Overestimation of snow depth and inorganic nitrogen wetfall using NADP data, Niwot Ridge, Colorado

We evaluated precipitation quantity and inorganic N deposition from wetfall at the Saddle site on Niwot Ridge, an alpine location in the Colorado Front Range of the Rocky Mountains. The chemical content of precipitation was collected with an Aerometrics wet-only deposition collector and precipitation amount was collected in a co-located Fergusson-type weighing gauge with a wind shield. Calculating the actual deposition of snow at a point in alpine areas is difficult because wind transport can cause under sampling or over sampling of the actual precipitation amount. We used a moisture sensor on the wet chemistry collector to account for blowing snow events, categorizing snow and rain collected in the Saddle precipitation gauge as precipitation events when the moisture sensor was “on” and blowing snow and rain events when the moisture sensor was “off”. Over a ten-year period, 61% of the winter precipitation was collected when the moisture sensor was “off”. Perhaps fortuitously, annual solid precipitation (October–May) of 1514 mm measured at the Saddle was 61% more than the 938 mm measured at an alpine site 2 km away and accounted for the difference in annual precipitation amount at the two sites. Annual inorganic N deposition at the Saddle site was then calculated as the measured summer deposition amount added to the measured winter deposition adjusted for over sampling from blowing snow events. These correction factors resulted in a 32% reduction 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.