Infiltration depth, rooting depth, and regolith flushing—A global perspective

In the vegetation root zone, infiltration (<i>Inf</i>) parts in two directions with distinct Earth-system functions. One goes up as evapotranspiration (<i>E</i> + <i>Tr</i>), returning <i>Inf</i> to the atmosphere (short-circuiting) and affecting short-term weather/climate and the carbon cycle. The other goes down as deep drainage (<i>DD</i>), flushing the regolith, mobilizing nutrients/contaminates and dissolved minerals into aquifers and rivers, eventually reaching the ocean (long-circuiting) thus regulating global biogeochemical cycles and long-term climate. We ask, what is the modern-day global structure in short- vs. long-circuiting? What forces and feedbacks create such structures? Synthesizing site-studies aided by global modeling, we found that: (i) long-circuiting prevails in evenly wet climates, in well-drained landscapes with a deep vadose zone, in substrates with deep conduits, and with plant biomass below natural equilibrium; (ii) soil B-horizons, via geochemical and vegetation feedbacks, enhance short-circuiting, while deep rock fractures enable long-circuiting even in dry climates; (iii) in dry climate/season and in uplands, plant roots follow <i>Inf</i> into deep vadose zone to tap wet-season <i>Inf</i>; (iv) plant water-use reinforces shallow <i>Inf</i>, reducing <i>DD</i> and regolith flushing in dry and season-dry climates; (v) where short-circuiting prevails, a dry soil zone separates modern surface processes from fossil groundwater; and (vi) the <i>E</i> + <i>Tr</i> supply depth, regolith flushing rate, and groundwater residence time vary greatly across the land, arising from multiscale drivers/feedbacks among climate, drainage, substrate, and biomass. These findings link site-based process discoveries to Earth-system level structures and functions of water belowground, shedding light on where/when/how the infiltrated rain influences the atmosphere above or the ocean downstream.