Variability of throughfall volume and solute inputs in wooded ecosystems

Throughfall is a critical component of the hydrological and biogeochemical cycles of wooded ecosystems with a characteristically large degree of temporal and spatial variability. The highly variable nature of throughfall is of importance to scientists and natural resource managers concerned with the effects of water and solute inputs to the subcanopy, including understory vegetation, soil moisture, soil solution chemistry, and the fate of atmospheric dryfall. The purpose of this study is to critically review and evaluate the present state of knowledge pertaining to the temporal and spatial variability of throughfall volume and solute inputs in wooded ecosystems. The authors are optimistic that this review will facilitate the advancement of science by exposing gaps in our current understanding and mitigating the duplication of unwarranted research efforts. Several key areas where current knowledge is weak are: (1) the effect of meteorological conditions on the variability of throughfall volume; a data gap exists concerning the effects of precipitation type (eg, rain, snow, snow-to-rain, rain-to-snow), incident rain drop size, intensity, duration, wind speed and direction, and wind run on the throughfall variability; (2) the effect of meteorological conditions on the variability of throughfall solute inputs; (3) the role of canopy structure on precipitation partitioning into throughfall and stemflow and the variability of throughfall volume and solute inputs; (4) effects of epiphytes on the spatial variation of throughfall volume and solute inputs; (5) the physics and fluid dynamics of water flow over vegetative surfaces and its impact on throughfall yield and chemical enrichment; and (6) intraspecific variation of throughfall water and solute inputs. Future research projects undertaken with the specific aim of addressing the deficiencies identified will improve our understanding of interactions among the biosphere-atmosphere-lithosphere and promote better stewardship of forest and water resources.