It is well recognized that the land surface soil moisture profile plays an important role in numerical weather and climate prediction, in addition to water resources and agricultural applications. Previous studies in the land surface community have been mostly focused on the inter-comparison of the simulated soil moisture fields by different land surface models (LSM) using observations as a benchmark. Since the total water budget is often the focus of these studies, the modeled and the observed soil moisture fields are often integrated vertically in a soil column or aggregated in a water basin to obtain the amount of water stored or the change in water storage in a much larger scale than the point representation of soil moisture.

This study focuses on using point values of the measured soil moisture field to examine the simulated soil moisture profiles by the Noah LSM. The soil moisture measurements provided by the Soil Climate Analysis Network (SCAN) are used as the bench mark for the evaluation. To make the study more focused on a specific geographic region, the Mississippi area is chosen since it has a larger number of well maintained SCAN sites. Noah is configured with twenty soil layers (as opposed to the usual four) to obtain a more accurate solution to the Richards equation. When compared to the in situ soil moisture measurements, the modeled moisture profiles are interpolated to the exact depth of each measurement for a point to point comparison. The simulation experiments are conducted for a two-year period to eliminate any possible spin-up effect.

The simulation results show that, in general, there is a better agreement between the modeled and observed soil moisture contents in the top soil layer than in the lower layers. Noah is found to systematically underestimate soil moisture in the lower soil layers at all the SCAN sites within the study area. This is attributed to the free drainage condition applied at the lower boundary of the two-meter simulation domain. This underestimation of soil moisture becomes even more severe in the late summer and the early fall seasons when less precipitation is measured. To correct this problem, a constant head (i.e. constant soil moisture) boundary condition, obtained by extrapolating measured moisture content at the lowest level from the SCAN sites, is implemented at the two meter depth. The results show that the modified boundary condition yields much wetter moisture contents in the lower part of the profile, leading to a better agreement with the observations at all sites. In conclusion, the free drainage condition used by many land surface models is not applicable in wet regions with high water tables; instead, a constant head boundary condition may be more appropriate based on the observations and the climate condition in the region. Though the conclusion may not be applied to other areas without further investigations, this approach and results of this study demonstrate that an in-depth examination of the modeled soil moisture field against observations at all levels can reveal deficiencies in model physics and result in more accurate soil moisture profile predictions.