Advancing a Hydrological Model to Quantify Groundwater Discharge of Forest Groundwater Dependent Ecosystems

Abstract

Worldwide, groundwater dependent ecosystems (GDEs) support rare/endemic species, providing essential habitat and access to water when rivers, wetlands and springs are present. Two decades of research have focused on improving knowledge to describe GDEs and identify their location, providing information to facilitate their protection, especially in areas affected by anthropogenic development of groundwater resources. However, a key knowledge gap persists in understanding water requirements of GDEs, providing challenges to sustainable use of groundwater. More specifically, groundwater management must account for timing of GDE groundwater use (i.e., monthly, seasonally), duration of groundwater reliance, and importantly, volumes of groundwater discharged, particularly by vegetation. Without this knowledge, difficulties remain in setting sustainable volumetric groundwater extraction limits at local, catchment and basin scales. Within, we leverage a unique, extensive on-ground Australian forest groundwater discharge dataset rare to GDE studies, to validate simulated groundwater discharge estimates from the ‘ABCD’ hydrological model, where monthly model outputs over multiple years provide vegetation groundwater discharge data to specifically examine vegetation groundwater requirements. On-ground forest data consist of multi-year, sub-monthly, plot scale data across 18 sites, taking a water balance approach underpinned by sap flow data and including soil moisture monitoring to quantify groundwater extraction across the ~2700 km² forest estate. The ABCD model requires low parameterisation and was further developed by incorporating the Budyko Framework to close the water balance (herein, ‘ABCD-BF model’). Forest groundwater discharge estimates correlated well with on-ground values (r of 0.7 and RMSE of 0.99 mm day⁻¹). Approximately half the sites returned correlations of > 0.8 and low RMSE of 0.46 mm day⁻¹, indicating the ABCD-BF model provides suitable groundwater discharge estimates. Thus, the development and application of the ABCD-BF model provide a pragmatic desktop approach to a significant knowledge gap impeding sustainable groundwater management, thereby enhancing GDE protection. Furthermore, broad-scale application of the model is possible by employing parameters globally available via remote sensing, enabling desktop impact assessments, enhancing GDE management and protection worldwide.