alert System produces information on the development of soil moisture
in Europe for up to ten days ahead. The forecasted soil moisture seven days ahead is compared
to the long-term average conditions of this date as derived from re-analysis data of the European
Centre for Medium-Range Weather Forecasts (ERA-40) for the period 1958-2001 (i.e., 44 years)
and provide a consistent set of forecasted meteorological parameters.
86