Agricultural Drought Indices - US Department of Agriculture

agriculture, which is one of the main economic activities in the country. Agricultural drought is
detected when continuous and intense soil moisture stress leads to significant yield reduction, as
shown in Figure 1.
3000
2500
Soybean Yield in Rio Grande do Sul State
Annual Yield Avg Yield Linear (Annual Yield)
Yield (kg ha -1 )
2000
1500
1000
500
Drought Impact
0
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
Figure 1. Historical soybean yield data for Rio Grande do Sul state, Brazil, showing the impact of
intense droughts on yield losses. Source: IBGE/Brazil.
According to Boken (2005), the most important kind of drought is agricultural drought, which can
cause serious disasters for food security, since crop yields are directly affected by soil moisture
shortage. Drought assessment, monitoring, and preparedness planning should be considered
essential components of integrated water resources management systems, as mentioned by
Wilhite (2005), to reduce societal vulnerability to future drought events.
Meteorological drought occurs when the seasonal or annual precipitation falls below its long-term
average. Hydrological drought develops when meteorological drought is prolonged and causes
shortages of surface and groundwater in the region. Agricultural drought is detected when
continuous and intense soil moisture stress leads to significant crop yield reduction. Finally, socioeconomic
drought is a manifestation of continued drought of severe intensity that causes economic
and sociopolitical instabilities in a region or country. Whereas meteorological drought is just an
indicator of precipitation deficiency, hydrological and agricultural droughts can be considered the
physical manifestations of meteorological drought, and socioeconomic drought results from the
impacts of hydrological and agricultural droughts on the society.
Even considering the importance of monitoring agricultural droughts, selecting indices for
monitoring is not easy because these indices will require information from different sources, like
climate (rainfall and evapotranspiration), soils (holding capacity and moisture), crops (species,
variety, root depth, and phenological phase), and crop management (sowing dates, crop rotation,
irrigation, no tillage, and intercropping), which should be integrated to produce a unique value. The
combination of these factors leads to different impacts on agriculture when a drought occurs,
making it difficult to decide which is the best index to quantify the drought for agriculture.
As there is an urgent need to mitigate the effects of agricultural droughts in the short term, the
recommendation has been made that the national or regional Meteorological and Hydrological
Services (M&HSs) provide assessment and monitoring of this natural disaster as part of their
products. The objective of this chapter is to give an overview of the agricultural drought indices in
current use in Brazil, considering the products provided by the M&HSs, as well as to discuss some
of their strengths and limitations.
61