A simple temperature-based method to estimate heterogeneous frozen ground within a distributed watershed model

• Published in: Hydrology and earth system sciences Vol. 22; no. 5; pp. 2669 - 2688
• Main Authors: Follum, Michael L, Niemann, Jeffrey D, Parno, Julie T, Downer, Charles W
• Format: Journal Article
• Language: English
• Published: Katlenburg-Lindau Copernicus GmbH 07-05-2018; Copernicus Publications
• Subjects: Air temperature; Annual variations; Computer simulation; Data processing; Elevation; Energy; Environmental aspects; Frost; Frozen ground; Ground cover; Hydrologic analysis; Hydrologic models; Hydrological analysis; Hydrology; Insulating materials; Insulation; Methods; Model testing; Permafrost; Radiation; Rivers; Snow accumulation; Snow depth; Snowpack; Soil; Soil conductivity; Soil moisture; Soil properties; Spatial variations; Temperature effects; Thermal conductivity; Watersheds
• ISSN: 1607-7938; 1027-5606; 1607-7938
• DOI: 10.5194/hess-22-2669-2018

Frozen ground can be important to flood production and is often
heterogeneous within a watershed due to spatial variations in the available
energy, insulation by snowpack and ground cover, and the thermal and moisture
properties of the soil. The widely used continuous frozen ground index (CFGI)
model is a degree-day approach and identifies frozen ground using a simple
frost index, which varies mainly with elevation through an elevation–temperature relationship. Similarly, snow depth and its insulating
effect are also estimated based on elevation. The objective of this paper is
to develop a model for frozen ground that (1) captures the spatial
variations of frozen ground within a watershed, (2) allows the frozen
ground model to be incorporated into a variety of watershed models, and
(3) allows application in data sparse environments. To do this, we
modify the existing CFGI method within the gridded surface subsurface
hydrologic analysis watershed model. Among the modifications, the snowpack
and frost indices are simulated by replacing air temperature (a surrogate for
the available energy) with a radiation-derived temperature that aims to
better represent spatial variations in available energy. Ground cover is also
included as an additional insulator of the soil. Furthermore, the modified
Berggren equation, which accounts for soil thermal conductivity and soil
moisture, is used to convert the frost index into frost depth. The modified
CFGI model is tested by application at six test sites within the Sleepers
River experimental watershed in Vermont. Compared to the CFGI model, the
modified CFGI model more accurately captures the variations in frozen ground
between the sites, inter-annual variations in frozen ground depths at a given
site, and the occurrence of frozen ground.