Brian L. McGlynn

The role of topography on catchment-scale water residence time

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.

The future of hydrology: An evolving science for a changing world

Human activities exert global-scale impacts on our environment with significant implications for freshwater-driven services and hazards for humans and nature. Our approach to the science of hydrology needs to significantly change so that we can understand and predict these implications. Such an adjustment is a necessary prerequisite for the development of sustainable water resource management strategies and to achieve long-term water security for people and the environment. Hydrology requires a paradigm shift in which predictions of system behavior that are beyond the range of previously observed variability or that result from significant alterations of physical (structural) system characteristics become the new norm. To achieve this shift, hydrologists must become both synthesists, observing and analyzing the system as a holistic entity, and analysts, understanding the functioning of individual system components, while operating firmly within a well-designed hypothesis testing framework. Cross-disciplinary integration must become a primary characteristic of hydrologic research, catalyzing new research and nurturing new educational models. The test of our quantitative understanding across atmosphere, hydrosphere, lithosphere, biosphere, and anthroposphere will necessarily lie in new approaches to benchmark our ability to predict the regional hydrologic and connected implications of environmental change. To address these challenges and to serve as a catalyst to bring about the necessary changes to hydrologic science, we call for a long-term initiative to address the regional implications of environmental change.

A review of the evolving perceptual model of hillslope flowpaths at the Maimai catchments, New Zealand

The Maimai catchment has been the site of ongoing hillslope research since the late 1970s. These studies have facilitated the development of a detailed perceptual model of hillslope hydrology at Maimai. This perceptual model has grown in complexity beyond analytical description; nonetheless it provides a very useful case study of hillslope hydrological processes and encapsulates much of what field hydrologists have come to recognize as the dominant hillslope runoff processes in steep, humid catchments. No single research approach has resolved the complexities of streamflow generation in this highly responsive catchment. Yet, each data set reviewed in this paper adds to the cumulative understanding of catchment behavior by providing alternative (and sometimes conflicting) interpretations of hillslope subsurface flow. Initial dye tracer studies of macropore flow provided insight into hillslope flow processes, but suffered from the limitations of a single-method approach. Subsequent water isotopic tracing studies showed clearly that stored soil water and groundwater comprised the majority of channel stormflow; notwithstanding, isotope-oriented approaches did not enable the development of a mechanistic understanding of hillslope processes. An integration of tensiometer recording and tracer techniques was required for later reconciliation of different process interpretations concerning the role of macropores and old/new water ratios. Although single throughflow pits continued to be the indicator of subsurface flow timing and magnitude for several published studies at Maimai, subsequent whole hillslope trench studies showed that flow varied widely across a slope section—making the single pit observations of the previous studies suspect. Most recent observations demonstrate that small depressions in the bedrock surface may exert a significant control on water mobility and mixing. In particular, the bedrock topography appears to determine spatially the pathway of rapid saturated subsurface water flow and tracer breakthrough at the hillslope scale. The Maimai catchments in New Zealand provide a historical perspective on the issues faced by hillslope/small catchment hydrologists since the mid 1970s and highlights the advantages of multiple repeat experiments for testing hypotheses and improving our mechanistic understanding of subsurface flow.

A new triangular multiple flow direction algorithm for computing upslope areas from gridded digital elevation models

[1] Gridded digital elevation data, often referred to as DEMs, are one of the most widely available forms of environmental data. Topographic analysis of DEMs can take many forms, but in hydrologic and geomorphologic applications it is typically used as a surrogate for the spatial variation of hydrological conditions (topographic indices) and flow routing. Here we report on a new flow routing algorithm and compare it to three common classes of algorithms currently in widespread use. The advantage of the new algorithm is that unrealistic dispersion on planar or concave hillslopes is avoided, whereas multiple flow directions are allowed on convex hillslopes. We suggest that this new triangular multiple flow direction algorithm (MD1) is more appropriate for a range of flow routing and topographic index applications.

Riparian zone flowpath dynamics during snowmelt in a small headwater catchment

The hydrology of the near-stream riparian zone in upland humid catchments is poorly understood. We examined the spatial and temporal aspects of riparian flowpaths during snowmelt in a headwater catchment within the Sleepers River catchment in northern Vermont. A transect of 15 piezometers was sampled for Ca, Si, DOC, other major cations, and δ18O. Daily piezometric head values reflected variations in the stream hydrograph induced by melt and rainfall. The riparian zone exhibited strong upward discharge gradients. An impeding layer was identified between the till and surficial organic soil. Water solute concentrations increased toward the stream throughout the melt. Ca concentrations increased with depth and DOC concentrations decreased with depth. The concentrations of Ca in all piezometers were lower during active snowmelt than during post-melt low flow. Ca data suggest snowmelt infiltration to depth; however, only upslope piezometers exhibited snowmelt infiltration and consequent low δ18O values,(while δ18O values varied less than 0.5‰ in the deep riparian piezometers throughout the study period. Ca and δ18O values in upslope piezometers during low streamflow were comparable to Ca and δ18O in riparian piezometers during high streamflow. The upland water Ca and δ18O may explain the deep riparian Ca dilution and consistent δ18O composition. The temporal pattern in Ca and δ18O indicate that upland water moves to the stream via a lateral displacement mechanism that is enhanced by the presence of distinct soil/textural layers. Snowmelt thus initiates the flux of pre-melt, low Ca upland water to depth in the riparian zone, but itself does not appear at depth in the riparian zone during spring melt. This is despite the coincident response of upland groundwater and stream discharge.

Flow velocity and the hydrologic behavior of streams during baseflow

[1] Diel variations in stream discharge have long been recognized, but are relatively little studied. Here we demonstrate that these diel fluctuations can be used to investigate both streamflow generation and network routing. We treat evapo-transpiration (ET) as a distributed impulse function in an advection model and analyze the effect of ET on diel fluctuations in discharge. We show that when flow velocity is high during high baseflow, discharge fluctuations tend to be in phase and constructive interference reinforces ET-generated signals resulting in strong diel fluctuations measured at a gauging station at the mouth of the watershed. As flow velocity slows with baseflow recession, ET-generated signals are increasingly out of phase so that fluctuations in discharge are masked by destructive interference. These results demonstrate that naturally produced fluctuations in discharge constitute discrete impulse functions that can be used to analyze eco-hydrologic behavior of whole-watersheds during baseflow periods. Citation: Wondzell, S. M., M. N. Gooseff, and B. L. McGlynn (2007), Flow velocity and the hydrologic behavior of streams during baseflow, Geophys. Res. Lett., 34, L24404, doi:10.1029/2007GL031256.

Seasonality in spatial variability and influence of land use/land cover and watershed characteristics on stream water nitrate concentrations in a developing watershed in the Rocky Mountain West

[1] In recent decades, the Rocky Mountain West has been one of the fastest growing regions in the United States. Headwater streams in mountain environments may be particularly susceptible to nitrogen enrichment from residential and resort development. We utilized stream water chemistry from six synoptic sampling campaigns combined with land use/land cover (LULC) and terrain analysis in geostatistical modeling to examine the spatial and seasonal variability of LULC impacts on stream water nitrate. Stream water nitrate was spatially correlated for longer distances during the dormant season than during the growing season, suggesting the importance of biological retention. Spatial linear models indicated that anthropogenic sources best predicted stream water nitrate in the dormant season, while variables describing biological processing were the best predictors in the growing season. This work demonstrates the importance of (1) incorporating spatial relationships into water quality modeling and (2) investigating stream water chemistry across seasons to gain a more complete understanding of development impacts on stream water quality.

Differential soil respiration responses to changing hydrologic regimes

[1] Soil respiration is tightly coupled to the hydrologic cycle (i.e., snowmelt and precipitation timing and magnitude). We examined riparian and hillslope soil respiration across a wet (2005) and a dry (2006) growing season in a subalpine catchment. When comparing the riparian zones, cumulative CO 2 efflux was 33% higher, and peak efflux occurred 17 days earlier during the dry growing season. In contrast, cumulative efflux in the hillslopes was 8% lower, and peak efflux occurred 10 days earlier during the drier growing season. Our results demonstrate that soil respiration was more sensitive to drier growing season conditions in wet (riparian) landscape positions.

The spatial and temporal evolution of contributing areas

Predicting runoff source areas and how they change through time is a challenge in hydrology. Topographically induced lateral water redistribution and water removal through evapotranspiration lead to spatially and temporally variable patterns of watershed water storage. These dynamic storage patterns combined with threshold mediation of saturated subsurface throughflow lead to runoff source areas that are dynamic through time. To investigate these processes and their manifestation in watershed runoff, we developed and applied a parsimonious but spatially distributed model (WECOH—Watershed ECOHydrology). Evapotranspiration was measured via an eddy-covariance tower located within the catchment and disaggregated as a function of vegetation structure. This modeling approach reproduced the stream hydrograph well and was internally consistent with observed watershed runoff patterns and behavior. We further examined the spatial patterns of water storage and their evolution through time by building on past research focused on landscape hydrologic connectivity. The percentage of landscape area connected to the stream network ranged from less than 1% during the fall and winter base flow period to 71% during snowmelt. Over the course of the 2 year study period, 90% of the watershed areas were connected to the stream network for at least 1 day, leaving 10% of area that never became connected. Runoff source areas during the event shifted from riparian dominated runoff to areas at greater distances from the stream network when hillslopes became connected. Our modeling approach elucidates and enables quantification and prediction of watershed active areas and those active areas connected to the stream network through time.

Calculating terrain indices along streams: a new method for separating stream sides.

There is increasing interest in assessing riparian zones and their hydrological and biogeochemical buffering capacity with indices derived from hydrologic landscape analysis of digital elevation da ...