Isotopic Composition Of Soil Water Research Articles

Partitioning of soil water among canopy trees in a seasonally dry tropical forest.

Little is known about partitioning of soil water resources in species-rich, seasonally dry tropical forests. We assessed spatial and temporal patterns of soil water utilization in several canopy tree species on Barro Colorado Island, Panama, during the 1997 dry season. Stable hydrogen isotope composition (δD) of xylem and soil water, soil volumetric water content (θv), and sap flow were measured concurrently. Evaporative fractionation near the soil surface caused soil water δD to decrease from about -15‰ at 0.1 m to -50 to -55‰ at 1.2 m depth. Groundwater sampled at the sources of nearby springs during this period yielded an average δD value of -60‰. θv increased sharply and nearly linearly with depth to 0.7 m, then increased more slowly between 0.7 and 1.05 m. Based on xylem δD values, water uptake in some individual plants appeared to be restricted largely to the upper 20 cm of the soil profile where θv dropped below 20% during the dry season. In contrast, other individuals appeared to have access to water at depths greater than 1 m where θv remained above 45% throughout the dry season. The depths of water sources for trees with intermediate xylem δD values were less certain because variation in soil water δD between 20 and 70 cm was relatively small. Xylem water δD was also strongly dependent on tree size (diameter at breast height), with smaller trees appearing to preferentially tap deeper sources of soil water than larger trees. This relationship appeared to be species independent. Trees able to exploit progressively deeper sources of soil water during the dry season, as indicated by increasingly negative xylem δD values, were also able to maintain constant or even increase rates of water use. Seasonal courses of water use and soil water partitioning were associated with leaf phenology. Species with the smallest seasonal variability in leaf fall were also able to tap increasingly deep sources of soil water as the dry season progressed. Comparison of xylem, soil, and groundwater δD values thus pointed to spatial and temporal partitioning of water resources among several tropical forest canopy tree species during the dry season.

Evaluation of evaporation rate from forested soil surface using stable isotopic composition of soil water in a headwater basin

The stable isotopic composition of rain water and soil water was analysed and the soil surface evaporation rate was evaluated using the isotope value of soil water taken from a forested headwater basin in central Japan. Rainfall, throughfall, soil water, groundwater and discharge water were sampled at intervals of one month to determine the isotopic compositions of deuterium (D) and oxygen-18 (18O) therein. In addition, tensiometric observation of soil water was performed continuously. The stable isotopic ratio of soil water was homogenized between the soil surface and a depth of 100 cm by means of active soil water movement above 100 cm in depth. The mean δ18O in deep soil water below a depth of 100 cm was 0·24‰ higher than the volume weighted mean δ18O in throughfall. Consequently, the mean evaporation rate from the soil surface was calculated as 5% of the evapotranspiration rate using the difference between the δ18O in throughfall and in deep soil water, based on the Rayleigh equation under equilibrium condition. Copyright © 1998 John Wiley & Sons, Ltd.

Evaluation of evaporation rate from forested soil surface using stable isotopic composition of soil water in a headwater basin

Evaluation of evaporation rate from forested soil surface using stable isotopic composition of soil water in a headwater basin

Oxygen isotopic composition of soil water: Quantifying evaporation and transpiration

The oxygen isotopic composition of soil water provides an extra quantitative dimension in water balance analysis which allows separation of evaporation from transpiration. Spatial and temporal variations in water content and oxygen isotopic composition in soils along an arid to humid transect in Hawaii reflect the processes of recharge by rain, mixing with antecedent moisture, and evapotranspiration. Rainwater is always more depleted in 18O than is the soil water with which it mixes. Input of 18O-depleted rain increases volumetric water content while lowering the soil-water δ 18O value. Evapotranspiration occurs continuously, leading to a decrease in the volumetric water content and an increase in the soil-water δ 18O value. These effects are most pronounced at the soil surface and decrease in a downward direction. The frequency of recharge determines temporal variability of these values within a given depth interval, while differences along the transect are due to climatic parameters. Results of a material balance model indicate that evaporation decreases, transpiration increases, and the ratio of evaporation to transpiration decreases with increasing annual rainfall and decreasing temperature.

Measurement of plant and soil water isotope composition by direct equilibration methods

Water contained in plant and soil samples can be analysed for 2H and 18O content by direct equilibration while contained within the sample matrix. Methods for this are described and compared with the commonly used azeotropic distillation of samples before isotope analysis. For δ 18O, direct equilibration with CO 2 gives results in good agreement with azeotropic distillation, i.e. within 0.5%o at natural abundance. Direct equilibration is a practical method for individual twig samples containing less than 0.5 ml of water, and offers significant operator time savings compared with azeotropic distillation. Batches of up to 100 samples can be prepared in less time than required for a single azeotropic distillation, and analysis by automated continuous-flow isotope ratio mass spectrometry after equilibration for 3 days again requires a minimum of operator time. Complete equilibration of plant water with H 2 for δ 2H measurement occurs only after the plant material has been heated to 100°C under vacuum. The method described here is barely precise enough for natural abundance measurements ( δ 2 H ± 15‰ ) but is well suited to field tracer studies with deuterium oxide.

Interlaboratory comparison of methods to determine the stable isotope composition of soil water

This paper presents results of an interlaboratory comparison of the effects of different techniques for extracting soil water on its measured 2H and 18O composition. In the comparison, four soils (a sand, a gypseous sand, and a clay soil at high and low water contents) were prepared and distributed to fourteen laboratories. Water was then extracted from these samples and analysed using each laboratory's standard method. A number of verification procedures was used to ensure that the experiment was truly a comparison of extraction techniques and that reported variations were not due to sample preparation, transport or measurement. The extraction techniques used included azeotropic, vacuum and microdistillation methods. The results show a large variation between laboratories in the isotopic composition of the water extracted (of up to 30‰ for 2H and 3.4‰ for 18O). The variation increased as the water content of the soil decreased and was greater for clays than sand at comparable soil matric suctions. The δ-value obtained was correlated with the final extraction temperature, with incomplete extraction being the most likely cause for the variation. The study highlights the need to develop standard protocols for the extraction of water from soils for isotopic analysis.

Effect of Catchment‐Scale Subsurface Mixing on Stream Isotopic Response

A 3.8‐ha watershed on the west coast of New Zealand was instrumented with suction lysimeters and automatic water samplers to determine the relationship between subsurface isotopic and chemical concentrations to those of rainfall and resulting streamflow. A t test showed that ±2‰ represented a significant difference between successive sample deuterium values. Eleven rainfall episodes were subdivided into two categories: (1) two events where stream isotopic composition did not deflect >2‰ from prestorm values, and (2) four events which demonstrated new water flushing. Detailed analysis of one 47‐mm rainfall (9.8‐mm runoff) event showed that old water dominated stream water exiting the watershed by 90% using a standard two‐component hydrograph separation for deuterium (corroborated by Cl and electrical conductivity). Three‐component hydrograph separation indicated that 12–16% was in the form of soil water, with <5% as on‐channel precipitation and 80% groundwater. Analysis of over 1000 water samples revealed systematic trends in soil water and groundwater isotopic composition both in a downslope and downprofile direction. Between‐storm suction lysimeter deuterium data showed a systematic dampened response to temporally variable rainfall deuterium concentrations. Multivariate cluster analysis revealed three distinct soil water/groundwater groupings, with respect to soil depth and geographic position within the watershed. Within‐storm suction lysimeter sampling preserved similar groupings, indicating that the subsurface reservoir is poorly mixed on short time scales. Understanding subsurface mixing response to rainfall should greatly improve models of episodic stream response and partitioning of storm flow into waters of different age.

A CASE STUDY OF SHALLOW FLOW PATHS IN A STEEP ZERO‐ORDER BASIN1

ABSTRACT: Soil water potentials, slope throughflow, runoff chemistry, and isotopic composition were monitored in a 97 m2 zero‐order basin within the Maimai 8 watershed on the South Island of New Zealand, for a natural rain storm and two artificial water applications. Contrary to results previously reported for other portions of the Maimai catchment, much of the runoff occurred as a shallow subsurface organic layer flow. For the 47 mm natural rain event, pre‐storm soil matric potential ranged from −60 to −150 cm H2O. No saturation was produced within the profile, and the majority of storm runoff emanated from flow within the organic horizon perched on the mineral soil surface. Hillslope applications corroborated this interpretation by showing >90 percent new water flushing with negligible mineral soil moisture response. Although the mechanisms cited in the text are not representative of the entire catchment, the study demonstrates: (1) the value of a combined physical‐chemical‐isotopic approach in quantifying slope processes, and (2) the heterogeneous nature and diversity of slope runoff pathways in a relatively homogeneous catchment.

Examining evapotranspiration in a semi-arid region using stable isotopes of hydrogen and oxygen

Daily variation of the isotopic composition, both deuterium and oxygen-18, of water in foliage of a mixed Eucalyptus foecunda - Eucalyptus socialis, ‘mallee’ community was studied in relation to the isotopic composition of soil water and atmospheric water vapour, transpiration and evapotranspiration, and environmental conditions. The isotopic composition of water in foliage and the soil surface were enriched over adjacent stem and deeper soil water respectively, characteristic of water subject to evaporation. The composition of foliar water varied systematically over the day, being highest in the late afternoon, in response to lower relative humidity. The foliar water composition fitted a steady-state model of leaf water fractionation. It was found that the fraction of leaf water equilibrated with the atmosphere was at a minimum during the middle of the day, fitting earlier observations of an inverse correlation of this fraction with transpiration rate. The equilibrated fraction was relatively high compared with previous transpiration studies, consistent with low transpiration rates. The soil water isotope-depth profiles had not achieved steady state. Using existing models, we conclude that the foliage and soil compartments are exchanging water vapour, but that some complicated evaporation processes are occurring in the soil, probably as a result of daily temperature cycling. Examining the relationship of the two isotope tracers in the different compartments, the vapour transport fits a transport model in which it is strongly diffusion-limited, both within the soil and leaf. Because of some uncertainty as to the transport mechanisms involved, it is not clear what the relative contributions from soil and foliage are. The vegetation may be having a considerable influence on the total evaporation from the community, since the effects of the root system on the soil structure and soil water transport are not clear.

An isotopic study of a fluvial-lacustrine sequence: The Plio-Pleistocene koobi fora sequence, East Africa

Stable isotopic analyses of Plio-Pleistocene and modern sediments in the fluvial-lacustrine system occupying the Turkana Basin, East Africa provide constraints on the paleoenvironmental and diagenetic histories of the Pliocene through the Recent sediments in the basin. The δ 13C values for carbonates in lacustrine sediments range from −15 to +22‰ relative to PDB, depending on the varying proportions of CO 2 from the atmospheric reservoir and from various metabolic sources. The δ 18O values of carbonates in lacustrine sediments indicate that the isotopic composition of paleolake water varied by over 10‰ from the Pliocene to the present. The δ 13C values for pedogenic carbonates record paleoccologic variations and suggest that C 4 plants did not become well established in the preserved depositional parts of the basin until about 1.8 myr ago. The δ 18O values pedogenic carbonates suggest a range of over 10‰ for the isotopic composition of soil water during this interval. They also suggest a period of major climatic instability from about 3.4 to 3.1 myr and at about 1.8 myr. Together, the δ 13C and δ 18O values of pedogenic carbonates indicate that the present conditions are as arid and hot as any that had prevailed during deposition of these Plio-Pleistocene sediments.

Climatic implications of the natural variations of D/H ratios in tree ring cellulose

Natural variations in the deuterium composition of carbon-bound hydrogen of cellulose nitrate from tree rings have been measured. Trees from Edmonton have been analysed in detail and the δD values of 5-year groups of rings show a strong temporal correlation with mean annual temperature at the growth site. This is shown to result from the cellulose isotopic composition being related to the isotopic composition of soil water used during photosynthesis. The soil water is in turn related to meteoric water which at this growth site appears to correlate temporally with mean annual temperature. Similar measurements from trees which grew in various areas of North America show a strong spatial correlation between δD of cellulose and mean annual temperature. The spatial functional relationship between δD values and mean annual temperature at the growth site ( t) is found to be δD = 5.5t - 130‰ swhich compares with the spatial relationship between δD of meteoric water and mean annual air temperature which is δD = 5.6t - 100‰.