Spatial Variability and Temporal Stability of Local Net Precipitation Patterns

Redistribution of precipitation water by plant canopies increases the spatial variation of net precipitation at the surface, affecting soil moisture patterns, localized preferential flow, and soil biogeochemical processes. This chapter reviews methods for assessing and the current state of knowledge on spatial patterns of the two net precipitation components: throughfall and stemflow. Spatial variation in throughfall is caused by canopy morphology, including creation of rain shadows due to canopy topography and dripping points. Stand scale throughfall is less than above-canopy precipitation (in the absence of fog), however, localized throughfall receipt at the surface ranges widely—from negligible beneath dense canopy areas to 10 times greater than gross precipitation. Coefficients of variation of throughfall (CVT) vary with canopy complexity, event size, and averaging period. In extreme cases, CVT > 1 for single events in regions with complex canopies have been observed, but they decrease to 2,500 times. Few studies select trees for stemflow measurement in a randomized fashion and in sufficient number; therefore, spatial CVS between individuals are typically not reported. In the studies available CVT > 1 are common and CVs of stemflow are typically much larger than those of throughfall. Differences between neighboring individuals are substantial, with tree species, tree size (e.g., canopy and stem size, number of branches) and crown architecture being reported as the most important drivers for observed variations. Both throughfall and stemflow contribute to comparatively stable net precipitation patterns over time, potentially much more so in stemflow compared to throughfall. For spatiotemporal patterns of solutes in net precipitation, little is known, and nothing is known about fine-scale patterns in particulates. Data collected to date are near-exclusively measured under overstory trees during rainfall, leaving the precipitation redistribution patterns that actually reach the surface beneath understory plants essentially unknown.