Session 7.9 Frequency of precipitation in the Humid Pampa of Argentina

Thursday, 23 June 2005: 10:30 AM
North & Center Ballroom (Hilton DeSoto)
Olga C. Penalba, Universidad de Buenos Aires, Buenos Aires, Argentina; and F. Robledo

Presentation PDF (530.1 kB)

Climate is one of the most important limiting factors for agricultural production, in particular the precipitation. Agriculture is quite sensitive to the ordinary rhythm of the climate, being adjusted to make use of the seasonal changes. Failure of the expected seasonal rainfall or any other kind of disturbance of the rhythm leads to economic losses. Crop diversification and production depend not only on total precipitation, but also on the distribution of precipitation during the growing season. Frequencies of daily precipitation events have important implications for agriculture as well as for management of natural resources.

The region under study, the humid Pampa, is located north of 40°S and east of 67°W. This zone plays a crucial role in the local economies. In particular, it is one of the largest food producers in the world. Cropping systems include corn, soybean and wheat-soybean doublecrop.

The objective of this work is to analyze the frequency of daily precipitation events throughout the year, with special interest in extreme and very heavy rainfall; and identify days with more frequent or less frequent precipitation.

Historical daily rainfall records were obtained from the National Weather Service for 20 pluviometric stations located in the humid Pampa. The shorter period analyzed was 1950-2003 and the longest one was 1908-2003.

The frequency of precipitation events for each day of the year was assessed over the whole period of each station. This frequency was determined for daily precipitation events of at least 0.1, 2, 5, 10, 30, 50, 80, 100 mm. These annual cycles were smoothing using 11-day running average. The persistence of rainday and no rainday was also analyzed by assessing the probability of rainday when the day before was a rain day and the probability of no rainday when the day before was a no rainday.

Annual precipitation shows a space variation, decreasing from northeast (1600 mm/year) to southwest (700 mm/year) across the humid Pampa. The rainfall mean annual cycle has a geographical component; varying from a defined annual cycle (rainfall maximum during the summer) in the western area to a uniform seasonal distribution with two maxima, during spring and autumn in the rest of the area. The annual cycle of the 50th, 75th and 90th percentiles of daily precipitation amount show a similar behavior as the rainfall mean annual cycle, showing that the rainfall variation is not caused by variations in the intensity.

The maximum values of the 90th percentile of daily precipitation occur during summer in the western area, being of 30 mm/day. Meanwhile, in the rest of the area, the maximum values occur during autumn, varying from 45 mm/day in NE area to 20 mm/day in SW area. The minimum values of 90th percentiles are observed during winter in the whole region, varying from 20 mm/day (NE) to 8 mm/day (SW).

The frequency of daily precipitation events exceeding 0.1 mm follows the temporal behavior of the rainfall mean annual cycle. The greatest differences among locations were observed during winter. Intense precipitation events, in excess of 50 and 80 mm/day were uncommon throughout the year. In particular rainfall events in excess of 100 mm are very rare. Total events in excess this value ranged from 4 in the southwest to 74 in the northeast. The persistence of rainday throughout the year shows high values during the wet season and low values during the drier season, caracteísitca de las zonas tropicales.

This regional and seasonal variability in precipitation must be considered when designing hydrologic systems and managing agricultural and natural resources.

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