Quantifying the base flow of the Colorado River: its importance in sustaining perennial flow in northern Arizona and southern Utah (USA)

Water in the Colorado River, USA, is known to be a highly over-allocated resource, yet managers and decision makers rarely consider one of the most important contributions to the existing water in the river, i.e. groundwater. This oversight may result from the contrasting results of base-flow studies conducted on the amount of streamflow into the Colorado River sourced from groundwater. Some studies rule out the significance of groundwater contribution, while others show groundwater contributing the majority of flow to the river. This study uses new and extant instrumented data (not indirect methods) to quantify the groundwater base-flow contribution to surface flow. The precipitation, streamflow, and base flow of 10 remote subbasins of the Colorado Plateau in southern Utah and northern Arizona were examined in detail. These tributaries have an annual average base-flow discharge of 0.45 km³/year (367,000 acre-feet per year) with an average base-flow fraction of 72% summing to more than 3% of the mean flow of the Colorado River at Phantom Ranch. The groundwater storage trend of the Colorado River Basin when measured with remote sensing is declining; however, when utilizing instrumented data, the average annual base-flow trend in the study area remains constant. This trend suggests that base-flow signatures in streams may have a delayed response from the decline observed in groundwater storage from remote sensing. The simple extant data measurement methods employed in this study can be applied to the entire drainage basin, revealing the quantity of base flow throughout the basin to better inform water resource management.