Weather variability in semiarid ecosystems: Consequences for ecosystem water balance and soil water availability

Background/Question/Methods: Water cycling and availability exert dominant control over ecological processes and the sustainability of ecosystem services in water-limited ecosystems. Consequently, dryland ecosystems have the potential to be dramatically impacted by hydrologic alterations emerging from global change. In addition to elevated temperature and altered precipitation, climate change is anticipated to include increasing weather variability.  However, the potential impact of increasing weather variability on water balance and ecohydrology in dryland ecosystems is not well understood. We utilized a daily time step, multiple soil layer, process-based ecosystem water balance model to simulate the ecohydrological consequences of increasing weather variability in the U.S. Great Plains and intermountain regions. Within each region, we focused on 49 sites, selected to span the range of temperature and precipitation within the region.  At each site, we simulated water balance for combinations of four treatments: climate (current, future A2 and future B1), duration of dry intervals (days; ambient, 50% of ambient and 150% of ambient), duration of wet or dry periods (years; ambient or doubled duration), and severity of wet or dry years (ambient or doubled mean deviation from the mean).  We simulated 500 years for each site and summarized primary components of ecosystem water balance: precipitation, interception, evaporation, transpiration and drainage. 

Results/Conclusions: Results indicated limited evidence of interactions among climate scenario or dry interval duration in their effect on water balance, so those treatment impacts are described independently.  While both A2 and B1 climate scenarios influenced snowfall and total precipitation, dry interval length did not consistently impacted rain or snowfall. The consequences of altered dry interval duration (days) as well as dry period (years) duration and severity, was, for many components of water balance, as substantial as the impact of climate change.  In both regions, longer dry intervals decreased interception and increased transpiration.  Longer dry intervals increased soil evaporation in the Great Plains while decreasing it slightly in the intermountain region.  Drainage increased with dry interval in the Great Plains, but showed little response in the intermountain region.  These results underscore the importance of temporal precipitation regime in dryland ecosystems and suggest that impact of altered precipitation regime may depend on the overarching climate, i.e. the Great Plains, which receives primarily summer precipitation, responded differently than the intermountain region, which receives a more uniform seasonal distribution of precipitation. Changes in precipitation timing stemming from altered weather variability represent an important, and often overlooked, component of climate change.