Partitioning of evapotranspiration through oxygen isotopic measurements of water pools and fluxes in a temperate grassland

Abstract

Stable isotopic measurements of water provide a promising tool for partitioning of ecosystem evapotranspiration (ET). This approach, however, is still facing some challenges due to the uncertainties in estimating the isotopic compositions of ET and its components. In this study, a tunable diode laser analyzer was deployed for in situ measurements of the oxygen isotopic compositions of water vapor. Using these measurements together with samples of water in plant and soil pools, we partitioned ET via estimating the oxygen isotopic compositions of ET (δET) and that of its two components, i.e., plant transpiration (δT) and soil water evaporation (δE). A new δT model was developed in this study, which illustrated consistent estimations with the traditional model. Most of the variables and parameters in the new model can be measured directly with high accuracy, making its potential to be used at other sites high. Our results indicate that the ratio of plant transpiration to evapotranspiration (T/ET) illustrates a "U" shape diurnal pattern. Mean T/ET at 0630-1830 during the sampling days was 83%. Soil depth of 15 cm is a reasonable depth for soil water sampling for estimating δE at this site. We also investigated the uncertainties in estimating these three terms and their effects on partitioning. Overall, in terms of partitioning, the uncertainties are relatively small from δT and δE but quite large from δET. Quantifying and improving the precision of δET should be a priority in future endeavors of ET partitioning via the stable isotopic approach. ©2014. American Geophysical Union. All Rights Reserved.