FIELD VALIDATION OF ISOTOPIC PROCEDURES FOR DETERMINING SOURCES OF WATER USED BY PLANTS IN A SEMIARID ENVIRONMENT

Methods for using stable isotopes of water (H-2, O-18) for determining the sources of water transpired by plants in a semi-arid field situat ion are validated. A comparison of the isotopic composition of the soi l water in zones of high soil water potential (and hence high plant wa ter availability) with that in plants shows that the overall approach is subject to a randomly distributed error of 5% in H-2 isotopic compo sition with no indication of any bias. This includes errors involved i n the extraction of water from plants and soil, in spatial heterogenei ty in soil profiles, in heterogeneity within plant canopies, in the ti me of travel of water through the plant, and in assumptions about frac tionation of isotopes during water uptake by plants. These errors are generally smaller than natural variations of isotope, thus allowing th e isotopic method to be used. Similarly, an estimate for the error in O-18 is 1%. A compartmental model was used to quantify errors associat ed with an inversion model using isotopic data of water coming from tw o soil layers. The error in the estimation of the fraction of water ta ken from each compartment was 20%. For many hydrological purposes, thi s error is acceptable. This is the first study estimating the errors a ssociated with the method.