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Precipitation runoff and water balance regimes variability along the Peruvian Pacific slope and coast : ENSO influence and sensivity to hydroclimatic change

Abstract : Climate variability and associated extreme events as El Niño phenomenon (ENSO) represent the most difficult episodes to deal with along the Peruvian Pacific slope and coast. In addition, a growing water concern takes place since seventies. In-depth documentation of precipitation and runoff regimes becomes a key part in any water management plan and this research offers the first hydroclimatic variability study at monthly and annual time step in the study area over the last four decades (1970?2010 period). First, an exhaustive database treatment was carried out overcoming some limitations due to Andean geographical conditions. Second, precipitation regime was studied with a regionalization approach under non-stationary time-series conditions. A combined process consisting in k-means clustering and regional vector methodology was proposed. Nine regions were identified with a homogeneous precipitation regime following a latitudinal and altitudinal gradient. Third, a hydroclimatic balance is done at catchment-scale addressing the issue of climate and anthropogenization and their potential influences over hydroclimatic time series. The theoretical Budyko-Zhang framework was used and allowed identifying 11 out of 26 catchments with both low climate and anthropogenization influence (i.e. unimpaired conditions). This hypothesis was verified with the use of land use and land cover remote sensing products as MODIS and LBA imagery. Then, runoff regime was studied under unimpaired conditions and an extension over 49 catchments of the Peruvian Pacific drainage was done. A regional runoff model is proposed via two conceptual lumped models at annual and monthly time scale (GR1A and GR2M respectively). A Differential Split-Sample Test (DSST) was used to cope with modelling robustness over contrasted climate conditions as dry and wet years according to the semi-arid conditions. These results also showed an increasing regional discharge from arid Peruvian Pacific coast towards the Pacific Ocean. Finally, the scope of the thesis covers (1) a revisitation of ENSO/precipitation relationship considering the regionalized precipitation and several ENSO indices in order to discriminate the influence of the two types of El Niño (the eastern Pacific (EP) El Niño and the central Pacific (CP) El Niño) as well as the influence of large-scale atmospheric variability associated with the Madden and Julian Oscillation, and of regional oceanic conditions. The proposed methodology consisting in principal component analysis, wavelets and coherence, running correlations and spatial covariance analysis, highlights the significant decadal modulation with the larger ENSO impact in particular in the 2000s, ENSO/precipitation relationship reverses compared to the previous decade. The two dominant co-variability modes between sea surface temperature in the tropical Atlantic and Pacific oceans and the nine regions show salient features of the ENSO influence: increased precipitation over downstream regions in northern Peru during EP El Niño and decreased precipitation over upstream regions along the Pacific slope during CP El Niño events. (2) The sensitivity to hydroclimatic change is explored by hydroclimatic trend analysis as changes indicators of regional hydroclimatology. According to significant upward trends in annual temperature found in all catchments, results showed a significant warming in the study area with a mean of 0.2°C per decade. Also, changes in trajectories in the Budyko space (i.e. direction and magnitude) over the 11 selected catchments revealed that six catchments were shown to be sensitive to climate variability (i.e. likely with high sensitivity to future climate) and land use changes, where precipitation and temperature are the main drivers of these environments changes.
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Pedro Rau Lavado. Precipitation runoff and water balance regimes variability along the Peruvian Pacific slope and coast : ENSO influence and sensivity to hydroclimatic change. Hydrology. Université Paul Sabatier - Toulouse III, 2017. English. ⟨NNT : 2017TOU30249⟩. ⟨tel-01925488⟩

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