Recent studies reveal that water moves through snowpack in other such preferential pathways, or macropores. These macropores can be present in both the horizontal and vertical. Horizontal pathways tend to form when impermeable ice lenses force the water to either pond or to move laterally until it finds another vertical pathway to follow to the bottom of the snowpack.
If the ground is unfrozen and unsaturated, it should be able to absorb the water quite easily as long as the snowmelt rate is less than the infiltration rate of the soil. In this case, the snowmelt would behave a lot like rainfall.
When water trickles down through a snowpack and reaches the ground below, its fate is determined by the conditions of the ground and the grounds surface. These are important considerations when forecasting runoff and possible flooding.
If the water trickling down through the snowpack encounters frozen soil, there is a possibility that water will pond and possibly refreeze at the soil surface, further impeding any infiltration of water into the soil in that region. In rapid snowmelt situations, this can cause significant flooding.
Permafrost is earth material that has temperature at or below 0◦C for at least two consecutive summers. Above the permafrost is the active layer, a zone that freezes in winter and thaws in summer. Ice-rich permafrost prevents infiltration of rainfall or snowmelt water, often maintaining a moist to saturated active layer where the permafrost table is shallow. Most hydrologic activities are confined above ground or in the thin active layer, which supplies summer moisture to plants and for evaporative flux. Limited storage capacity of the thawed zone does not support extended baseflow in a stream, though the proportion of baseflow increases as the percentage of permafrost extent decreases.
Permafrost regions have long winters associated with lengthy periods of darkness at higher latitudes, and extended stretches of daylight during summer. With little or no solar radiation
and considerable radiative cooling during the long winters, the net radiation becomes negative. The air is much colder than the ground or its overlying snow, producing a strong negative ground heat flux, and the freezing front descends into the active layer. The frost-table, which represents the lower limit of the seasonally thawed zone, also rises as freeze-back continues.
The next process is water movement to the freezing front. The process of water migration induced by the thermal gradient at the front. Soil moisture redistributes with a greater concentration near the surface.
Translocated water freezes and form lenses of ice, which are impervious to meltwater percolation in spring
As spring starts (during at the end of April), snow melts. The cold Arctic snow usually produces flow fingers that are confined zones of concentrated flow that move ahead of the general front of meltwater advance. Meltwater delivered to the base of the snow by the flow fingers can penetrate the frozen soil, which is made pervious by dilation and dessication cracks and by the voids left by the partially melted ice lenses.
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infiltration rate is inversely proportional to the moisture content, particularly the ice content near the surface. Hydraulic conductivity of frozen soil is two orders of magnitude less than that of the same soil in unfrozen condition.. In continuous permafrost areas, the considerable supply of meltwater in spring, coupled with a shallow, thawed zone to hold this water, invariably leads to extensive surface ponding and runoff.
The thawing of the active layer is governed by the ground heat flux. Runoff can occur as surface flow, groundwater flow and interflow.
Surface flow
In spring when the thawed zone in the active layer is shallow, most of the slope runoff occurs as flow through snow or as surface discharge. As water is discharged below a snow patch, surface flow may or may not be seen depending on the position of the water-table in the active layer. When the watertable rises above the ground surface, both overland flow and flow channelled in rills are generated. This is particularly prominent in spring because a frozen substrate close to the ground surface hinders percolation at a time when water supply is abundant. Bafore runoff is generated, however, depression storage has to be satisfied. The demand of depression storage depends on the microtopography of the slope. So, during the wet period, a rise of the water-table above the slope in low-lying areas causes surface runoff. The surface flow is variable in time and in areal extent, depending on the amount of water supply and the proximity of the prost-table to the surface. There, the bulk of total annual runoff is discharged in spring when the slopes are at least partially snow covered. Expressed as a ratio of the available snow water equivalent, surface runoff in the Arctic and the subarctic environment varies from a negligible quantity to over 70%.
On different segments of a slope, surface and subsurface flows can occur together or they can alternate, depending on whether water emerges or submerges in the active layer.
Subsurface water can be distinguished as subpetmafrost, intrapermafrost and suprapermafrost groundwater, depending on whether the water is found beneath, within, or above the permafrost. In continuous permafrost areas, suprapermafrost groundwater provides the most common source for subsurface flow. Total grounflow is seldom more than 10% of annual runoff.
Subpermafrost groundwater may be considered to be relict in origin except where there are unfrozen zones along which meteoric water can penetrate to the base of the permafrost. This source of groundwater occurs in surficial deposits or in bedrock and artesian flow is quite common. In the discontinuous permafrost zone, it emerges in springs or seeps through river and lake beds.
Intrapermafrost groundwater is found in perennially unfrozen zones (called talik) within the permafrost. Taliks are sometimes maintained by thermal springs, but more often they are kept open by the flow of warm groundwater
Suprapermafrost groundwater is meteoric in origin and, if not drained before winter, may be stored as ground-ice in the active layer.
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