Temporal dynamics of source water contributions to evapotranspiration from sky island ecosystems with ephemeral snow pack: A case study using Pseudotsuga menziesii (Douglas Fir)

Lejon Hamann1, Shirley A. Papuga2

1Department of Hydrology and Atmospheric Sciences, The University of Arizona, Tucson, Arizona

2School of Natural Resources and the Environment, The University of Arizona, Tucson, Arizona

In the water-limited desert southwest, ephemeral snow pack in sky island ecosystems is a primary water resource to communities lower in the watershed.  Because evapotranspiration (ET) is the largest component of the water budget in these regions, changes in ET will have major downstream implications. Climate scientists predict more intense and less frequent precipitation events in the southwestern US, which will alter the soil-plant-atmosphere continuum.  Understanding how water currently moves within that continuum is imperative in preparing for these predicted changes.  Currently, information is lacking on (1) where trees retrieve water and 2) whether that source varies with season.  We hypothesize that trees in sky island ecosystems use deep moisture during the snowmelt season and shallow moisture during the summer monsoon season.  We present a preliminary analysis of two years of water stable isotope (δ18O and δD) data from the Santa Catalina Mountains CZO.  We highlight the probable source waters for Douglas Fir and how they vary throughout the year.  A shift in precipitation could alter the source water of these trees such that they may become increasingly water stressed which will have important consequences for water resources in this region if they are unable to continue to thrive.

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