The Effects of Frozen Soil on Snowmelt Runoff and Soil Water Storage

This paper addresses the impacts of frozen soil permeability on the magnitude and seasonality of runoff and soil water storage in cold regions. The National Center for Atmospheric Research (NCAR) Community Land Model (CLM) neglected the supercooled unfrozen water, i.e., existence of a certain amount of liquid water below the freezing point, in the parameterization of frozen soil. As such, the ice fraction of a soil layer always becomes 100% when the cold content is sufficient to freeze all the liquid water. In addition, the hydraulic conductivity for frozen soil is parameterized as a function of the unfrozen water using the Clapper-Hornberger relationship. Thus, when there is no unfrozen water, the soil surface permeability becomes extremely low, such that the snowmelt water flows mainly as surface runoff, resulting in much earlier spring runoff with higher peaks than the monthly GRDC runoff products. This paper presents modifications to the CLM2.0 frozen soil parameterization to produce more accurate magnitude and seasonality of runoff and the soil water storage. These modifications includes: (1) introduction of the supercooled water and (2) a new scheme of the hydraulic conductivity and soil matric potential for frozen soil under the assumption that a fractional permeable area exists in a GCM grid-cell, where water can infiltrate through air-filled macropores. The proposed modifications have been tested in the Sleepers River basin and the six largest river basins in cold regions (Lena, Yenisei, Mackenzie, Ob, Churchill&Nelson, and Amur). The proposed modifications are demonstrated to produce favorable changes in reproducing the GRDC runoff and the GRACE measured terrestrial water storage change.