The goal of this paper is to explore the process controls underpinning regional patterns of variations of runoff regime behavior, i.e., the mean seasonal variation of runoff within the year, across the continental United States. The ultimate motivation is to use the resulting process understanding to generate insights into the physical controls of 5 Flow Duration Curves, in view of the close connection between these two alternative signatures of runoff variability. To achieve these aims a

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... wn modeling approach is adopted; we start with a simple two-stage bucket model, which is systematically enhanced through addition of new processes on the basis of model performance assessment in relation to observations, using rainfall-runoff data from 197 United States 10 catchments belonging to the MOPEX dataset. Exploration of dominant processes and the determination of required model complexity are carried out through model-based sensitivity analyses, guided by a performance metric. Results indicated systematic regional trends in dominant processes: snowmelt was a key process control in cold mountainous catchments in the north and north-west, whereas snowmelt and vege-15 tation cover dynamics were key controls in the north-east; seasonal vegetation cover dynamics (phenology and interception) were important along the Appalachian mountain range in the east. A simple two-bucket model (with no other additions) was found to be adequate in warm humid catchments along the west coast and in the south-east, with both regions exhibiting strong seasonality, whereas much more complex models 20 are needed in the dry south and south-west. Agricultural catchments in the mid-west were found to be difficult to predict with the use of simple lumped models, due to the strong influence of human activities. Overall, these process controls arose from general east-west (seasonality) and north-south (aridity, temperature) trends in climate (with some exceptions), compounded by complex dynamics of vegetation cover and to 25 a less extent by landscape factors (soils, geology and topography). 7036 catchments, the analysis involves systematic model development and assessment of model predictions and performance in comparison to observed data. This downward or top-down approach to model development (Jothityangkoon et al., 2001; Farmer et al., 2003; Sivapalan et al., 2003) commenced with the development of a simple two-bucket model (hereafter referred to as the "base model"). This model was initially applied to 25 all 197 catchments, and its performance assessed. Guided by alternative hypotheses regarding the reasons for the poor fits against regime curves estimated from observed runoff data, the model was enhanced step by step through addition of new processes 7037 HESSD 9, 2012 initially left out of the base model. Model development was continued until the model performance could not be improved any longer. The complete model was then utilized in sensitivity studies to (a) decipher the dominant process controls on the regime curve, and (b) the minimum complexity of models (i.e., the mix of processes required) needed to achieve a satisfactory fit to the empirical regime curves. In this way it is hoped to 5 25 remained a challenging problem. A synthesis of these two top-down and bottom-up approaches is possibly the key to developing new understanding and new theories of hydrologic responses at catchment scales. The present study is a step in this direction. 7038 Abstract 25 7039 Abstract HESSD 9, 7035-7084, 2012 Abstract HESSD 9, 7035-7084, 2012 Abstract HESSD 9, 7035-7084, 2012 Abstract HESSD 9, 7035-7084, 2012 Abstract HESSD 9, 7035-7084, 2012 Abstract HESSD 9, 7035-7084, 2012 Abstract 9, 7035-7084, 2012 Abstract HESSD 9, 7035-7084, 2012 Abstract HESSD 9, 7035-7084, 2012 Abstract 9, 7035-7084, 2012 Abstract 9, 7035-7084, 2012 Abstract 9, 7035-7084, 2012 Abstract 9, 7035-7084, 2012 Abstract 9, 7035-7084, 2012 Abstract HESSD 9, 7035-7084, 2012 Abstract HESSD 9, 7035-7084, 2012 Abstract Florida catchments. A more detailed classification system, such as that developed by Coopersmith et al. (2012), may be needed to group catchments more accurately. 7063 Abstract influence of high flows and low flows. In general, due to the removal of time dependence of flows in the construction of FDCs, the information we gain from the FDCs is 7064 Abstract processes for modeling cold, mountainous forested catchments is snowmelt; for cold, forested catchments near the east coast, however, they include both snowmelt and 7065 HESSD 9, 7035-7084, 2012 Abstract HESSD 9, 7035-7084, 2012