SCALING XYLEM SAP FLUX AND SOIL-WATER BALANCE AND CALCULATING VARIANCE - A METHOD FOR PARTITIONING WATER FLUX IN FORESTS

To partition evapotranspiration between canopy and subcanopy component s in a 12-m-tall Pinus taeda forest and to assess certain aspects of e nvironmental regulation of canopy transpiration, we quantified water f lux in a forest using three approaches. 1) measuring water flux in xyl em of trees, and scaling to stand transpiration of canopy trues (E-C); 2) measuring soil water content and saturated conductivity, and model ing drainage to estimate total evapotranspiration (E-T) during rainles s days based on a local water balance (LWB); and 3) using an eddy corr elation approach to estimate total E-T. We calculated variances for ea ch estimate, and proposed an approach to test for differences between estimates of E-C and E-T. Diurnal 'patterns' in water uptake were simi lar using direct measurements in stem xylem and LWB. However, LWB was found to be inappropriate for estimating 'absolute' E-T diurnally when changes in soil moisture between consecutive measurements were small. Eddy correlation estimates of E-T are of a higher temporal resolution than xylem flux measurements made in branches. Diurnal flux patterns in branches are more similar to the pattern generated by eddy correlat ion than those in stems. However, differences between the patterns ind icate that patchiness in branch transpiration may preclude using branc h xylem flux measurements to estimate canopy conductance. In one stand , daily E-C accounted for ca 70 % of total E-T estimated by either LWB tin a separate study) or the eddy correlation approach; the differenc e between E-T and E-C was significant based on variances calculated to account for spatial variation in each. Regardless of the vapor pressu re deficit, E-C decreased linearly with soil moisture from 2.5 to 1.5 mm d(-1) over a 9-d drying cycle, as soil moisture in the rooting zone (ca 0.35 m depth) declined by 23 mm. ((C) Inra/Elsevier, Paris.).