Parameterizing subsurface karst geology within the Noah Land Surface Model (LSM)

Karst geology is characterized by highly fractured, soluble bedrock or unconsolidated materials covering roughly 25% of the United States. Karst allows for rapid drainage from the bottom of the soil column, effectively increasing the "baseflow" by 10-15%, which affects the amount of moisture available at the surface of the column for fluxes. This can affect soil moisture memory, which is crucial to land-atmosphere coupling. Despite this influence, underlying geology is largely overlooked within land surface model (LSM) parameterizations used in weather and climate models. These models parameterize water flow at the bottom of a soil column as "baseflow," that is defined simply as a function of mean surface slope and the moisture in the lowest model soil layer, interacting in some models directly with the water table. We present a new LSM parameterization for the effects of underlying karst terrain. Continental United States karst information was obtained as polygons from the USGS, gridded to match the North American Land Data Assimilation (NLDAS) model grid at 0.125 degree, and incorporated into version 2.8 of the NCEP Noah LSM. Control and test runs have been conducted to quantify the effect of karst on soil moisture distribution and memory.