MAPPING 2023 GROUNDWATER STRESS ZONES IN COASTAL LOWLAND AQUIFERS USING DOWNSCALED GRACE SATELLITE DATA

Coastal Lowlands Aquifer is a vital water source supplying approximately one billion gallons daily for public and private use. However, extensive withdrawals have led to significant issues such as saltwater encroachment and land subsidence in some areas. The study applied downscaled GRACE (Gravity Recovery and Climate Experiment) satellite data to assess Groundwater Storage (GWS) and identify the potential groundwater stress zone for 2023. A Random Forest Model (RFM) was employed to generate the monthly 4-km Groundwater Storage Anomaly (GWSA) map. The model incorporated variables such as monthly anomaly of total precipitation, mean temperature, normalized differences of vegetation Index (NDVI), evapotranspiration of 2023; Shuttle Radar Topography Mission (SRTM) DEM; slope angle; dominant soil type; and lithology to downscale the monthly Total Water Storage Anomaly (TWSA) by GRACE-FO of 2023 from 3 degree (~333 km) to 4 km. The RFM was then reapplied with all the parameters, including the downscaled monthly TWSA, to further downscale the monthly GWSA. This GWSA was derived by subtracting the sum of the monthly anomalies of Root Zone Soil Moisture (RZSM), Plant Canopy Surface Water (PCSW), and Snow Depth Water Equivalent (SWE) for 2023 from the TWSA by GRACE-FO of 2023, using data from Global Land Data Assimilation System (GLDAS). Validation of the downscaled monthly GWSA showed high R² values (0.92 to 0.98) and low RMSE values, indicating accurate predictions. The downscaled monthly GWSA map of the study area showed that the western part of the area is more depleted in GWS than the eastern part. These results agree with groundwater level data of 2023 by USGS that also showed excessive groundwater depletion in most of the western part of the area, especially in Baton Rouge, Louisiana and Houston, Texas area. Therefore, the methodology demonstrated in this study can be utilized to monitor changes in potential GWS stress zones over time using downscaled GRACE and GRACE-FO data, providing valuable insights for groundwater management.