SCHRIFTENREIHE Institut für Pflanzenernährung und Bodenkunde ...

as snow. The snow routine predicts up to 15 mm snow depth (Fig. 6a), however,
the simulated runoff after air temperature increasing above 0°C is negligible (Fig.
6c). This might relate with that the snow routine does not account for surface
runoff from the frozen soil layer since the code cannot consider the subsurface
soil freezing and thawing process. In fact, it is likely that surface runoff is
generated during snowmelt while soil is fully or at least partially frozen (Fig. 6b).
Unexpectedly, the freezing model, which can account for the subsurface freezing
and thawing processes, also does not compute surface runoff during winter (Fig.
2c). Instead, we found that the simulated SWC by freezing model is higher than
the measured values in the transition time when soil begins to thaw (Figs. 4a and
5a). This implies that the freezing model might overestimate water content and
underestimate surface runoff after spring snowmelt. Therefore, the freezing
model seems still not sensitive enough to estimate surface runoff accompanied
with snowmelt from the soil frozen layer. This might relate to the fact that the
freezing model we applied adopts soil surface temperature as the atmospheric
boundary condition instead of air temperature, which neglects the lag-effects of
energy transfer. Consequently, the freezing model may incorrectly partition all
the snowmelt into infiltration as both soil thawing and snow melting happen
simultaneously. Therefore, to solve this, a transferable and double-layered
boundary condition (e.g., one accounting for air temperature and other
accounting for considering soil temperature) should be introduced. Additionally,
the gradual release of water from the frozen soil profile also might reduce the
maximum rate of runoff.
130
In contrast to the freezing model, an ‘‘implicit’’ frozen soil module that used
in other studies might totally stop water infiltration inside the soil (Mitchell and
Warrilow, 1987) by specifying that the meltwater has to run off when snow melts
while soil is still frozen. However, the soil ice is subsequently melting when the
snow is melting (Luo et al., 2003). Thus the frozen soil layer moves downward
and leaves the upper soil layer available for infiltration. In contrast to this, an
‘‘explicit’’ frozen soil module like the freezing model that we used is theoretically
reasonable as it only reduces the infiltration rate based on soil temperature, soil