Precipitation events drive ecosystem water fluxes across an elevation gradient in New Mexico, USA

Background/Question/Methods

Precipitation events drive many ecosystem processes in the southwestern United States. Surface processes including evaporation, nutrient production and carbon assimilation are often better correlated to soil moisture availability than they are to precipitation events, however, and the way soil moisture responds to precipitation is often nonlinear and may be influenced by a number of discrete precipitation events. The persistence of single precipitation events and the aggregation of events through time induces variability in soil moisture availability and resulting ecosystem productivity. Characterizing responses to precipitation variability and event aggregation through time may enable better ecosystem-scale climate change predictions. In this study, we explored how soil moisture and water fluxes respond to precipitation across an elevation gradient in New Mexico, USA. Our primary goals were to test the hypotheses that the magnitude of soil moisture required to induce heightened water fluxes differs across the elevation gradient, and that related ecosystems experience differing average precipitation event persistence.

Results/Conclusions

To test these hypotheses, we analyzed precipitation, soil moisture and water flux data from Chihuahuan Desert grassland, piñon-juniper woodland and coniferous forest sites in New Mexico, USA, for the 2008-2011 growing seasons. Results indicate that differences exist in how precipitation events drive soil moisture and ecosystem water flux dynamics. The magnitude and depth of soil moisture required to induce increased water fluxes was significantly (P < 0.05) different between sites, with Chihuahuan Desert grasslands exhibiting the smallest magnitude and depth and coniferous forests exhibiting the largest. The average temporal memory of baseline soil moisture does not differ across sites. Instead, the duration of water flux responses to precipitation events of similar magnitude differ significantly between sites (P < 0.05), where drier, lowland sites show a stronger response to large events than upland sites do, and large precipitation events persist for longer periods in lowland sites. These analyses show that southwestern US ecosystems exhibit differing precipitation input and response dynamics and characterizes these dynamics across sites of analysis, and suggest that changes to precipitation delivery during the growing season will likely have predictable but variable effects on ecosystems in the southwestern United States.