Regional hydro-climatic changes in the Southern Amazon Basin (Upper Madeira Basin) during the 1982–2017 period

Jhan Carlo Espinoza,Anna A Sörensson,Josyane Ronchail,Jorge Molina-Carpio,Hans Segura,Omar Gutierrez-Cori,Romina Ruscica,Thomas Condom,Sly Wongchuig-Correa

doi:10.1016/j.ejrh.2019.100637

Abstract

Study regionUpper Madeira Basin (975,500 km2) in Southern Amazonia, which is suffering a biophysical transition, involving deforestation and changes in rainfall regime. Study focusThe evolution of the runoff coefficient (Rc: runoff/rainfall) is examined as an indicator of the environmental changes (1982–2017). New hydrological insights for the regionAt an annual scale, the Rc at Porto Velho station declines while neither the basin-averaged rainfall nor the runoff change. During the low-water period Rc and runoff diminish while no changes are observed in rainfall. This cannot be explained by increase of evapotranspiration since the basin-averaged actual evapotranspiration decreases. To explain the decrease of Rc, a regional analysis is undertaken. While the characteristic rainfall-runoff time-lag (CT) at Porto Velho basin is estimated to 60 days, CT is higher (65–75 days) in the south and lower (50 days) over the Amazon-Andes transition regions. It is found that 1) the southern basin (south of 14 °S) best explains low-level Porto Velho runoff, 2) in the south, rainfall diminishes and the frequency of dry days increases. Both features explain the diminution of the runoff and the Rc in Porto Velho. Moreover, the increasing dryness in the south compensates for the rainfall and frequency of wet days (>10 mm) increase north of 14 °S and explains the lack of basin-averaged rainfall trends of the upper Madeira basin.

Highlights

The upper Madeira Basin at Porto Velho extends over 975,500 km2 and is characterized by an altitude variation from 50 to 6450 masl, setting the scene for a wide range of climatic, geomorphological and biological features
These results suggest a change in the runoff-rainfall relationship in the upper Madeira Basin characterized by a runoff diminution during the low-water season that is not observed in the basin averaged rainfall, producing a significant diminution of the runoff coefficient (Rc)
We document that the runoff coefficient (Rc: runoff/rainfall) in the upper Madeira Basin, an indicator of environmental changes over the basin, decreases during the 1982–2017 period and in particular during the dry season

Summary

Introduction

The upper Madeira Basin at Porto Velho extends over 975,500 km and is characterized by an altitude variation from 50 to 6450 masl, setting the scene for a wide range of climatic, geomorphological and biological features. The Madeira river basin at Porto Velho station consists of four main tributaries: Madre de Dios arriving from the Peruvian Andes, Beni and Mamoré from the central Bolivian Andes and the Guaporé River from the southeastern part of the basin (Brazilian Shield). While the Andes cover around 23% of the upper Madeira surface, a vast floodplain, the Llanos de Mojos, extends over the lowlands between the Beni, Mamoré and Guaporé rivers (Guyot, 1993, 1996). The Llanos de Mojos floodplain is characterized by a mean altitude of 150 masl, a slope of less than 10 cm/km (Guyot, 1993) and a floodable surface up to 150,000 km (Hamilton et al, 2004; Ovando et al, 2016; Parrens et al, 2019, among others)

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