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.
Experimental data recorded over a natural tallgrass prairie during the later stages of drying in the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment-1987 ...showed (1) that the total daily values of evaporation exhibited a kind of second stage of drying behavior with a t-1/2 dependency at the daily timescale and (2) that this day-to-day evolution was modulated by the available energy at the surface, that is, the hourly radiation input. This allowed a simple description of the phenomenon by combining a desorptive diffusion-type parameterization for the total daily evaporation or for its dimensionless counterpart (such as Priestley and Taylor's alpha, the evaporative fraction, and a few others), with an assumption of self-preservation in the surface energy budget during the daytime hours. The resulting formulation, which involves two timescales, a daily and an hourly, was able to reproduce daytime hourly flux values over a 2-week period of intensive drying. The method can also be useful in the disaggregation of daily, or even weekly, evaporation into hourly values
