No abstract

[1] The age, or residence time, of water is a fundamental descriptor of catchment hydrology, revealing information about the storage, flow pathways, and source of water in a single integrated measure. While there has been tremendous recent interest in residence time estimation to characterize watersheds, there are relatively few studies that have quantified residence time at the watershed scale, and fewer still that have extended those results beyond single catchments to larger landscape scales. We examined topographic controls on residence time for seven catchments (0.085-62.4 km 2 ) that represent diverse geologic and geomorphic conditions in the western Cascade Mountains of Oregon. Our primary objective was to determine the dominant physical controls on catchment-scale water residence time and specifically test the hypothesis that residence time is related to the size of the basin. Residence times were estimated by simple convolution models that described the transfer of precipitation isotopic composition to the stream network. We found that base flow mean residence times for exponential distributions ranged from 0.8 to 3.3 years. Mean residence time showed no correlation to basin area (r 2 < 0.01) but instead was correlated (r 2 = 0.91) to catchment terrain indices representing the flow path distance and flow path gradient to the stream network. These results illustrate that landscape organization (i.e., topography) rather than basin area controls catchment-scale transport. Results from this study may provide a framework for describing scale-invariant transport across climatic and geologic conditions, whereby the internal form and structure of the basin defines the first-order control on base flow residence time.

[1] Field studies in watershed hydrology continue to characterize and catalogue the enormous heterogeneity and complexity of rainfall runoff processes in more and more watersheds, in different hydroclimatic regimes, and at different scales. Nevertheless, the ability to generalize these findings to ungauged regions remains out of reach. In spite of their apparent physical basis and complexity, the current generation of detailed models is process weak. Their representations of the internal states and process dynamics are still at odds with many experimental findings. In order to make continued progress in watershed hydrology and to bring greater coherence to the science, we need to move beyond the status quo of having to explicitly characterize or prescribe landscape heterogeneity in our (highly calibrated) models and in this way reproduce process complexity and instead explore the set of organizing principles that might underlie the heterogeneity and complexity. This commentary addresses a number of related new avenues for research in watershed science, including the use of comparative analysis, classification, optimality principles, and network theory, all with the intent of defining, understanding, and predicting watershed function and enunciating important watershed functional traits.

No abstract

[1] Analysis of subsurface stormflow from 147 storms at the 20 m long trench in the Panola Mountain Research Watershed by Tromp-van Meerveld and McDonnell (2006a) showed that there was a distinct 55 mm precipitation threshold for significant subsurface stormflow production. This second paper in the series investigates the processes responsible for this threshold response. We installed a dense spatial array of maximum rise crest stage gauges and recording wells on the hillslope and studied the temporal and spatial patterns of transient saturation at the soil-bedrock interface and its relation to subsurface stormflow measured at the trench face. Results show that while transient groundwater developed on parts of the hillslope during events smaller than 55 mm, it was not until more than 55 mm of rain fell before bedrock depressions on the hillslope were filled, water spilled over microtopographic relief in the bedrock surface, and the subsurface saturated areas became connected to the trench. When connectivity was achieved, the instantaneous subsurface stormflow rate increased more than fivefold compared to before the subsurface saturated areas were connected to the trench face. Total subsurface stormflow was more than 75 times larger when connectivity was achieved compared to when connectivity was not achieved. The fill and spill hypothesis presented in this paper is a process explanation for the observed threshold behavior of Tromp-van Meerveld and McDonnell (2006a), thereby linking patterns and processes.

Simultaneous observations of rapid preferential flow through macropores and isotopically "old" water displacement remain unresolved in the Maimai (M8) catchment. Continuous, three-dimensional soil moisture energy conditions were monitored in two discrete catchment positions for a series of storm events in 1987. TenSiometric response was related to the soil water characteristic curve, hillslope throughflow, and total catchment runoff. For events yielding <<2 mm hr -1 peak runoff, near-stream valley bottom groundwater systems discharged water volumes sufficient to account for storm period streamflow. This process was assisted by regular low (<-40 cm H20) matric potential conditions and rapid filling of available soil water storage. For events yielding >2 mm hr-1 peak storm flow, hillslope hollow drainage into steeply sloping first-order channels dominated old water production and most of the catchment storm flow. Highly transient macropore-driven processes of crack infiltration (bypass flow), slope water table development, and lateral pipe flow enabled large volumes of stored water to be delivered to the first-order channel bank at the appropriate time to satisfy catchment storm flow volumes and water isotopic and chemical composition. INTRODUCTIONProgress in understanding processes of storm runoff generation in humid headwater catchments is hampered by discrepancies often found between physical, chemical, and isotopic approaches. Furthermore, the results from chemical and natural isotope separations of stream water into old (pre event) and new (event) water sources often appear to contradict results from hydrometric studies of pathways of water movement on hillslopes.Recent work in a highly responsive catchment at Maimai (M8) on the west coast of New Zealand has yielded conflicting results regarding the importance of new versus old water in through-flow transmission. Hydrometric studies employing dye tracing and subsurface flow measurement [Mosley, 1979[Mosley, , 1982 concluded that rapid flow of new water through macropores was capable of accounting for storm period streamflow. In more recent natural stable isotope and chemical tracing studies, Pearce et al. [1986] and SMash et al. [1986] explicitly refuted this earlier interpretation by indicating that old water dominated throughflow in all storm events monitored. On the basis of previous computer simulations [SMash and Farvolden, 1979] and physical principles [Gillham, 1984], they suggested that a rapid matrix flow displacement mechanism, occurred through (1) saturated wedges on the lower slopes and (2) groundwater ridges in the valley bottoms. This process involved capillary fringe response [Ragan, 1968], which increased local hydraulic gradients and promoted increased gravity drainage of old water to the stream channel.For this study area it now appears that there may be no need for rapid macropore flow transport of new water to explain the subsurface transmission of water downslope, because stored water is discharged into the stream channel. However, the c...

[1] The dialog between experimentalist and modeler in catchment hydrology has been minimal to date. The experimentalist often has a highly detailed yet highly qualitative understanding of dominant runoff processes; thus there is often much more information content on the catchment than we use for calibration of a model. While modelers often appreciate the need for ''hard data'' for the model calibration process, there has been little thought given to how modelers might access this ''soft'' or process knowledge. We present a new method where soft data (i.e., qualitative knowledge from the experimentalist that cannot be used directly as exact numbers) are made useful through fuzzy measures of model simulation and parameter value acceptability. We developed a three-box lumped conceptual model for the Maimai catchment in New Zealand, a particularly well-studied process-hydrological research catchment. The boxes represent the key hydrological reservoirs that are known to have distinct groundwater dynamics, isotopic composition, and solute chemistry. The model was calibrated against hard data (runoff and groundwater levels) as well as a number of criteria derived from the soft data (e.g., percent new water, reservoir volume, etc.). We achieved very good fits for the three-box model when optimizing the parameter values with only runoff (R eff = 0.93). However, parameter sets obtained in this way showed in general a poor goodness of fit for other criteria such as the simulated new water contributions to peak runoff. Inclusion of soft data criteria in the model calibration process resulted in lower R eff values (around 0.84 when including all criteria) but led to better overall performance, as interpreted by the experimentalist's view of catchment runoff dynamics. The model performance with respect to soft data (like, for instance, the new water ratio) increased significantly, and parameter uncertainty was reduced by 60% on average with the introduction of the soft data multicriteria calibration. We argue that accepting lower model efficiencies for runoff is ''worth it'' if one can develop a more ''real'' model of catchment behavior. The use of soft data is an approach to formalize this exchange between experimentalist and modeler and to more fully utilize the information content from experimental catchments.

No abstract