Research “Hylight”: CO2‐vegetation feedbacks and other climate changes implicated in reducing base flow by Ralph Trancoso et al.

Recently, Ralph Trancoso and colleagues published a paper on CO2‐vegetation feedbacks and river flow. We decided to ask him a couple of questions.

Q: Where are you from, where are you based, and what are your current research interests?

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Ralph Trancoso

A: I am from Brazil, and have moved to Brisbane in Australia, about four years ago, to undertake my PhD at the University of Queensland. Having finished PhD a couple of months ago, I am now working for the Queensland government. My current main research focus is catchment ecohydrology. By integrating hydrology, remote sensing and ecosystem sciences, I explore the spatio-temporal variability of catchments biophysical properties to generate new insights into their hydrological functioning and changes. I compare the water and energy exchanges of many catchments spanning large extents to investigate large-scale ecohydrological patterns.

Q: What is the take home message of your paper?

A: Climate forcings (precipitation and potential evapotranspiration) and eCO2-driven (elevated CO2) vegetation feedbacks are decreasing catchment baseflow – the portion of streamflow derived from groundwater storage and other delayed sources that plays a critical role in water supply to agriculture, urban areas and ecosystems. We show that the reductions in baseflow not associated with climate forcings are explained by increasing photosynthesis activity, which is consistent with vegetation feedbacks from increased atmospheric CO2. The intensity of these impacts changes across a gradient of aridity and streamflow regimes. The regions already experiencing water scarcity (i.e., water-limited) are more vulnerable to the combined impact of changing climate forcings and CO2 fertilization of vegetation; and are experiencing greater reductions in surface water availability.

Q: Your paper focusses on the effects of elevated CO­2 in water-limited catchments. Would some of the effects you report here also be relevant for more humid regions?

The study catchments actually constrain a large climatic gradient across the east coast of Australia, spanning Tropical, Sub-tropical, Temperate and Alpine climates. Therefore, while the majority of catchments are water-limited, we have also assessed energy-limited and equitant catchments (i.e., catchments that swap being water- and energy-limited over the years). Our findings show a clear gradation of the climate and eCO2-vegetation feedbacks impacts across the aridity spectrum (see Figure 4-a in the paper for details). We highlight the water-limited catchments because they are far more vulnerable to these combined impacts. However, humid catchments are also experiencing significant baseflow reductions, but as they have more water to begin with, the reduction relative to the initial condition is lower.

Q: Can your results be used to better constrain hydrologic predictions (e.g. modeling efforts) now we have a better idea of vegetation greenness on baseflow?

Yes, absolutely. The ecohydrological feedbacks investigated in this paper are yet poorly constrained in the current modelling efforts. That implicates in misleading conclusions and disagreements with observational studies, which confuses the progress of knowledge. Observational evidences like ours seeking to understand and quantify these mechanistic links between water, vegetation and climate at catchment scale are the first step to improve hydrological predictions. Folks who develop and use hydrological models can now better constrain these feedbacks by implementing, calibrating and validating them.

Q: You mention that your findings have “broad implications for water resource management, especially in the world’s water-limited regions”. Can you elaborate a bit more on this?

Previous studies have shown that global water-limited areas have been experiencing rainfall reduction, yet getting greener because of the CO2 fertilization of vegetation. We show that these combined forcings led to an acute reduction in the surface water availability in Australia. Given, the consistency of our findings with global evidences of rainfall reduction and vegetation greening, other water-limited regions across the globe may be experiencing baseflow reductions as well. More than a third of the world’s population inhabit water-limited regions. The reduction in surface water directly affects society and ecosystems, challenging the water resource management to adapt to keep providing a range of water-related societal services under increasing demand in regions where water resources is already scarce.

[END] – Interviewed by Wouter Berghuijs

This interview is part of the new YHS Research “Hylights” series to showcase interesting and outstanding work by early career scientists. Selection criteria are not set in stone, but reasons to select work can include e.g. novelty and relevance of findings, fun of reading, unique collaborations, media coverage and generated controversy. Selected work will be provided with a short layman summary, and a short written or video interview with the (first) author(s). Tips can be sent to young or

Reference to paper: Trancoso, R., J. R. Larsen, T. R. McVicar, S. R. Phinn, and C. A. McAlpine (2017), CO2-vegetation feedbacks and other climate changes implicated in reducing base flow, Geophys. Res. Lett., 44, doi:10.1002/2017GL072759.

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