The Water Balance of a Subarctic Town

SR98_10
Page Navigation
102

High-latitude towns experience unique water management problems related to the storage of pre-

cipitation as snow for upwards of five months each winter. By providing an overview of pathways

and the relative importance of different hydrological parameters, a water balance is valuable to the

design and operation of urban water management systems. Urban water balances differ from their

rural counterparts, specifically due to the extreme spatial heterogeneity and artificial sources and

flow paths. Urban catchments are generally characterized by high peak discharges and fast re-

sponse times. Snow further complicates urban water balances. For instance, snow accumulation

and melt is influenced not only by topography but also by the presence of buildings and snow

handling measures.

The monthly water balance of Lulea, Sweden, is investigated. Half the annual precipitation is snow;

thaw is usually at the end of April. The study period was June 1992June 1996. Of interest were the

seasonal differences in runoff volumes, flow pathways, and flow through the urban pipe system to

the wastewater treatment plant and natural receiving waters. Data available included daily precipi-

tation, air temperature, and inflow to the Uddebo wastewater treatment plant, and monthly potential

evapotranspiration, groundwater levels, and long-term water supply statistics.

The snowpack was simulated with a degree-day temperature index. This method is unsuitable for

urban hydrology as it represents spatial and temporal scales that are discordant to the processes in

operation; however, as the results are reported as monthly values, inaccuracy is assumed to be

minimal. Snowmelt-induced runoff from urban areas is largely new water. During thaw, urban soils

rapidly become saturated and can freeze, leading to overland flow and increased storm- and waste-

water flow. Soil was modeled with a single-layer bucket model. Water can enter pipes either

directly through inlets or indirectly as infiltration. Sewer infiltration was approximated by remov-

ing the supply and direct stormwater component from the measured inflow at Uddebo. This method

does not account for water lost to CSO or pump station overflows; however, observations show that

most overflows occur in April.

It was found that the volume of discharge is greatest during spring thaw, but autumn rains coupled

with lowered evapotranspiration cause a secondary flow peak; rainwater on permeable surfaces

infiltrates whereas the sheer quantity of water during thaw causes inundation and increased storm-

water; the urban drainage system is most likely to overflow during thaw and the weeks after.

Recharge is most likely to occur after melt, but ground frost can limit flow some years. There is a

bi-modal pattern with a second peak in autumn.

Department of Water Resources Engineering, Lund University, Box 118, 22100 Lund, Sweden