James P. McNamara

An approach to understanding hydrologic connectivity on the hillslope and the implications for nutrient transport

[1] Hydrologic processes control much of the export of organic matter and nutrients from the land surface. It is the variability of these hydrologic processes that produces variable patterns of nutrient transport in both space and time. In this paper, we explore how hydrologic ‘‘connectivity’’ potentially affects nutrient transport. Hydrologic connectivity is defined as the condition by which disparate regions on the hillslope are linked via subsurface water flow. We present simulations that suggest that for much of the year, water draining through a catchment is spatially isolated. Only rarely, during storm and snowmelt events when antecedent soil moisture is high, do our simulations suggest that mid-slope saturation (or near saturation) occurs and that a catchment connects from ridge to valley. Observations during snowmelt at a small headwater catchment in Idaho are consistent with these model simulations. During early season discharge episodes, in which the mid-slope soil column is not saturated, the electrical conductivity in the stream remains low, reflecting a restricted, local (lower slope) source of stream water and the continued isolation of upper and mid-slope soil water and nutrients from the stream system. Increased streamflow and higher stream water electrical conductivity, presumably reflecting the release of water from the upper reaches of the catchment, are simultaneously observed when the mid-slope becomes sufficiently wet. This study provides preliminary evidence that the seasonal timing of hydrologic connectivity may affect a range of ecological processes, including downslope nutrient transport, C/N cycling, and biological productivity along the toposequence. A better elucidation of hydrologic connectivity will be necessary for understanding local processes as well as material export from land to water at regional and global scales. INDEX TERMS: 1615 Global Change: Biogeochemical processes (4805); 1860 Hydrology: Runoff and streamflow; 1866 Hydrology: Soil moisture; 1899 Hydrology: General or miscellaneous; KEYWORDS: carbon and nitrogen transport, hydrologic connectivity, TOPMODEL

An analysis of streamflow hydrology in the Kuparuk River Basin, Arctic Alaska: a nested watershed approach

A hydrologic monitoring program was implemented in a nest of watersheds within the Kuparuk River basin in northern Alaska as part of an interdisciplinary effort to quantify the flux of mass and energy from a large arctic area. Described here are characteristics of annual hydrographs and individual storm hydrographs of four basins draining areas of 0.026 km2, 2.2 km2, 142 km2, and 8140 km2; an assessment of the influence that permafrost has on those characteristics; and comparisons to rivers in regions without permafrost. Snowmelt runoff dominated the annual runoff in each basin. A typical storm hydrograph in the Kuparuk River basin had a fast initial response time, long time lags between the hyetograph and hydrograph centroids, an extended recession, and a high runoff/precipitation ratio due to the diminished storage caused by permafrost. The seemingly contradictory results of fast response times and extended recessions can be explained by the presence of a large saturated area occupied by hillslope water tracks. This saturated area provides a partial-source area for fast runoff generation that bypasses the storage capacity of organic soils and tundra vegetation.

Arctic Landscapes in Transition: Responses to Thawing Permafrost

Observations indicate that over the past several decades, geomorphic processes in the Arctic have been changing or intensifying. Coastal erosion, which currently supplies most of the sediment and carbon to the Arctic Ocean [Rachold et al., 2000], may have doubled since 1955 [Mars and Houseknecht, 2007]. Further inland, expansion of channel networks [Toniolo et al., 2009] and increased river bank erosion [Costard et al., 2007] have been attributed to warming. Lakes, ponds, and wetlands appear to be more dynamic, growing in some areas, shrinking in others, and changing distribution across lowland regions [e.g., Smith et al., 2005]. On the Arctic coastal plain, recent degradation of frozen ground previously stable for thousands of years suggests 10–30% of lowland and tundra landscapes may be affected by even modest warming [Jorgenson et al., 2006]. In headwater regions, hillslope soil erosion and landslides are increasing [e.g., Gooseff et al., 2009].

Hydrograph separations in an Arctic watershed using mixing model and graphical techniques

Storm hydrographs in the Upper Kuparuk River basin (142 km2) in northern Alaska were separated into source components using a mixing model and by recession analysis. In non-Arctic regions, storm flow is commonly dominated by old water, that is, water that existed in the basin before the storm. We suspected that this may not be true in Arctic regions where permafrost diminishes subsurface storage capacity. Streamflow during the snowmelt period was nearly all new water. However, all summer storms were dominated by old water. Storms in a neighboring basin were dominated by new water but much less than was the snowmelt event. Thus a large increase in old water contributions occurred following the snowmelt period. This increase continued moderately through the summer in 1994 but not in 1995. We credit the seasonal changes in old water contributions to increased subsurface storage capacity due to thawing of the active layer.

Application of time-lapse ERT imaging to watershed characterization

Time-lapse electrical resistivity tomography ERT has many practical applications to the study of subsurface properties and processes. When inverting time-lapse ERT data, it is useful to proceed beyond straightforward inversion of data differences andtakeadvantageofthetime-lapsenatureofthedata.Weassess various approaches for inverting and interpreting time-lapse ERTdataanddeterminethattwoapproachesworkwell.Thefirst approachismodelsubtractionafterseparateinversionofthedata from two time periods, and the second approach is to use the inverted model from a base data set as the reference model or prior information for subsequent time periods. We prefer this second approach. Data inversion methodology should be considered when designing data acquisition; i.e., to utilize the second approach, it is important to collect one or more data sets for which the bulk of the subsurface is in a background or relatively unperturbed state.Athird and commonly used approach to time-lapse inversion,invertingthedifferencebetweentwodatasets,localizes the regions of the model in which change has occurred; however, varying noise levels between the two data sets can be problematic. To further assess the various time-lapse inversion approaches,weacquiredfielddatafromacatchmentwithintheDry Creek Experimental Watershed near Boise, Idaho, U.S.A. We combined the complimentary information from individual static ERTinversions,time-lapseERTimages,andavailablehydrologicdatainarobustinterpretationschemetoaidinquantifyingseasonalvariationsinsubsurfacemoisturecontent.

An analysis of an arctic channel network using a digital elevation model

Abstract Drainage basins possess spatial patterns of similarity that can be characterized by universal qualities in the fractal dimension and the cumulative area distribution. Features called water tracks often drain hillslopes in basins with permafrost and impose significant control on the hydrologic response of watersheds. We analyzed the arrangement of channel networks and water tracks in Imnavait Creek in Northern Alaska to determine if basins with permafrost possess the same universal characteristics as basins without permafrost. Using digital elevation models (DEMs), we explored the hillslope/channel scaling regimes, the spatial distribution of mass through the cumulative area distribution, and the fractal characteristics of channel networks in the Kuparuk River basin in Northern Alaska. Fractal analysis, slope–area analysis, and field mapping suggest that water tracks are positioned on the hillslopes where channels should occur. Fully-developed channel networks, however, possess certain universal characteristics in aggregation patterns that are manifested in a common cumulative area distribution. Imnavait Creek possesses those universal characteristics only above the scale of the hillslope water track, or when the drainage areas reach the main channels in the valley bottom. Our interpretation is that a rudimentary channel network formed on the hillslopes, but never developed into a mature channel network because permafrost is limiting erosion. Consequently, the undissected hillslopes are extensive. Given the dependence of permafrost on a cold climate, a warming climate and subsequent degradation of permafrost may have significant impacts on the erosional development of channel networks in the Arctic.

Effects of Changes in Arctic Lake and River Ice

Climatic changes to freshwater ice in the Arctic are projected to produce a variety of effects on hydrologic, ecological, and socio-economic systems. Key hydrologic impacts include changes to low flows, lake evaporation regimes and water levels, and river-ice break-up severity and timing. The latter are of particular concern because of their effect on river geomorphology, vegetation, sediment and nutrient fluxes, and sustainment of riparian aquatic habitats. Changes in ice phenology will affect a wide range of related biological aspects of seasonality. Some changes are likely to be gradual, but others could be more abrupt as systems cross critical ecological thresholds. Transportation and hydroelectric production are two of the socio-economic sectors most vulnerable to change in freshwater-ice regimes. Ice roads will require expensive on-land replacements while hydroelectric operations will both benefit and be challenged. The ability to undertake some traditional harvesting methods will also be affected.

Hydrological Partitioning in the Critical Zone: Recent Advances and Opportunities for Developing Transferable Understanding of Water Cycle Dynamics

Hydrology is an integrative discipline linking the broad array of water-related research with physical, ecological, and social sciences. The increasing breadth of hydrological research, often where subdisciplines of hydrology partner with related sciences, reflects the central importance of water to environmental science, while highlighting the fractured nature of the discipline itself. This lack of coordination among hydrologic subdisciplines has hindered the development of hydrologic theory and integrated models capable of predicting hydrologic partitioning across time and space. The recent development of the concept of the critical zone (CZ), an open system extending from the top of the canopy to the base of groundwater, brings together multiple hydrological subdisciplines with related physical and ecological sciences. Observations obtained by CZ researchers provide a diverse range of complementary process and structural data to evaluate both conceptual and numerical models. Consequently, a cross-site focus on “critical zone hydrology” has potential to advance the discipline of hydrology and to facilitate the transition of CZ observatories into a research network with immediate societal relevance. Here we review recent work in catchment hydrology and hydrochemistry, hydrogeology, and ecohydrology that highlights a common knowledge gap in how precipitation is partitioned in the critical zone: “how is the amount, routing, and residence time of water in the subsurface related to the biogeophysical structure of the CZ?” Addressing this question will require coordination among hydrologic subdisciplines and interfacing sciences, and catalyze rapid progress in understanding current CZ structure and predicting how climate and land cover changes will affect hydrologic partitioning. This article is protected by copyright. All rights reserved.

Spatially distributed temperatures at the base of two mountain snowpacks measured with fiber-optic sensors

Snowpack base temperatures vary during accumulation and diurnally. Their measurement provides insight into physical, biological and chemical processes occurring at the snow/soil interface. Recent advances in Raman-spectra instruments, which use the scattered light in a standard telecommunications fiber-optic cable to infer absolute temperature along the entire length of the fiber, offer a unique opportunity to obtain basal snow temperatures at resolutions of 1 m, 10 s, and 0.18C. Measurements along a 330 m fiber over 24 hours during late-spring snowmelt at Mammoth Mountain, California, USA, showed basal snow temperatures of 0 � 0.28C using 10 s averages. Where the fiber- optic cable traversed bare ground, surface temperatures approached 408C during midday. The durability of the fiber optic was excellent; no major damage or breaks occurred through the winter of burial. Data from the Dry Creek experimental watershed in Idaho across a small stream valley showed little variability of temperature on the northeast-facing, snow-covered slope, but clearly showed melting patterns and the effects of solar heating on southwest-facing slopes. These proof-of-concept experiments show that the technology enables more detailed spatial and temporal coverage than traditional point measurements of temperature.

Past and Future Changes in Arctic Lake and River Ice

Paleolimnological evidence from some Arctic lakes suggests that longer ice-free seasons have been experienced since the beginning of the nineteenth century. It has been inferred from some additional records that many Arctic lakes may have crossed an important ecological threshold as a result of recent warming. In the instrumental record, long-term trends exhibit increasingly later freeze-ups and earlier break-ups, closely corresponding to increasing air temperature trends, but with greater sensitivity at the more temperate latitudes. Broad spatial patterns in these trends are also related to major atmospheric circulation patterns. Future projections of lake ice indicate increasingly later freeze-ups and earlier break-ups, decreasing ice thickness, and changes in cover composition, particularly white-ice. For rivers, projected future decreases in south to north air-temperature gradients suggest that the severity of ice-jam flooding may be reduced but this could be mitigated by changes in the magnitude of spring snowmelt.