A comprehensive approach to analyze discrepancies between land surface models and in-situ measurements: a case study over US and Illinois with SECHIBA forced by NLDAS
The purpose of this study is to test the ability of the Land Surface Model SECHIBA to simulate water budget and particularly soil moisture at two different scales: regional and mesoscale. The model is forced by NLDAS data set at eighth degree resolution over the 1997-1999 period. SECHIBA gives satisfying results in terms of evapotranspiration and runoff over US compared with four other land surface models, all forced by NLDAS data set for a common time period. The simulated soil moisture is compared to in-situ data from the Global Soil Moisture Database across Illinois by computing a soil wetness index. A comprehensive approach is performed to test the ability of SECHIBA to simulate soil moisture with a gradual change of the vegetation parameters closely related to the experimental conditions. With default values of vegetation parameters, the model overestimates soil moisture, particularly during summer. Sensitivity tests of the model to the change of vegetation parameters are performed and show that the roots extraction parameter has the largest impact on soil moisture, others parameters such as LAI, height or soil resistance having a minor impact. Moreover, a new computation of evapotranspiration including bare soil evaporation under vegetation has been introduced into the model. The results point out an improvement of the simulation of soil moisture when this effect is taken into account. Finally, uncertainties in forcing precipitation to simulate a realistic soil moisture are addressed and it is shown that soil moisture observations can be rather different depending on the method to measure field capacity. When the observed field capacity is deducted from the observed volumetric water profiles, simulated soil wetness index is closer to the observations. Excepted for one station, the monthly mean correlation is around 0.9 between observation and simulation.