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 (T_s), 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) T_s, 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.