chapter - Atmospheric and Oceanic Science

Clarke 1998). This aspect may have contributed to the observed streamflow
changes in addition to the effect of rainfall variation. However, the amplification of
the precipitation change in the streamflow in the first case, where a significant use
of land change can be discarded and the persistence of approximately the same factor
of amplification in the different time scales indicate that without dismissing a
possible effect of the use of land change, the major part of the change in the streamflows
between 1951/1970 and 1981/1999 was caused by the change in precipitation
As stated, in all cases the variability in precipitation is considerably amplified
in the corresponding river streamflow. In any given year (or long period) a significant
amount of the water precipitated over the basin is either evaporated or infiltrated
in a way that does not runoff to the rivers. In the examples of table 5.1, the evaporated
plus infiltrated fraction accounts approximately for 70% of the precipitated
water, but this fraction is larger during relatively dry years and smaller during the
rainy ones. On the other hand, although the streamflow accounts only for approximately
30% of the precipitated water, its interannual or interdecadal variability is
larger (in absolute values) than the evaporation plus infiltrated water, and consequently,
its relative variability is even larger. This implies that in the La Plata basin
the balance between precipitation, streamflow, evaporation and infiltration are such
that the interannual variability in precipitation is mostly translated to the river discharge
while only a smaller fraction of it is converted in evaporation or infiltration
(see also chapter 7).
The relative changes in streamflow in all examples are considerably large,
about 17% as the average in the El Niño/La Niña composites, but as large as 44%
for the 1998/1999 case, and 35% between the two 20-year periods. These relative
changes are more impressive when they refer to the streamflow of one of the largest
rivers of the world. At the same time, these examples show the great sensitivity of
the streamflows of the major rivers within La Plata basin to the rainfall variability.
This sensitivity transforms into vulnerability for the activities that depend on river
water supply and become a matter of concern since during the last half of the past
century, the region showed a great interannual and interdecadal variability in the
precipitation that can be repeated in the future.
5.5. Summary
Regional precipitation trends
The rainfall positive trend in subtropical Argentina seems part of a global pattern
found during the 20th Century. It has been stronger than in other parts of the
world, and has been intensified in the last 30 to 40 years due to changes in the
atmospheric circulation.
As a consequence of the increased precipitation in semiarid regions of
Argentina, the agricultural frontier has been extended towards the West. By con-
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