WATER-BALANCE DELINEATES THE SOIL LAYER IN WHICH MOISTURE AFFECTS CANOPY CONDUCTANCE

To link variation in canopy conductance to soil moisture in the rootin g zone, measurements of throughfall (P-T), volumetric soil moisture (t heta) 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 t aeda. Total evapotranspiration (E-T) was estimated from P-T, changes i n volumetric soil water content within a defined soil volume (BS), and drainage out of that volume (Q), calculated from unsaturated soil hyd raulic conductivity and theta. Our calculations suggest that over 145 growing-season days, Q was negligible, and most of P-T, averaging simi lar to 2 mm/d, was partitioned between soil moisture recharge (0.4 mm/ d) and E-T (1.6 mm/d, not including similar to 0.4 mm/d of interceptio n, I, by canopy trees), of which E-C was estimated from direct measure ments at nearly 1.2 mm/d. Evapotranspiration by the subcanopy componen t 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 fr om 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 t o a in the upper soil layer once theta decreased below 0.22.