Modeling land surface processes of the midwestern United States : predicting soil moisture under a warmer climate

Abstract

This dissertation seeks to quantify the response of soil moisture to climate change in the midwestern United States. To assess this response, a dynamic global vegetation model, Integrated Biosphere Simulator, was coupled to Regional Climate Model version 3 (RegCM3- IBIS). IBIS has several key advantages over the native surface physics scheme used in RegCM3, Biosphere-Atmosphere Transfer Scheme 1e (BATS1e), most notably superior subsurface hydrology and partitioning of runoff. A series of 22-year numerical experiments were completed to evaluate the ability of RegCM3-IBIS and RegCM3-BATS1e to simulate the energy and water budgets of the American Midwest. Several errors in both RegCM3 and IBIS were identified and corrected, including a significant warm bias, an underestimation of root zone soil moisture, and an overestimation of incident surface shortwave radiation, net long wave radiation, and total runoff. In addition, an agricultural plant functional type was added to RegCM3-IBIS to better represent the current vegetation cover of the midwestern United States. The sensitivity of latent heat flux to available energy plays an important role in determining the effects of climate change on regional hydrologic cycles. An intuitive framework based on the Penman-Monteith equation was developed that identifies deficiencies in model parameterizations of latent heat flux and provides a consistent comparison of the sensitivity of evapotranspiration between models and observations. For Illinois, RegCM3-IBIS and RegCM3-BATS1e tend to overestimate the sensitivity of latent heat flux to available energy in May and June, but underestimate the sensitivity of latent heat flux to available energy in the late summer. The response of soil moisture in RegCM3-IBIS and RegCM3-BATS1e to a surrogate climate change scenario and the ECHAM5 GCM A1B climate change scenario was evaluated. RegCM3-IBIS and RegCM3-BATS1e simulate increased rainfall, evapotranspiration, and runoff during the spring and summer. Soil moisture is unchanged throughout the growing season as enhanced precipitation offsets increased evaporative demand. Negligible changes in soil moisture are robust across surface physics schemes, large-scale forcings, and convective closure assumptions.