Coupling the ParFlow Integrated Hydrology Model within the NASA Land Information System: a case study over the Upper Colorado River Basin

Understanding, observing, and simulating Earth's water cycle is imperative for effective water resource management in the face of a changing climate. While NASA's Land Information System (LIS)/Noah-MP is widely used for land surface modeling, its ability to represent groundwater processes is limited. In contrast, the ParFlow hydrologic model explicitly simulates subsurface water movement. This study explores the effectiveness and usefulness of the newly coupled modeling framework, ParFlow-LIS/Noah-MP (PF-LIS/Noah-MP) over the Upper Colorado River Basin (UCRB). The framework integrates the strengths of both models to provide a physically based representation of surface and subsurface processes and their interactions. Unlike standalone LIS/Noah-MP, the coupled system enables three-dimensional groundwater flow simulations by solving the Richards' equation, improving the realism of subsurface hydrologic processes. We evaluate PF-LIS/Noah-MP over the UCRB by comparing its simulations against in-situ and satellite observations, including soil moisture, streamflow, and groundwater storage. In general, the results show that PF-LIS/Noah-MP produces soil moisture simulations comparable to those of LIS/Noah-MP across the entire UCRB, with nearly identical root mean squared error and correlation coefficients. However, further analysis - when these metrics are averaged over areas with complex topography - revealed that in regions with high elevation gradients, PF-LIS/Noah-MP slightly outperforms standalone LIS/Noah-MP in soil moisture simulation. The coupled model's ability to simulate groundwater storage and lateral subsurface flow introduces new hydrologic prediction capabilities that were not possible within the standalone LIS/Noah-MP model.