An analysis of trends in baseflow recession and low-flows in rain-dominated coastal streams of the pacific coast

Abstract

The work presented here centers on the identification and calculation of indices representative of annual low-flow conditions and baseflow recession form, and the evaluation of trends in these indices with time for rain-dominated streams of the Pacific coastal mountain ranges. Two different baseflow recession analysis techniques are employed, which when combined are capable of modeling the varied dry-season flow conditions that exist over the broad range of catchments included in the study area. Results indicate that over the past 40–80 years widespread trends of increasing rates of baseflow recession and decreasing annual low-flow conditions exist throughout the region. Of streamgages analyzed, 44% were identified as having a statistically significant trend in either low-flow conditions or recession form with time. While spring flow conditions show little change over the study period, trends of decreasing late-summer flow conditions and increasing rates of recession are particularly common. Northern California and Oregon are especially impacted locations, with upwards of 60% of study gages exhibiting decreasing trends in late summer flow conditions. Detailed conceptual explanations for the connections between trends in recession form and indices of low-flow are also presented.