Separating physical and biological controls on evapotranspiration fluctuations in a teak plantation subjected to monsoonal rainfall

Abstract:

Evapotranspiration (ET), especially in the mainland of the Indochina peninsula, can impact and is impacted by the Asian monsoonal (AM) system, thereby prompting interest in its long-term variability. To separate physical and biological factors controlling ET variability in a tropical deciduous forest under the AM influence, 7-year eddy-covariance and ancillary measurements were collected and analyzed. The 7-year mean rainfall (P_r) and ET along with their standard deviations were 1335 ± 256 and 1027 ± 77.9 mm (about 77% of P_r), respectively, suggesting close coupling between these two hydrologic fluxes. However, other physical and biological drivers de-couple seasonal and annual variations of ET from P_r. To explore them, a big-leaf model complemented by perturbation analysis was employed. The big-leaf model agreed well with measured ET at daily to multi-year time scales, lending confidence in its skill to separate biological from physical controls on ET. Using this formulation, both first-order and second-order Taylor series expansion of total ET derivatives were applied to the big-leaf model and compare with measured changes in ET (dET). Higher-order and joint-terms in the second-order expansion were necessary for matching measured and analyzed dET. Vapor pressure deficit (D) was the primary external physical controlling driver on ET. The leaf area index (LAI) and bulk stomatal conductance (g_s) were shown to be the main significant biological drivers on the transpiration component of ET. It can be surmised that rainfall variability controls long-term ET through physical (mainly D) and biological (mainly LAI and g_s) in this ecosystem.