OVERESTIMATION OF SNOW DEPTH AND INORGANIC NITROGEN WETFALL USING NADP DATA, NIWOT RIDGE, COLORADO

We evaluated precipitation quantity and inorganic N deposition from we tfall at the Saddle site on Niwot Ridge, an alpine location in the Col orado Front Range of the Rocky Mountains. The chemical content of prec ipitation was collected with an Aerometrics wet-only deposition collec tor and precipitation amount was collected in a co-located Fergusson-t ype weighing gauge with a wind shield. Calculating the actual depositi on of snow at a point in alpine areas is difficult because wind transp ort can cause under sampling or over sampling of the actual precipitat ion amount. We used a moisture sensor on the wet chemistry collector t o account for blowing snow events, categorizing snow and rain collecte d in the Saddle precipitation gauge as precipitation events when the m oisture sensor was ''on'' and blowing snow and rain events when the mo isture sensor was ''off''. Over a ten-year period, 61% of the winter p recipitation was collected when the moisture sensor was ''off''. Perha ps fortuitously, annual solid precipitation (October-May) of 1514 mm m easured at the Saddle was 61% more than the 938 mm measured at an alpi ne site 2 km away and accounted for the difference in annual precipita tion amount at the two sites. Annual inorganic N deposition at the Sad dle site was then calculated as the measured summer deposition amount added to the measured winter deposition adjusted for over sampling fro m blowing snow events. These correction factors resulted in a 32% redu ction of annual inorganic N in wetfall for the period 1986-1995, from an arithmetic mean of 4.7 to 3.2 kg ha(-1) yr(-1). (C) 1998 Elsevier S cience Ltd. All rights reserved.