Analysis of difference between direct and geodetic mass balance measurements at South Cascade Glacier, Washington

Net mass balance has been measured since 1958 at South Cascade Glacier usin g the 'direct method,' e.g. area averages of snow gain and firn and ice los s at stakes. Analysis of cartographic vertical photography has allowed meas urement of mass balance using the 'geodetic method' in 1970, 1975, 1977, 19 79-80, and 1985-97. Water equivalent change as measured by these nearly ind ependent methods should give similar results. During 1970-97, the direct me thod shows a cumulative balance of about -15 m, and the geodetic method sho ws a cumulative balance of about -22 m. The deviation between the two metho ds is fairly consistent, suggesting no gross errors in either, bur rather a cut mulative systematic error. It is suspected that the cumulative error i s in the direct method because the geodetic method is based on a non-changi ng reference, the bedrock control, whereas the direct method is measured wi th reference to only the previous year's summer surface. Possible sources o f mass loss that are missing from the direct method are basal melt, interna l melt, and ablation on crevasse walls. Possible systematic measurement err ors include under-estimation of the density of lost material, sinking stake s, or poorly represented areas.