Further evidence of changes in the hydrological regime of the River Paraguay: part of a wider phenomenon of climate change?

Analysis of flow measured at 20 sites, rainfall measured at 36 sites, and a 95-year record of water-level at one site in the basin of the River Paraguay (area 1095 X 10(3) km2) showed that the flow regime during the approximate period 1960-1970 differed substantially from the flow regime both before and after. The long record of Ladario water-levels suggested that the changes between one period and the next were considerably abrupt, and that the periods differed not only in terms of mean water-levels but also in terms of the year-to-year correlational structure within the record. Despite the fragmentary nature of rainfall records from 36 sites, an explanation for the increased flows since 1970 was found in the increases of rainfall, as assessed in terms of the frequency of annual rainfalls more than the long-term mean rainfall. There was some degree of consistency in the change of rainfall pattern across the Paraguay basin as a whole. A detailed examination of daily rainfall characteristics at two gauges where records were fairly complete showed that during the 1960-1970 period, when river flows were low, dry spells were more persistent and, on days when rain did fall, the amounts of rain were generally smaller. The results obtained were compared with results obtained by other researchers using flow records from the Rivers Paraguay, Parana, Negro and Uruguay in the la Plata basin, and rainfall records from other parts of South America. There is now strong evidence of changes in the runoff regime of the la Plata basin during the last 40 years, not all of which can be attributed to land-use change, as there is complementary evidence of change in rainfall regime. The results were also compared with findings from the Congo basin, which appears to exhibit changes in flow regime that are a mirror image of those found for the Paraguay at Ladario.