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The semi-arid Pampas region of Argentina is economically important, generating a considerable proportion of the country’s wheat production. The region suffers frequent drought periods, being climatically semi-arid and dry subhumid. Precipitation is scarce and erratic and the soils are generally shallow, with a low soil-water holding capacity.

The Palmer drought severity index (PDSI) was applied to evaluate the historical sequence of wet and dry periods since the beginning of the century. Data for this study cover the period 1912-1990, though several locations have longer records. The region has dry, wet, and normal periods during 38 per cent, 32 per cent and 30 per cent of the time, respectively. Extreme intensity monthly values of PDSI (more than +4 and less than -4) occur in episodes of more than a year’s length.

Return periods of 20 years were calculated for the extreme drought (-4) from the series of minimum annual PDSI values. Extreme wet intensity, such as that registered in 1986 (+7) has a recurrence interval of 50 years.

Correlation coefficients for the PDSI indicate a good association between station pairs, increasing as separation distances decrease. Another feature of the coefficients is scattering, which may be related to precipitation gradients.

From principal components analysis of monthly PDSI it is concluded that the locations selected are fairly well associated and correspond to a homogeneous region from a climatic view point.