EMMA AND VOLUMETRIC STREAM GAUGING REVEAL MULTIPLE TIME-VARYING CONTRIBUTIONS TO AGRICULTURAL DRAINS ALONG RIO GRANDE, CENTRAL NM

Volumetric flow gauging and end member mixing analysis (EMMA) of sampled environmental tracers were performed on waters from irrigation drains and the Rio Grande in the Belen District of Middle Rio Grande Conservatory District (MRGCD) to determine the proportion and provenance of groundwater inflows to the Rio Grande River and associated riparian aquifer system. The 70-km project area overlies the distal end of the Albuquerque Basin of the Rio Grande Rift, where basin and sub-basin structure drives hydraulic flow to converge at the southern end of the basin. Previous work has generally focused on deep groundwater flow along the axis of the basin, or shallow modern flow. This study considers those end members as well as lateral groundwater inputs. Quantifying the volumetric contribution and spatial distribution of distinct water sources to the floodplain aquifer informs water management decisions, drought sensitivity, and salinization mechanics. EMMA was performed on hydrochemical datasets collected at various times throughout the 2021 water year. Analysis results indicate that lateral groundwater inputs from the West Central hydrochemical zone accounts for as much as 70% of volumetric flow in irrigation drains during pre-irrigation season (March), and volumetric inflows of groundwater were either static or increased into the irrigation season (late May) despite an overall increase in total volumetric drain flow. These results highlight the dynamic nature of the hydraulic system and imply that lateral groundwater inputs provide a significant component to Rio Grande baseflow.