Precipitation or no precipitation persistent over time often became extreme weather conditions with greater regional economic impact. The temporal and spatial variability of these variables, together with the evapotranspiration, is crucial in the east‐northeast of Argentina, where rain‐fed agricultural production is carried out. In this work, the frequency of precipitation was studied from dry spells and complemented with an analysis of the accumulated precipitation and evapotranspiration. In particular, dry sequences longer than 15 days, return period and severity, were the focus of this study. Finally, the impact of the amount and frequency of precipitation on soil water storage was assessed through a decadal analysis. The region of study is characterized by northeast‐southwest gradient in accumulated precipitation and east–west gradient in winter long dry sequences. During summer, higher and more frequent precipitations (lower probability of long dry sequences and lower return period of 15 days and severity) were presented whereas the opposite was found in winter. However, the stations located to the west presented the highest probability of long dry sequences with higher severity and lower accumulated precipitation. This result highlights the vulnerability of the agriculture activity in the western stations. Regarding the impact of long dry sequences over soil water storage, the seasonality of evapotranspiration is also involved. The impact is stronger during austral summer because of higher values of evapotranspiration and it is lower during winter, in spite of the higher probability of long sequences. Decadal analysis suggested that soil water storage responds to precipitation amount or frequency depending on the magnitude of the anomalies. In this sense, the impact of precipitation over soil water storage depends on how it is distributed.

Soil water surplus and deficit occur frequently in Buenos Aires province in Argentina. This paper analyses the soil water surplus in a sub-area, the Salado River basin, in the period 1968-2008. This basin is divided in seven drainage areas, delimitated according to the National Water Resources. The series of soil water surplus data were adjusted by means of the theoretical normal cubic-root probability distribution, and the mean areal soil water surplus value of 300 mm was considered as a threshold above which floods can cause severe damage. An increase in the frequency of extreme events and in their tendency exists during the recent years, coherent with the increase of precipitation recorded in the region. The statistical significance of the results was assessed using the Mann Kendall and MAKESENS tests. The results showed a relevant temporal variability, but did not show significant tendencies.

The deficit of daily precipitation, persistent over time (long dry sequences [LDS]), causes economic losses in the east–northeast of Argentina where agriculture production is the main economic activity. For this reason, an improvement in the seasonal prediction of the frequency of precipitation is required. Among several forcings influencing the region of study, El Niño–Southern Oscillation (ENSO) has been identified as responsible for a great part of the inter‐annual variability of precipitation. Therefore, in this study, we assess the ENSO signal on LDS and the response of soil water storage under the different phases. In order to assess this issue, daily data were used from 30 meteorological stations. Based on this information, dry sequences were quantified considering a dry day when the accumulated precipitation of two consecutive days was lower than 5 mm. From the dry sequences, two different thresholds were used to identify LDS. On one hand, the 85th percentile was selected as a spatially variable threshold and, on the other hand, 15‐day length was used as a fixed one. Based on this selection, the severity and the number of LDS were analysed. In general terms, La Niña (El Niño) phase presents higher (lower) severity and higher (lower) probability in the occurrence of more than one LDS per season, except in winter. This result is in concordance with negative (positive) anomalies of accumulated precipitation under La Niña (El Niño) phase. Regarding the impact on the water balance, soil water storage responds to accumulated precipitation anomalies in all the seasons.

El fenómeno de “El Niño - Oscilación del Sur” (ENOS) es uno de los principales responsables de la variabilidad climática inter-anual, con influencia sobrela región pampeana argentina. El objetivo de este trabajo fue analizar el impacto del ENOS sobre las variaciones anuales de la producción de maíz. La región fue dividida en 10 zonas en las cuales los rendimientos tienen una variabilidad inter anual homogénea. Se utilizaron diferentes índices del ENOS. Los resultados indicaron que, en la mayor parte de la región, los desvíos de los rendimientos son, en general, positivos durante eventos “El Niño” y negativos durante “La Niña”. Sin embargo, el impacto es muy fuerte al norte y centro de la región analizada, y se debilita hacia el sur. En el sudeste de la provincia de Buenos Aires el comportamiento es diferente que en el resto. Los índices promediados de mayo a julio son, en general, un buen indicador de la influencia del ENOS sobre la productividad de los cultivos, y brindan una adecuada información para la toma de decisiones de la empresa agropecuaria, con suficiente anticipación respecto del comienzo de la campaña.

(3) Faculty of A g ro n o m y, Buenos A i res University (4) Wa t e r R e s o u rces Sub Secre t a ry, Serrano 669 (1414) Buenos A i res, A r g e n t i n a e-mail: olga@mpero . c y t . e d u . a r ABSTRACT: Forecasting of interannual and seasonal variability of hydrological processes is very important when planification of water