Ming-ko Woo

Permafrost hydrology in North America 1

Abstract This paper reviews hydrologic processes in the permafrost regions of northern North America. Much work has recently been done at specific experimental plots to parallel the progress in laboratory investigations, improving our understanding of the heat and water fluxes in thawed and frozen grounds, infiltration in frozen soils, evaporation in a cold environment, interaction between snow and its frozen substrate, and the dynamics of storage in the active layer. Field research on permafrost slopes and in northern research basins adds to our knowledge of permafrost groundwater hydrology, runoff generating processes, river freeze‐up and breakup processes and allows more precise definition of basin water balance. Sufficient hydrometric data are now available to analyse the streamflow characteristics in an area with permafrost, and more work should be done along this line. It is urged that process studies be continued to gain a better understanding of the effect of permafrost upon the hydrologic cycle. ...

Hydrology of subarctic Canadian shield: soil-filled valleys

The Canadian Shield landscape includes bedrock uplands and valleys infilled with soil. A site near Yellowknife in subarctic Canada was studied to elucidate the hydrological behaviour of soil-filled valleys. A suite of hydrological processes was found to be important in the studied valley, including snowmelt and rainfall, ground frost development, evaporation, infiltration, lateral inflow from adjacent uplands, and surface and subsurface flows. Valley storage requirements during snowmelt are met by local meltwater infiltration, but lateral inflows in the summer are needed to satisfy the storage before runoff can be generated from the valley. The valleys perform three functions of collecting vertical and lateral water inputs, retaining and losing the water held in storage, and transferring water down the valley and generating local outflow. Unlike channel flows in humid areas, runoff from the upper valley has to meet the storage demands of the lower valley segments, often causing seepage loss along flow paths to render the flows intermittent. A fill-and-spill runoff system is proposed in which the valley physiography results in a series of segments with varying storage conditions. As water is supplied to the valley, each segment has to be filled until its storage threshold for runoff is exceeded. Then, subsurface or surface flows will be generated, but these flows may be arrested to furnish water to satisfy the storage requirements of the segments downstream. Such a flow system applies also to other valleys in the Shield environment.

Slope runoff processes and flow generation in a subarctic, subalpine catchment

Hillslope runoff was studied in a subarctic, subalpine environment to improve understanding of runoff generation processes and the mechanisms whereby water moves from hillslopes to the stream. Runoff characteristics of four hillslopes were examined between 1997 and 1999, each with distinct soils, frost, topography and vegetation. Lateral fluxes were confined to hillslopes with porous organic soils overlying less permeable mineral substrates, setting up a two-layer flow system whereby most drainage occurs as quickflow in the porous organic layer as matrix flow and/or as preferential flow in pipes, rills and interconnected surface depressions. During snowmelt, meltwater infiltrated and percolated the porous frozen organic layers with little resistance. Percolation ceased at the organic/mineral interface due to the impermeable nature of frozen mineral soils, forming a perched saturated zone and initiating runoff. Snowmelt runoff was greater on slopes with greater snow water equivalent and reduced organic layer thickness. In summer, runoff was greatest on slopes where wet conditions were sustained by inflow. Stormflow hydrographs responded rapidly to rainfall while exhibiting extended recessions compared with temperate regions. Where sustained inflow occurred, the recession limb showed two segments to reflect different source areas of stormflow production. Recession analysis was used to quantify contributing areas, which were highly variable and controlled largely by hillslope wetness and organic-layer properties. Results indicate that the concept of variable source area for runoff generation applies to subarctic, subalpine catchments.

The role of permafrost and seasonal frost in the hydrology of northern wetlands in North America

Abstract Wetlands are a common landscape feature in the Arctic, Subarctic, and north Temperate zones of North America. In all three-zones, the occurrnce of seasonal frost results in similar surface-water processes in the early spring. For example, surface ice and snow generally melt before the soil frost thaws, causing melt water to flow into depressions, over the land surface and at times, across low topographic divides. However, evapotranspiration and ground-water movement differ among the three climatic zones because they are more affected by permafrost than seasonal frost. The water source for plants in the Arctic is restricted to the small volume of subsurface water lying above the permafrost. Although this is also true in the Subarctic where permafrost exists, where it does not, plants may receive and possibly reflect, more regional ground-water sources. Where permafrost exists, the interaction of wetlands with subsurface water is largely restricted to shallow local flow systems. But where permafrost is absent in parts of the Subarctic and all of the Temperature zone, wetlands may have a complex interaction with ground-water-flow systems of all magnitudes.

Comparison of weather station snowfall with winter snow accumulation in high arctic basins

Abstract Most water balance studies in the High Arctic indicate that the weather stations underestimate annual precipitation, but the magnitude of such error is unknown. Based on up to seven years of field measurements, this study provides a comparison of snowfall at weather stations with the winter snow accumulation in their nearby drainage basins. Snowfall is the major form of precipitation in the polar region for nine months every year. Without vegetation, snowdrift is controlled by the local terrain. By establishing the snow characteristics for different terrain types, total basin snow storage can be obtained by areally weighting the snow cover for various terrain units in the basin. Such a method was successfully employed to compute total winter snowfall in the drainage basins near Resolute, Eureka and Mould Bay. Results show that the basins had 130 to 300per cent more snow than the weather stations recorded. Using revised snowfall values that are reinforced by Koerners snow core measurements from i...

Changes in rainfall characteristics in northern Nigeria

This study examines recent changes in several rainfall characteristics in northern Nigeria. The records at 25 locations were analyzed for the occurrence of abrupt changes and trends using the Pettitt and the Mann-Kendall tests. Variables analyzed included annual total rainfall and number of rain days, the dates of onset, termination and duration of the rainy season as well as monthly rainfall, monthly number of rain days and various categories of rainfall above certain intensities. An abrupt change occurred in the time series of annual rainfall, number of rain days and affected areas north of latitude 11° N. However, the sub-periods prior to and after the change points may be considered to be homogenous. The series of variables related to the duration of the rainy season exhibit no significant trends or jumps. It is concluded that recent changes in rainfall over the Sahel were driven by a reduction in the frequency of rain days of high rainfall intensities during the months of August and September. The fact that the high intensity rainfall does not contribute significantly to crop growth may explain the continuation of agricultural activities in the Sahel despite massive reductions in annual rainfall.

The role of soil pipes as a slope runoff mechanism, Subarctic Yukon, Canada

Pipeflow in subarctic slopes provides a preferential runoff mechanism that bypasses the soil matrix, rapidly conveying water to the stream. Extensive soil piping occurs in the central Wolf Creek basin, Yukon, at the interface of the organic and mineral horizons. Flow in these pipes are ephemeral, transmitting water only when the water table is within or above the zone where pipes occur. During snowmelt, pipeflow began several days after the onset of surface runoff. Pipeflow contribution increased until ground thaw lowered the water tables, leaving matrix flow within the organic layer as the dominant mode of runoff. Pipeflow accounted for 21% of runoff during the 15 day melt period of 1997. Following melt, pipeflow recurred only during two intense summer rainstorms, each yielding different pipeflow response characteristics. During melt, pipeflow closely followed the daily snowmelt cycles and responded earlier than the integrated slope runoff. In the summer, pipeflow hydrographs rose and fell much quicker than direct storm runoff from the entire slope, which was dominated by fast matrix flow within the organic layer. Pipeflow contributing areas were relatively small, but their extent changed with hillslope wetness. Analysis revealed that the Manning flow formula can be combined with contributing areas to simulate pipeflow discharges. Unlike temperate environments where frozen ground is not a factor, the frost table position significantly controls the position of the phreatic surface, and must be considered in any models of pipeflow in permafrost slopes.

Hydrology of a wetland in the continuous permafrost region

Abstract This study examines the hydrological system of a northern wetland in the continuous permafrost region. Water balance computation quantifies the relative importance of various hydrological processes including snowmelt, rainfall, inflows, evaporation, surface and subsurface outflows. The existence of this wetland is closely related to a lake upslope which provides the bulk of the water input. The ability of northern wetlands to absorb water input is limited by the frozen ground and the high specific retention of the peat, rendering the wetland a poor regulator of streamflow. Storage capacity increases in summer as evaporation reduces the moisture content of the peat. Subsurface flow remains insignificant because of the low hydraulic gradient and conductivity. Thus, when the water table lies within the peat layer, there is little wetland contribution to streamflow but when the water table lies above the ground during some intense storms, streamflow is rapidly increased by surface flow on the wetlands.

Multiple Effects of Changes in Arctic Snow Cover

Snow cover plays a major role in the climate, hydrological and ecological systems of the Arctic and other regions through its influence on the surface energy balance (e.g. reflectivity), water balance (e.g. water storage and release), thermal regimes (e.g. insulation), vegetation and trace gas fluxes. Feedbacks to the climate system have global consequences. The livelihoods and well-being of Arctic residents and many services for the wider population depend on snow conditions so changes have important consequences. Already, changing snow conditions, particularly reduced summer soil moisture, winter thaw events and rain-on-snow conditions have negatively affected commercial forestry, reindeer herding, some wild animal populations and vegetation. Reductions in snow cover are also adversely impacting indigenous peoples’ access to traditional foods with negative impacts on human health and well-being. However, there are likely to be some benefits from a changing Arctic snow regime such as more even run-off from melting snow that favours hydropower operations.

Impacts of peat and vegetation on permafrost degradation under climate warming

[1] Simulations of maximum annual thaw at a continuous and a discontinuous permafrost site in Canada were performed using Community Land Model version 3 (CLM3) and randomized historical climate records from these sites, superimposed with United Nations Intergovernmental Panel on Climate Change (IPCC), Report on Emission Scenarios (SRES) A2 scenario of climate change. A positive trend in permafrost degradation was simulated for the 2000 to 2100 period in response to climate warming. Surface cover condition and soil properties play a dominant role in affecting ground thaw. In particular, a thin peat layer or surface organic cover can significantly buffer the permafrost against severe degradation. The occurrence of vegetation and extensive presence of a peat and organic layer in the circumpolar areas will modulate the regional impact of climate warming on permafrost thaw.