Using stable water isotopes in a two-layer soil moisture conceptual framework to understand transpiration dynamics in a semiarid shrubland

Abstract:

Semiarid shrublands and other dryland ecosystems are highly sensitive to precipitation pulses. Because the frequency and magnitude of precipitation events have been projected to change for these ecosystems, the nature of these pulses and how they are distributed as moisture in the soil profile are also expected to change. Previous research has suggested that transpiration dynamics in drylands are associated with deep soil moisture, which accumulates after large rainfall events. Because transpiration is the productive component of evapotranspiration in that it is water used toward biomass accumulation, a hypothetical decrease in large rainfall events would have major consequences for the health and functioning of dryland ecosystems. Furthermore, as drylands account for nearly 40% of terrestrial biomes, these cascading changes have the potential to impact global water and carbon budgets. Still, in pulse-dependent dryland ecosystems, the relative contribution of transpiration to evapotranspiration and the temporal dynamics of this contribution are not well understood. The objective of this research is to better characterize the temporal dynamics of transpiration in dryland ecosystems. We present the relative contribution of transpiration to evapotranspiration over the course of a year from eddy covariance and sap flow measurements taken at a creosotebush-dominated shrubland ecosystem in southern Arizona. We analyze soil moisture and stable water isotopes within the context of a two-layer soil moisture conceptual framework in an attempt to identify the source water for transpiration. We use these results to explain the temporal dynamics of transpiration in this semiarid shrubland. Finally, we put our results in the context of regional climate projections to suggest how this dryland ecosystem might be impacted in the future. We expect our study will contribute to understanding where precipitation pulses are distributed in the soil moisture profile and when these pulses are used for transpiration in dryland ecosystems; ultimately these findings should improve how drylands are represented in models of land-atmosphere exchange.