Unravelling groundwater's role in soil-plant-atmosphere continuum: Integrated ecohydrological modelling approach using STEMMUS-SCOPE and MODFLOW 6

Abstract. Soil-plant-atmosphere continuum (SPAC) models are commonly used to investigate various components’ role(s) in ecosystem functioning. Yet, in most SPAC models, groundwater is ignored or at best represented in an over-simplified manner, leading to misunderstanding of its critical role in simulating soil-vegetation dynamics. This study investigates the groundwater’s role in soil-plant-atmosphere processes. To this end, an integrated ecohydrological modelling (IEM) framework is developed by coupling the STEMMUS-SCOPE SPAC model to the MODFLOW 6 integrated hydrological model. The standalone STEMMUS-SCOPE (ST-SC) and coupled STEMMUS-SCOPE-MODFLOW 6 (ST-SC-MF6) models were applied over an 8-year period (1 April 2016 – 31 March 2024) to three sites in the Netherlands (Loobos, Cabauw and Veenkampen). Simulated various essential variables, including soil moisture (θ), soil temperature (Ts), groundwater level, groundwater temperature, evapotranspiration (ET), gross primary productivity, net ecosystem exchange (NEE), and sun-induced chlorophyll fluorescence (SIF) were then compared to corresponding in-situ observations to evaluate the ST-SC and ST-SC-MF6 setups. Results indicated that the groundwater contribution is spatially and temporally variable. ST-SC-MF6 showed better agreement with observations than ST-SC for: a) Ts, and ET at Loobos, b) θ, ET, NEE, and SIF at Cabauw, and c) θ, and ET at Veenkampen. Notably, benefits of ST-SC-MF6 simulation were particularly prominent during dry periods, when shallow groundwater mitigated vegetative water stress. Overall, the proposed ST-SC-MF6 IEM helped to: (1) incorporate groundwater as a key component in the water, energy and carbon cycles, and (2) define the important role groundwater dynamics play in soil-plant-atmosphere continuum for deepening our understanding of ecosystem functioning.