Implementing deep soil and dynamic root uptake in Noah-MP (v4.5): Impact on Amazon dry-season transpiration

Abstract. Plant roots act as critical pathways of moisture from the subsurface to the atmosphere. Deep moisture uptake by plant roots can provide a seasonal buffer mechanism in regions with a well-defined dry season such as the southern Amazon. Most existing state-of-the-art earth system models cannot fully capture the required subsurface-to-atmosphere processes, including groundwater dynamics, a sufficiently deep soil column, dynamic root water uptake, and a fine model spatial resolution (<5 km).

To address this, we present DynaRoot, a dynamic root water uptake (RWU) scheme implemented within the Noah-MultiParameterization (Noah-MP) land surface model, a widely used model for studying kilometer-scale regional land surface processes. Our modifications include the implementation of DynaRoot, eight additional resolved soil layers reaching a depth of 20 m, and soil properties that vary with depth. DynaRoot is computationally efficient and ideal for regional- or continental-scale climate simulations. We perform four 20 year uncoupled Noah-MP experiments for a region in the southern Amazon basin. Each experiment incrementally adds physical processes. The experiments include default Noah-MP with free drainage (FD); addition of a groundwater scheme that resolves water table variations (GW); addition of eight soil layers and soil properties that vary with depth (SOIL), and addition of DynaRoot (ROOT).

Our results show that DynaRoot allows mature forests in upland regions to avoid water stress during dry periods by taking up moisture from the deep vadose zone (where antecedent precipitation is still draining downward). Conversely, RWU in valleys can take up moisture from groundwater (while remaining constrained by the water table). Temporally, we capture a seasonal shift in RWU from shallower layers in the wet season to deeper soil layers in the dry season, particularly over regions with dominant evergreen broadleaf (forest) vegetation. Compared to the control case, there is a domain-average increase in transpiration of about 29 % during dry months in the ROOT experiment. Critically, the ROOT experiment performs best in simulating the temporal evolution of dry-season transpiration and evapotranspiration (ET) compared with an observational ET product. Future work will explore the effect of the DynaRoot uptake scheme on atmospheric variables in a coupled modeling framework.