The two water worlds hypothesis: Addressing multiple working hypotheses and proposing a way forward

Recent studies using water isotopes have shown that trees and streams appear to return distinct water pools to the hydrosphere. Cryogenically extracted plant and soil water isotopic signatures diverge from the meteoric water lines, suggesting that plants would preferentially use bound soil water, while mobile soil water that infiltrates the soil recharges groundwater and feeds streamflow all plots on meteoric water lines. These findings have been described under the ?two water worlds? (TWW) hypothesis. In spite of growing evidence for the TWW hypothesis, several questions remain unsolved within the scope of this framework. Here, we address the TWW as a null hypothesis and further assess the following: (a) the theoretical biophysical feasibility for two distinct water pools to exist, (b) plant and soil processes that could explain the different isotopic composition between the two water pools, and (c) methodological issues that could explain the divergent isotopic signatures. Moreover, we propose a way forward under the framework of the TWW hypothesis, proposing alternative perspectives and explanations, experiments to further test them, and methodological advances that could help illuminate this quest. We further highlight the need to improve our sampling resolution of plants and soils across time and space. We ultimately propose a set of key priorities for future research to improve our understanding of the ecohydrological processes controlling water flows through the soil?plant-atmosphere continuum.

Fil: Berry, Z. Carter. The University of New Hampshire. Department of Natural Resources and the Environment; Reino Unido

Fil: Jaivime, Evaristo. University of Saskatchewan. Global Institute for Water Security and School of Environment and Sustainability; Canadá

Fil: Moore, Georgianne. Texas A&M University. Department of Ecosystem Science and Management; Estados Unidos

Fil: Poca, María. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto Multidisciplinario de Biología Vegetal. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Instituto Multidisciplinario de Biología Vegetal; Argentina

Fil: Steppe, Kathy. Ghent University. Faculty of Bioscience Engineering. Department of Applied Ecology and Environmental Biology. Laboratory of Plant Ecology; Bélgica

Fil: Verrot, Lucile. Stockholm University. Department of Physical Geography; Suecia. University of Aberdeen. School of Geosciences; Reino Unido

Fil: Asbjornsen, Heidi. The University of New Hampshire. Department of Natural Resources and the Environment; Reino Unido

Fil: Borma, Laura S.. National Institute for Space Research. Earth System Science Centre; Reino Unido

Fil: Bretfeld, Mario. Smithsonian Tropical Research Institute; Panamá

Fil: Hervé-Fernández, Pedro. Ghent University. Faculty of Bioscience Engineering. Department of Forest and Water management. Laboratory of Hydrology and Water Management; Bélgica. Ghent University. Faculty of Bioscience Engineering. Department of Applied Analytical and Physical Chemistry. Isotope Bioscience Laboratory; Bélgica

Fil: Seyfried, Mark. USDA ARS Northwest Watershed Research Center; Estados Unidos

Fil: Schwendenmann, Luitgard. The University Of Auckland; Nueva Zelanda. The University of Auckland. School of Environment; Nueva Zelanda

Fil: Sinacore, Katherine. The University of New Hampshire. Department of Natural Resources and the Environment; Reino Unido. Smithsonian Tropical Research Institute; Panamá

Fil: De Wispelaere, Lien. Ghent University. Faculty of Bioscience Engineering. Department of Applied Analytical and Physical Chemistry. Isotope Bioscience Laboratory; Bélgica

Fil: McDonnell, Jeffrey. University of Saskatchewan. Global Institute for Water Security and School of Environment and Sustainability; Canadá. University of Aberdeen. School of Geosciences; Reino Unido