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The use of groundwater to supplement surface-water supplies for the Bureau of Reclamation Klamath Project in the upper Klamath Basin of Oregon and California markedly increased between 2000 and 2014. Pre-2001 groundwater pumping in the area where most of this increase occurred is estimated to have been about 28,600 acre-feet per year. Subsequent supplemental pumping rates have been as high as 128,740 acre-feet per year. During this period of increased pumping, groundwater levels in and around the Bureau of Reclamation Klamath Project have declined by about 20–25 feet. Waterlevel declines are largely due to the increased supplemental pumping, but other factors include increased pumping adjacent to the Klamath Project and drying climate conditions. This report summarizes the distribution and magnitude of supplemental groundwater pumping and groundwater-level declines, and characterizes the relation between the stress and response in subareas of the Klamath Project to aid decision makers in developing groundwater-management strategies. Introduction Groundwater has been used for irrigation in the upper Klamath Basin of Oregon and California for many decades. The use of groundwater to supplement surface-water supplies on the Bureau of Reclamation Klamath Project (hereinafter referred to as the “Project”), however, increased markedly after 2000 as a result of changes in surface-water allocations. Much of the increased supplemental pumping has been associated with a pilot water bank and similar agency-sponsored programs under which well owners have been contracted to supply water to the Project. The increased pumping in the Project has caused localized increases in seasonal drawdown and generally continuous year-to-year water-level declines. These effects are a cause for concern among groundwater users and agencies tasked with maintaining stable water levels and managing aquifers for sustainable use. There is an extensive dataset of supplemental pumping volumes and corresponding water-level changes that can be used to inform groundwater-management strategies and decisions. Groundwater use in the upper Klamath Basin can be categorized as primary, meaning that it is the sole source of irrigation water for a parcel of land, or supplemental, meaning it is used only when a primary surface-water source is not available. Groundwater pumping not associated with the Project is referred to hereinafter as background pumping. Groundwater pumping for the Project contracted by Reclamation or the Klamath Water and Power Agency is all supplemental and is referred to hereinafter as Project pumping.

Groundwater levels, trends, and relations to pumping in the Bureau of Reclamation Klamath Project, Oregon and California