Integration of a Groundwater Model to the Noah Land Surface Model for Aquifer-Soil Interaction

Soil water-groundwater interactions are important in determining the moisture profile in a soil column. However, the Noah Land Surface Model (Noah LSM), the land surface component of several general circulation models, does not consider groundwater effects. The present study investigates the impact of integrating a groundwater model into the Noah LSM. By performing experiments forced by observations at the surface, insights into the characteristics of soil moisture evolution with and without the aquifer are investigated. In the absence of surface precipitation, soil moisture is shown to exhibit an exponential decay rate without the aquifer (termed as Noah-Cntl). This is because evapotranspiration dominates other processes governing the soil moisture evolution. Based on scale analysis, we derived an analytical equation that could well represent the aforementioned exponential variation in soil moisture. Presence of the aquifer (termed as Noah-GW) makes the capillary processes important as well. The result is a slower decay of soil moisture as compared to Noah-Cntl without surface precipitation. Soil moisture evolution in both the models is a function of vegetation and soil types. As a result, the aforementioned decay timescale displays significant heterogeneity over the Indian region, with Central India exhibiting relatively faster decay rates. We also performed 24-year long experiments with observed interannually varying forcing that indicates an enhancement in the annual mean soil moisture at all levels due to groundwater effects. This enhancement is particularly prominent during the post-monsoon season, reaffirming the results of the drain-out experiments mentioned above. Plain Language Summary Land surface processes are integral parts of weather and climate. Hence, an accurate representation of these processes is crucial from prediction and understanding points of view. However, state-of-the-art land surface models do not incorporate some essential components that make these models erroneous. This study integrates a groundwater model into an existing land surface model and characterizes its impact on the evolution of soil moisture. We discuss factors affecting soil moisture decay rate over the Indian subcontinent without surface forcing. We show that the post-monsoon and annual mean soil moisture increases in the presence of a groundwater model. However, spatial heterogeneity of groundwater effects is dependent on parameters like soil type and surface rain rate.