✍ Scribed by Carlos M. Krepper; Norberto O. García; Phil D. Jones
No coin nor oath required. For personal study only.
The use of river flow as a surrogate to study climatic variability implies the assumption that changes in rainfall are mirrored and likely amplified in streamflow. This is probably not completely true in large basins, particularly those that encompass different climatic regions, like the Paraguay river basin. Not all the signals present in precipitation are reflected in river flow and vice versa. The complex relationship between precipitation and streamflow could filter some signals and introduce new oscillatory modes in the discharge series. In this study the whole basin (1 095 000 km 2 ) was divided into two sub-basins. The upper basin is upstream of the confluence with the River Apa and the lower basin is between the Apa river confluence and the Puerto Bermejo measuring station. The rainfall contribution shows a clear wet season from October to March and a dry season from April to September. A singular spectrum analysis (SSA) shows that there are trends in rainfall contributions over the upper and lower basins. Meanwhile, the lower basin only presents a near-decadal cycle (T ∼ 10 years). To determine the flow response to seasonal rainfall contributions, an SSA was applied to seasonal flow discharges at Puerto Bermejo. The seasonal flows, Q(t) O-M and Q(t) A-S , present high significant modes in the low-frequency band, like positive trends. In addition, Q(t) O-M presents a near-decadal mode, but only significant at the 77% level for short window lengths (M ≤ 15 years). Really, the Paraguay river flow is not a good surrogate to study precipitation variation. The low-frequency signals play an important role in the flow behaviour, especially during extreme events from the second half of the last century onwards.
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