Identifying the Canary in the Lower Rio Grande Basin: Hydrologic Signals to Support Ground Water Management in a Transboundary Stream-Aquifer System

Within the Lower Rio Grande Basin is an extensive stream-connected aquifer, tapped by neighboring countries, neighboring states, and within the states of Texas and New Mexico, by political jurisdictions with sometimes non-aligned interests, for example, municipalities and irrigation districts.  Adding complexity to water resource allocation in the region is the Rio Grande itself, a transboundary river with flow obligations specified by international treaty; and, interstate allocation implemented by the historic operations of the Rio Grande (irrigation) Project.  Controversy and conflict over water allocation has characterized the region for decades; although a recently adopted Rio Grande Project operation agreement includes solutions acceptable to multiple parties.  The operation agreement does not restrict groundwater pumping that may impact river flows; instead, New Mexico entities carry the burden of ensuring that inflow-dependent surface water deliveries at the Texas stateline are maintained.   The stateline flow requirement and corresponding silence on groundwater challenges the upstream New Mexico entities to develop a good understanding of surface water/groundwater interactions, particularly in times of extended drought, to ensure that an unsustainable reliance on groundwater is not developed.   New Mexico, with cooperation of numerous entities, has developed a groundwater model for use in water resource administration and management, including understanding groundwater/surface water interactions.  This model and supporting data analysis can be used to identify potential triggers that may signal surface water impacts that would endanger satisfaction of the river operations agreement or require unpractical or overly expensive mitigation.   Potential triggers include drain flows, river efficiency, and shallow groundwater levels, among others. The historic relationships of these physical properties to groundwater pumping is examined under a variety of climatic/inflow conditions to provide insight on system changes and aquifer recovery dynamics following high-stress periods.