Increasing contribution of alluvial groundwater to riparian cottonwood forest water use through warm and dry summers

Abstract

In semi-arid regions, riparian forests access alluvial groundwater to supplement the limited local precipitation. Understanding the sensitivity of riparian forests to variations in weather and climate, and water resources management is essential for guiding riparian conservation measures, especially on rivers impacted by dams and water withdrawal. We anticipated a seasonal trade-off in ecosystem water sources, with increasing relative groundwater use through the summer to compensate for the reduced availability of the shallow soil moisture. To test this hypothesis, we assessed the temporal patterns of alluvial groundwater uptake by a riparian cottonwood (Populus species) forest along a regulated river in southern Alberta, Canada over a three-year interval with relatively warm and dry weather, and low river flows. In support of our hypothesis, groundwater evapotranspiration, as calculated from diurnal fluctuations in the alluvial water table, displayed relative seasonal consistency until the autumnal leaf senescence period. In contrast, the ecosystem-scale evapotranspiration, measured by eddy covariance, gradually receded through the summer as precipitation inputs that fed the shallow soil layers became limited. The consistent uptake rate of alluvial groundwater by cottonwoods enables riparian forests to maintain high rates of water use and productivity in this semi-arid prairie region. We conclude the consistent groundwater use through the warm and dry summer reflected: (1) perennial river flow that replenished the floodplain aquifer; (2) adaptations of cottonwoods to the riparian hydrology and the semi-arid climate; and (3) environmental flow operations that maintained the minimum river and alluvial groundwater levels. Our study provides information on the riparian forest water sources and water use patterns that could help water managers effectively regulate river flows to support healthy downstream riparian ecosystems.