Water balance delineates the soil layer in which moisture affects canopy conductance

To link variation in canopy conductance to soil moisture in the rooting zone, measurements of throughfall (P(T)), volumetric soil moisture (θ) to 0.7 m, transpiration from trees >10 mm in diameter (E(C)), and vapor pressure deficit (D) were made in a forest dominated by Pinus taeda. Total evapotranspiration (E(T)) was estimated from P(T), changes in volumetric soil water content within a defined soil volume (ΔS), and drainage out of that volume (Q), calculated from unsaturated soil hydraulic conductivity and θ. Our calculations suggest that over 145 growing-season days, Q was negligible, and most of P(T), averaging ~2 mm/d, was partitioned between soil moisture recharge (0.4 mm/d) and E(T) (1.6 mm/d, not including ~0.4 mm/d of interception, I, by canopy trees), of which E(C) was estimated from direct measurements at nearly 1.2 mm/d. Evapotranspiration by the subcanopy component accounted for slightly over 0.4 mm/d, about a third of E(T) (a fourth if I is included). Most of the water used for E(T) (>90%) was taken from the upper soil layer (top 0.35 m, or less). Canopy leaf conductance, calculated from E(C), D, and canopy leaf area, was strongly related to θ in the upper soil layer once θ decreased below 0.22.

Duke Scholars

Author Gabriel G. Katul Civil and Environmental Engineering