Vegetation source water identification using isotopic and hydrometric observations from a subhumid mountain catchment

Authors: DWIVEDI, R. - University Of Arizona; EASTOE, C. - University Of Arizona; Knowles, John; WRIGHT, W.E. - University Of Arizona; HAMANN, L. - Us Geological Survey (USGS); MINOR, R. - University Of Arizona; MITRA, B. - University Of Arizona; MEIXNER, T. - University Of Arizona; MCINTOSH, J. - University Of Arizona; FERRE, P.A. - University Of Arizona; CASTRO, C. - University Of Arizona; NIU, G-Y - University Of Arizona; BARRON GAFFORD, G.A. - University Of Arizona; ABRAMSON, N. - University Of Arizona; PAPUGA, S.A. - Wayne State University; STANLEY, M. - University Of Arizona; HU, J. - University Of Arizona; CHOROVER, J. - University Of Arizona

Submitted to: Ecohydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/26/2019
Publication Date: 1/3/2020
Citation: Dwivedi, R., Eastoe, C., Knowles, J.F., Wright, W., Hamann, L., Minor, R., Mitra, B., Meixner, T., Mcintosh, J., Ferre, P., Castro, C., Niu, G., Barron Gafford, G., Abramson, N., Papuga, S., Stanley, M., Hu, J., Chorover, J. 2020. Vegetation source water identification using isotopic and hydrometric bservations from a subhumid mountain catchment. Ecohydrology. 13(1). https://doi.org/10.1002/eco.2167.
DOI: https://doi.org/10.1002/eco.2167

Interpretive Summary: In many areas, mountains provide sustaining water resources to increasingly arid regions downstream. A fundamental question remains, however, as to whether mountain forests are maintained by tapping into the same subsurface waters that contribute to streamflow. As a result, accurate predictions of current and future water resources are predicated upon a thorough understanding of vegetation water dynamics in the mountains. This study used a combination of long-term hydrological observations and stable water isotope data from precipitation and stream, soil, vegetation, and groundwater to identify the water sources that support vegetation and streamflow in the Santa Catalina mountains near Tucson, Arizona, USA. The results showed that different tree species accessed different pools of water, and that observed differences or similarities between stream and vegetation water depended on the method that was used to isolate the different water sources. These results constrain the influence of vegetation on seasonal stream and groundwater replenishment and are applicable to other dryland mountain locations with implications for water resources management.

Technical Abstract: Recent research presents conflicting evidence to support the assertion that water sources supporting vegetation water demand and streamflow are different i.e., the ecohydrological water source separation (EWSS) hypothesis, across a range of ecosystem types. To constrain the impact of vegetation on subsurface water allocations, the current study characterized the water sources that support dry season vegetation transpiration and streamflow at a subhumid, i.e., aridity index between 0.5 and 0.65, montane forested catchment in southern Arizona, U.S.A., using both long-term hydrometric and stable water isotope data. The hydrometric data include long-term observations of catchment-scale hydrologic fluxes and soil water storage, while the stable water isotope data include observations of stream water, soil water (at various depths), deep groundwater and Arizona pine and Douglas-fir sap water samples. The results showed that (1) tightly-bound soil water was sufficient to meet dry period vegetation water demand when the former was defined in terms of field capacity; (2) there were two different vegetation source waters, of which only one type contributed significantly to streamflow; (3) different tree species occupied different ecohydrological niches in terms of subsurface water use during dry periods; and (4) subsurface water was an incomplete mixture of various water sources that support both vegetation and streamflow. These results can be considered to both support and contradict the EWSS hypothesis at this site, and they highlight how alternative definitions of tightly-bound water can influence interpretation of data. The identification of contributing source waters herein is applicable to other subhumid montane locations and critical zones that are disproportionately important to the management of groundwater supply and contribute to a better understanding of how and when vegetation mediates surface and groundwater interactions.