Factors controlling changes in evapotranspiration, runoff, and soil moisture over the conterminous U.S.: Accounting for vegetation dynamics

While remotely-sensed data reveal enhanced vegetation growth under a warming climate, how such vegetation dynamics could alter regional hydrology is not fully understood. In this study, we investigated how changes in leaf area index (LAI) and climatic variability alter long-term trends and seasonal variability of evapotranspiration (ET), runoff (R), and soil moisture (SM) over the conterminous United States (CONUS) during the period 1983-2009 by using the Variable Infiltration Capacity (VIC) model with transient remotely-sensed LAI inputs. Factor-controlled simulation experiments reveal that while precipitation is the dominant factor controlling long-term trends and inter-annual fluctuations of water fluxes, other factors (temperature, LAI, and atmospheric CO2) make significant contributions to long-term trends, particularly over specific regions. Changes in LAI result in minor overall contributions to the inter-annual variability of ET, R, and SM over the CONUS as a whole; however, they do make a more significant contribution over certain regions, such as the Midwest. Furthermore, changing LAI is the second most important factor in controlling variability in ET and SM, particularly over the cool season. Anomalies of LAI during El Nino Southern Oscillation (ENSO) phases result in no significant correlations with anomalies in R and SM, but result in positive correlations with ET, indicating that vegetation dynamics can have a significant impact on the hydrologic cycle over continental scales.