Exploring the physical controls of regional patterns of flow duration curves – Part 2: Role of seasonality, the regime curve, and associated process controls

摘要

The goal of this paper is to explore the process controlsunderpinning regional patterns of variations of streamflow regime behavior,i.e., the mean seasonal variation of streamflow within the year, across thecontinental United States. The ultimate motivation is to use the resultingprocess understanding to generate insights into the physical controls ofanother signature of streamflow variability, namely the flow duration curve(FDC). The construction of the FDC removes the time dependence of flows. Thusin order to better understand the physical controls in regions that exhibitstrong seasonal dependence, the regime curve (RC), which is closelyconnected to the FDC, is studied in this paper and later linked back to theFDC. To achieve these aims a top-down modeling approach is adopted; we startwith a simple two-stage bucket model, which is systematically enhancedthrough addition of new processes on the basis of model performanceassessment in relation to observations, using rainfall-runoff data from 197United States catchments belonging to the MOPEX dataset. Exploration ofdominant processes and the determination of required model complexity arecarried out through model-based sensitivity analyses, guided by aperformance metric. Results indicated systematic regional trends in dominantprocesses: snowmelt was a key process control in cold mountainous catchmentsin the north and north-west, whereas snowmelt and vegetation cover dynamicswere key controls in the north-east; seasonal vegetation cover dynamics(phenology and interception) were important along the Appalachian mountainrange in the east. A simple two-bucket model (with no other additions) wasfound to be adequate in warm humid catchments along the west coast and inthe south-east, with both regions exhibiting strong seasonality, whereasmuch more complex models are needed in the dry south and south-west.Agricultural catchments in the mid-west were found to be difficult topredict with the use of simple lumped models, due to the strong influence ofhuman activities. Overall, these process controls arose from generaleast-west (seasonality) and north-south (aridity, temperature) trends inclimate (with some exceptions), compounded by complex dynamics of vegetationcover and to a less extent by landscape factors (soils, geology andtopography).