A case study for land model evaluation: Simulation of soil moisture amplitude damping and phase shift

Observations have described soil moisture profile variability in terms of phase shift, fluctuation damping, and persistence increasing with soil depth [Wu et al., 2002]. This variability as a function of soil depth couples to climate variability. Whether or not land models can reproduce this variability should be a good test of their parameterizations in soil hydrology both physically and numerically. A widely used multilayer land surface model was applied to simulate the soil moisture profile variability documented from observations to explore the sensitivity to various parameters and to evaluate the model performances through the detailed analysis of a case study. Sensitivity experiments assumed changes of (1) the initial soil moisture field; (2) the root sink term; (3) the soil texture; and (4) the atmospheric forcing at upper boundary. Their impacts on the soil moisture profile phase shift, amplitude damping, and corresponding evapotranspiration were examined. The key land surface prognostic variables, i.e., soil moisture and evapotranspiration, were evaluated against observations prior to the sensitivity integrations. All the factors that affected the soil moisture profile variability of amplitude damping and phase shift also influenced the amplitude and phase of evapotranspiration, suggesting that the simulation of soil moisture profile variability might be more important in the context of timescales than the soil wetness field itself.