Evaluation of GRACE data for water resource management in Iberia: a case study of groundwater storage monitoring in the Algarve region

Study region: Iberia, Algarve basin, South Portugal. Study focus: This study evaluates the performance of several GRACE products in Iberia using the closure of the water budget. Then, it focusses on the Algarve region and explores the potential of GRACE as a tool of quantitative groundwater monitoring capable of bridging gaps in the existing ground-based network. Monthly data from GRACE, ancillary datasets from E-OBS, GLEAM, GRUN and ERAS, and groundwater level measurements from 12 karst-porous aquifers in the Algarve basin (5000 km(2)) are analyzed from 2004 to 2014. New Hydrological Insights for the Region: When considering the closure of the water budget at the Iberian scale, GRACE Mascon solutions perform remarkably well and better than the products based on spherical harmonics. When considering only the Algarve region, the results are similar to the ones obtained for Iberia, but the GRACE solution that performs the best is the average of the CSR and JPL Mascon products. In spite of the Algarve's extremely small area when compared to the GRACE footprint, the satellite is capable of capturing the regionally averaged seasonal and deseasonalized variations in observed groundwater storage (correlation between GRACE-derived and regionally averaged ground-based measurements is 0.82). For the first time ever at the regional Algarve scale, bounds are placed on the aquifer's storage properties which vary from 3.65 x 10(-3) to 4.92 x 10(-2).