A WATER-BALANCE MODEL FOR A SUB-ARCTIC SEDGE FEN AND ITS APPLICATION TO CLIMATIC-CHANGE

A model to calculate the water balance of a hummocky sedge fen in the northern Hudson Bay Lowland is presented. The model develops the poten tial latent heat flux (evaporation) as a function of net radiation and atmospheric temperature. It is about equally sensitive to a 2% change in net radiation and a 1 degrees C change in temperature. The modelle d potential evaporation agrees well with the Priestley-Taylor formulat ion of evaporation under conditions of a non-limiting water supply. Th e actual evaporative heat flux is modelled by expressing actual/potent ial evaporation as a function of potential accumulated water deficit. Model evaporation agrees well with energy balance calculations using 7 years of measured data including wet and dry extremes. Water deficit is defined as the depth of water below reservoir capacity. Modelled wa ter table changes concur with measurements taken over a 4 year period. When net radiation, temperature and precipitation measurements are av ailable the water balance can be projected to longer time periods. Ove r a 30 year interval (1965-1994) the water balance of the sedge fen sh owed the following. During the growing season, there was an increase i n precipitation, no change in temperature and a decrease in net radiat ion, evapotranspiration and water deficit. There was also a decrease i n winter snow depths. The fen was brought back to reservoir capacity d uring final snowmelt every year but one. Summer rainfall was the most important single factor affecting the water balance and the ratio actu al/potential evaporation emerged as a linear function of rainfall amou nt. A 2 x CO2 climate warming scenario with an annual temperature incr ease of 4 degrees C and no precipitation change indicates lesser snow amounts and a shorter snow cover period. A greater summer water defici t, triggered mainly by greater evaporation during the month of May, is partially alleviated by lesser evaporation magnitudes in July. The gr eater water deficit would be counterbalanced by a 23% increase in summ er rainfall. On average, the fen's water reservoir would still be rech arged after winter snowmelt but the ground would remain at reservoir c apacity for a shorter time. The warming scenario with a 10% decline in summer rainfall would create a large increase in the longevity and se verity of the water deficit and this would be particularly evident dur ing drier years. The carbon budget and peat accumulation and breakdown rates are strongly affected by changes in the water balance. Some evi dence implies that greater water deficits lead to an increase in net c arbon emissions. This implies that the sedge peatland could lose bioma ss under such conditions. An example is given where increased water de ficit results in large decreases in local wetland streamflow.