Calculating Surface Water and Groundwater Interactions for Water Right Transfers—How Far Do We Go?

The fundamental concepts for the interaction between surface water and groundwater have been well studied and defined, and used for calculating stream-depletion effects of water-right transfers. The New Mexico Office of the State Engineer has adopted administrative criteria for calculating stream depletion effects of water rights transfers for many of the groundwater basins in New Mexico. Should these methods for calculating stream depletion effects be prescriptive or case-by-case? How do we know the discrepancies between the calculation and reality? What should be considered significant or insignificant? Three case studies are used to explore the answers to these questions: (1) groundwater transfer to surface water on tributary to Rio Grande, (2) new groundwater appropriation in tributary groundwater basin to the Middle Rio Grande, and (3) bulk surface water purchases for offsetting stream depletion from groundwater pumping. In all cases, the assumptions for calculating stream depletion overstate the actual effect and limit the amount of water that can be transferred and reduce the quantity of the water right. Average streamflow conditions and continuous hydraulic connection between surface water and groundwater are some of the assumptions that lead to over-prediction of streamflow depletion. The lack of data also leads to overly conservative analysis. Performing gain-loss measurements on stream and developing a simple monitoring program consisting of stream gauging and water-level monitoring will help marry calculated effects with reality.