A Reassessment of Areal Variability of Throughfall Deposition Measurements

Throughfall deposition in a low-elevation red spruce-balsam fir stand in east-central Maine was measured for 10 chemical constituents over four sampling seasons to evaluate areal variability of throughfall deposition measurements. Forty collectors within one stand were sampled weekly and combined to provide 40 monthly samples. Areal variation in measurements of throughfall deposition differed greatly among chemical constituents and from month to month. Sulfate showed the least areal variability of the chemical constituents; K⁺ showed the most. Water volume measurements consistently showed less areal variability than any of the chemical constituents. Mean SO₄^2- deposition calculated from 15 collectors was within 20% of the mean value calculated from 40 collectors (with 90% confidence) in 22 out of 23 sampling months; mean K⁺ deposition calculated from 28 collectors was within 20% of the mean of the 40 collectors (with 90% confidence) in 21 out of 23 mo. Chemical deposition showed less areal variability when calculated on a seasonal basis than when calculated on a monthly basis. Mean seasonal SO₄^2- deposition calculated from 15 collectors was within 12.1% of the mean 40 collectors (90% confidence); mean K⁺ deposition calculated from 28 collectors was within 21% of the mean of 40 collectors (90% confidence) for the same three seasons. Contrary to the conclusions of previous studies, excessively large numbers of collectors are not needed to obtain the accuracy and precision adequate for most forest deposition studies. This is significant in light of the growing dependence on throughfall measurements for estimating atmospheric deposition.