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Moffat_Emma_Final_Project.pdfProject Overview
Groundwater is a vital resource, supplying 30% of the global freshwater and supporting over 115 million people in the United States. In central Texas, where droughts are common, groundwater is a reliable water source for municipal, agricultural, and industrial needs. The San Gabriel Watershed, a key contributor to the Edwards Aquifer, plays a crucial role in replenishing groundwater through its unique karst features like fractures and sinkholes.
This project uses GIS and the Analytical Hierarchy Process (AHP) to map groundwater potential zones in the San Gabriel Watershed. By analyzing factors such as land cover, lineament density, drainage density, rainfall, and geology, my project identifies areas with high or low potential for groundwater recharge, discharge, and storage.
Methodology
The flow diagram on the right visualizes the methodology of my project, which include the GIS processing steps, AHP, and weighted overlay analysis.
Study Area
The study focuses on the San Gabriel Watershed in central Texas, a critical recharge zone for the Edwards Aquifer. Spanning urban, suburban, and rural areas, the watershed's geology, hydrology, and land use were analyzed to identify groundwater potential zones.
Data and Preprocessing
Spatial datasets for geology, rainfall, drainage density, lineament density, and land cover were collected from reliable sources and processed in ArcGIS Pro. All datasets were converted into raster formats compatible with the weighted overlay analysis tool. Continuous data (e.g., drainage density, rainfall) were reclassified using natural breaks, while categorical data (e.g., geology, land cover) were ranked based on their influence on groundwater potential.
Map of the San Gabriel Watershed
Analytical Hierarchy Process
The Analytical Hierarchy Process is a method used to manage complex decision-making created by Thomas Saaty in 1980. It uses hierarchy and pairwise comparisons to rank the importance of criteria and calculate their weights to achieve a specific goal. High weights calculated from the AHP indicate the criteria has more influence on groundwater potential, while lower weights indicate the criteria has less influence on groundwater potential. For my project, geology (44%) and rainfall (30%) were identified as the most influential factors, followed by drainage density (15%), lineament density (7%), and land cover (4%). More information on this process is seen on pages 7 to 10 of my report.
Comprehensive Map showing Groundwater Potential Zones in the San Gabriel Watershed
Results and Findings
The final comprehensive map on the right was created by combining the reclassified and weighted geology, rainfall, drainage density, lineament density, and land cover layers shown on the left into the weighted overlay analysis tool in ArcGIS Pro. It was found that 35.7% of the watershed has low groundwater potential, 37.7% of the watershed has moderate groundwater potential, 26.2% of the watershed has high groundwater potential, and 0.4% of the watershed has very high groundwater potential.
My groundwater potential map shows that areas with high groundwater potential are concentrated in the east part of the watershed, where high rainfall and permeable lithologies such as alluvium and gravel facilitate infiltration. The west side of the watershed exhibits low groundwater potential due to lower rainfall, semi-confining lithologies including marl, and a lack of lineaments. Areas on top of the aquifer vary between low and high groundwater potential. This is likely due to a combination of favorable geology, such as karstified limestone, and the impermeable surfaces of built areas.
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