General circulation models predict alterations to the hydrologic cycle due to global climate change. Warming of the atmosphere increases evaporation rates and the water holding capacity of the air, leading to more intense precipitation. Increases in the intensity of rainfall may occur without an increase in overall amount, leading to longer dry periods between storms. This may have significant ecological consequences for water limited ecosystems, such as grasslands. Ongoing field work at the Konza Prairie Long-Term Ecological Research station in northeastern Kansas investigates various ecosystem responses to altered rainfall patterns using Rainfall Manipulation Plots (RaMP's). Changing water inputs during the growing season to fewer but more intense events, lead to a ~10% decrease in aboveground net primary productivity over a 4 year period compared to the control (natural rainfall amounts applied), despite receiving the same seasonal total.
Using a land surface model, the Community Land Model 3.0 coupled with a dynamic vegetation model (CLM-DGVM), this project attempts to further examine the relationship between altered precipitation patterns and grassland productivity. In addition to the vegetative response, the model also simulates the impacts on hydrology and surface energy fluxes, both not measured in the field. We applied the same rainfall manipulation scheme as the RaMP's over a five year period (2003 through 2007), using ambient and altered treatments. Ambient simulations received the same frequency and intensity as the natural rainfall, while altered simulations decreased the frequency and increased the intensity of rain by extending the interval between storms by 50%. We then examined the impacts of altered rainfall on various ecosystem processes.
Results/Conclusions
The model produced a consistent decrease in annual net primary production over the five year period which matched the magnitude of the RaMP's, but failed to capture the interannual variability. In addition, productivity was positively correlated with mean soil water content, and negatively correlated with soil moisture variability, another similarity to the RaMP's. By examining surface hydrology, we found increased runoff due to larger events, and decreased evapotranspiration in the altered treatment. As more water leaves the system through runoff, evapotranspiration and the latent heat flux both decreased as a consequence of reduced soil water. This model simulation demonstrated that changing the size and timing of rainfall in the tallgrass prairie results in a complex ecosystem response involving hydrology, soil moisture dynamics, plant productivity and surface energy fluxes.
