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This paper reviews and summarizes all known previous water balance studies in Svalbard. An updated water balance computation was then done for the three water catchments with the best data: Bayelva, De Geerdalen and Isdammen/Endalen for 10 hydrological years 1990‐2001. The computations were based on the best available data and correction methods. Special emphasis was put on correction of precipitation data, both for catch errors and gradients in precipitation. Areal precipitation in the three catchments is more than two times the measured precipitation at the closest meteorological station: 548 mm/year in De Geerdalen, 486 mm/year in Endalen/Isdammen and 890 mm/year in Bayelva. Compared to this, average measured precipitation is only 199 mm/year at Svalbard Airport, close to Endalen/Isdammen and De Geerdalen, and 426 mm/year in Ny‐Ålesund, close to Bayelva. Evaporation is not well understood in Svalbard; the best estimates indicate an average annual evaporation of ca. 80 mm/year from glacier‐free areas, and no net evaporation from glaciers. Glacial mass balance has in general been negative in Svalbard during the last 40 years, leading to a significant contribution to the water balance, on the order of 450 mm/year on average. Annual runoff ranges from 545 mm in Endalen/Isdammen, 539 mm/year in De Geerdalen up to 1050 mm/year in Bayelva. Runoff computed from water balance compares well with observed runoff, and average error in water balance is less than ±30 mm/year in all three catchments.

Chemical denudation rates and carbon dioxide withdrawal were calculated for the Londonelva basin, an ice-free karst catchment on Svalbard. Runoff was recorded 12 June to 11 October 2000 and water samples were collected daily in August. The chemical denudation rate that month amounted to 2.2 m 3 km À2 . The estimated chemical denudation rate for 2000 was 5.8 m 3 km À2 yr À1 or 303 S meq þ m À2 yr À1 , but this rate may be unusually low because basin runoff was only 158 Â 10 3 m 3 , the lowest since recording began in 1992. For other discharge years estimated chemical denudation rates range from 5.8 to 13.3 m 3 km À2 yr À1 or 300-690 S meq þ m À2 yr À1 . The CO 2 withdrawal in August 2000 amounted to 560 kg C km À2 and for the hydrological year 2000 it was 1560 kg C km À2 yr À1 . In 1993, the highest recorded runoff year, withdrawal of CO 2 may have reached 3800 kg C km À2 yr À1 . Figure 2 Mean daily discharges (Q) in Londonelva in the period of runoff recording (12 June to 11 October 2000) with mean daily air temperature (t a ) and precipitation (P) at Ny-Å lesund in the background.Figure 5 Transient carbon dioxide drawdown (TCD) in relation to crustal solute fluxes (CSF) in glacierized Bayelva basin (filled triangles) and ice-free Londonelva catchment (circles).

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