Modelling the effects of soil type and root distribution on shallow groundwater resources

Vegetated, shallow groundwater environments typically have high environmental and economic value. A sound understanding of the complex interactions and feedbacks between surface vegetation and groundwater resources is crucial to managing and maintaining healthy ecosystems while responding to human needs. A vegetated shallow groundwater environment was modelled using the software HYDRUS 2D to investigate the effects of several combinations of soil type and root distributions on shallow groundwater resources. Three rainfall regimes coupled to both natural and anthropogenically affected groundwater conditions were used to investigate the effect that combinations of four soil types and five root distributions can have on (a) groundwater level drops, (b) groundwater depletion, (c) groundwater recharge and (d) water stress conditions. Vegetation with roots distributed across the whole unsaturated zone and vegetation with dimorphic root systems (i.e.roots having larger concentrations both near the surface and the capillary fringe) behaved differently from vegetation growing roots mainly near the saturated zone. Specifically, vegetation with roots in the unsaturated zone caused water-table drops and groundwater depletions that were half the amount due to deep-rooted vegetation. Vegetation with a large portion of roots near the soil surface benefited from rainfall and was less vulnerable to water-table lowering; as such, the fraction of the total area of roots affected by water stress conditions could be 40 smaller than in the case with deep-rooted vegetation. However, roots uniformly distributed in the unsaturated zone could halve groundwater recharge rates observed in bare soils. Our analysis provided insights that can enable the formulation of site- and purpose-specific management plans to respond to both human and ecosystem water requirements.