Twenty Years of Trials and Insights: Bridging Legacy and Next Generation in ParFlow and Land Surface Model Coupling

Abstract. Groundwater plays a vital role in terrestrial water and energy cycles, yet it remains oversimplified in most Earth system models (ESMs), limiting their ability to represent key land-atmosphere interactions, including evapotranspiration partitioning, drought propagation, and boundary layer development. The original coupling of ParFlow with the Common Land Model (CoLM) in 2005 not only demonstrated the feasibility of integrating physically based groundwater models into ESMs, but also revealed emergent behaviors – such as lateral moisture redistribution, along with the buffering effects that emerge from enhanced subsurface connectivity – that cannot be captured by traditional land surface models (LSMs). This study reviews key findings from two decades of ParFlow–land/atmosphere coupled modeling efforts, highlighting how groundwater–land–atmosphere interactions shape surface energy balance and hydrologic connectivity across three dimensions: upward feedbacks, downward influences, and the critical zone of coupling. Given the substantial advances in LSMs such as CoLM over the past two decades, a renewed recoupling effort is warranted to enhance our understanding of groundwater's role across a broader range of Earth system processes. Preliminary efforts to recouple ParFlow with the updated water and energy modules of CoLM demonstrate improved performance when evaluated against reanalysis and observational data. To ensure long-term sustainability, we propose a modular and maintainable coupling framework addressing functional extensibility, data/code interoperability, and parallel computing needs, in which area, TerrSysMP2 has taken early steps and may be considered an initial forerunner. Finally, we summarize existing ParFlow-based coupled systems and highlight the need for a community-led model intercomparison project (PLCMIP) to benchmark performance, evaluate process coupling under varied configurations, and foster cross-community collaboration.