Streamflow extremes alter air-water relationships in South American
river-floodplain systems of the Uruguay River Basin
Abstract
Extreme hydroclimate events affect the thermal regimes of freshwater
systems. In a four-year period spanning periods of flooding and severe
drought in Southeastern South America (2019-2023), we monitored
temperature variability in streams and floodplain lakes in an undammed
tributary of the Lower Uruguay River, using Onset HOBO dataloggers and
modeled daily streamflow (Q). We compare the performance of linear and
nonlinear regression and generalized additive models (GAM) to evaluate
how air temperature (T air) variability affects water
temperature (T water) at daily, weekly and monthly time
scales and under different Q percentile ranges. Mean daily temperatures
ranged from 6.4ºC to 30.2ºC with daily maxima coinciding with record
highs >40 ºC in T air and record low
streamflow in summer months. Daily and weekly air-water relationships
for rivers, streams and lakes were best fit by nonlinear sinusoidal
models (Nash Sutcliffe Error - NSE ≥ 0.68). Extremely low-flow periods
(<10% Q) showed a linear relationship to T
air, while high-flow periods (>90%) showed
a nonlinear sinusoidal air-water relationship. Moreover, high-flow
events showed weaker T air-T water model
perforamce than moderate to low flow periods. Preliminary comparisons
between basin land-cover and T water suggest that forest
cover favors lower T min. This study fills a
geographical gap in thermal regime data in freshwater ecosystems in
South America and suggests that extreme hydroclimate events have
important implications for the thermal behavior of freshwater systems.