Deuterium tracing for the estimation of transpiration from trees Part 1. Field calibration

Flow rates calculated using the deuterium tracing method were compared with measurements of direct water uptake of excavated trees which had been inserted into water containers. Taking into account that the tracing method provides an estimate of the weighted mean flow rate over the period that the tracer pulse is present (the weighting factor is the relative concentration of the tracer as measured at the sampling point) agreement was obtained within the experimental error (about 10%) associated with the tracing method.     

Development of saline ground water through transpiration of sea water

As vegetation usually excludes salt during water uptake, transpiration will increase the salinity of the residual water. If the source water is sea water, then the residual water may become highly saline. In the unconfined coastal aquifer of the tropical Burdekin River delta, northeastern Australia, areas of highly saline ground water with chloride concentrations up to almost three times that of sea water occur up to 15 km from the present coastline, and are attributed to transpiration by mangrove vegetation during periods of high sea level. Radiogenic ((14)C) carbon isotope analyses indicate that ground water with chloride concentrations between 15,000 and 35,000 mg/L is mostly between 4000 and 6000 years old, at which time sea level was 2 to 3 m higher than present. Stable isotope analyses of oxygen-18 and deuterium show no evidence for evaporative enrichment of this water. Oxygen-18, deuterium, and stable (delta(13)C) carbon isotope analyses of ground water and soil water point to a recharge environment beneath the mangrove forests during this postglacial sea level high stand. During that period, transpiration of the mangrove forests would have led to high chloride concentrations in the residual ground water, without inducing isotopic fractionation. Due to the higher density, this hypersaline water moved downward through the aquifer by gravity and has formed lenses of highly saline ground water at the bottom of the unconfined aquifer.     

Canopy transpiration obtained from leaf transpiration,sap flow and FAO‐56 dual crop coefficient method

With a maize seed planting area of about 67 000 hm², Zhangye city supplies the seeds for more than 40% of the maize planting area in China. Irrigation water is often overused to ensure the quality of the maize seeds, leading to serious water shortage problems in recent years. An accurate and convenient estimate of canopy transpiration is of particular importance to ease the problem. In this paper, leaf transpiration and sap flow in a maize field were measured in 2012 using a portable photosynthesis system and a heat balance sap flow system. Based on a large amount of meteorological data and relevant maize plant‐growing parameters, canopy transpiration was up‐scaled from both leaf transpiration (T_l) and sap flow (T_f), and also calculated by the FAO‐56 dual crop coefficient method (T). Comparing these three types of transpiration, T_f was proved to be more reliable than T_l. Taking T_f as a benchmark, the basal crop coefficient (K_cb, the key parameter of FAO‐56 dual crop coefficient method) was further adjusted and verified for the maize plants in this region. In addition, the errors when using up‐scaling methods and FAO‐56 dual crop coefficient method are summarized. Copyright © 2014 John Wiley & Sons, Ltd.     

Deuterium tracing for the estimation of transpiration from trees Part 3. Measurements of transpiration from Eucalyptus plantation, India

Measurements of transpiration from individual trees of Eucalyptus from plantations at four different sites in Karnataka, Southern India, are presented. These show large (as much as tenfold) differences in the transpiration between premonsoon and postmonsoon periods, a reflection of the effects of soil-moisture stress in the premonsoon periods. For trees with diameters at breast height (DBH) less than 10 cm the transpiration rate of individual trees is proportional to the square of the DBH. For trees which are not experiencing soil-water stress the daily transpiration rate of individual trees, q, is well represented by the relation: q = (6.6 ± 0.3)g (m³ day⁻¹ where g (m²) is the tree basal area. On a unit ground area basis the transpiration rate, expressed as a depth per day is given by the relation: E_t = (0.66 ± 0.03)G (mm day⁻¹ where g(m²ha⁻¹) is the total basal area per hectare. For all the sites studied, although there is evidence for the ‘mining’ of soil water as roots penetrate deeper depths in the soil each year, there is no evidence for direct abstraction from the water table.     

Two-point ray tracing in 3-D

Algorithm for determination of all two-point rays of a given elementary wave by means of the shooting method is presented. The algorithm is designed for general 3-D models composed of inhomogeneous geological blocks separated by curved interfaces. It is independent of the initial conditions for rays and of the initial-value ray tracer. The algorithm described has been coded in Fortran 77, using subroutine packages MODEL and CRT for model specification and for initial-value ray tracing.     

Hydrologic implications of the isotopic kinetic fractionation of open-water evaporation

The kinetic fractionation of open-water evaporation against the stable water isotope H_2 ~(18)O is an important mechanism underlying many hydrologic studies that use ~(18)O as an isotopic tracer. A recent in-situ measurement of the isotopic water vapor flux over a lake indicates that the kinetic effect is much weaker(kinetic factor 6.2‰) than assumed previously(kinetic factor14.2‰) by lake isotopic budget studies. This study investigates the implications of the weak kinetic effect for studies of deuterium excess-humidity relationships, regional moisture recycling, and global evapotranspiration partitioning. The results indicate that the low kinetic factor is consistent with the deuterium excess-humidity relationships observed over open oceans.The moisture recycling rate in the Great Lakes region derived from the isotopic tracer method with the low kinetic factor is a much better agreement with those from atmospheric modeling studies than if the default kinetic factor of 14.2‰ is used. The ratio of transpiration to evapotranspiration at global scale decreases from 84±9%(with the default kinetic factor) to 76±19%(with the low kinetic factor), the latter of which is in slightly better agreement with other non-isotopic partitioning results.     

Effect of vertical resolution on predictions of transpiration in water-limited ecosystems

Water-limited ecosystems are characterized by precipitation with low annual totals and significant temporal variability, transpiration that is limited by soil-moisture availability, and infiltration events that may only partially rewet the vegetation root zone. Average transpiration in such environments is controlled by precipitation, and accurate predictions of vegetation health require adequate representation of temporal variation in the timing and intensity of plant uptake. Complexities introduced by variability in depth of infiltration, distribution of roots, and a plant's ability to compensate for spatially heterogeneous soil moisture suggest a minimum vertical resolution required for satisfactory representation of plant behavior.To explore the effect of vertical resolution on predictions of transpiration, we conduct a series of numerical experiments, comparing the results from models of varying resolution for a range of plant and climate conditions. From temporal and spatial scales of the underlying processes and desired output, we develop dimensionless parameters that indicate the adequacy of a finite-resolution model with respect to reproducing characteristics of plant transpiration over multiple growing seasons. These parameters may be used to determine the spatial resolution required to predict vegetation health in water-limited ecosystems.     

Determination of regional distribution of crop transpiration and soil water use efficiency using quantitative remote sensing data through inversion

As is widely known, there is a severe shortage of water resources in North China. There have been frequent droughts in recent years. Developing water saving measures, especially in agricul-ture, has become an urgent task. In water-saving agriculture, one …     

On the information content of forest transpiration measurements for identifying canopy conductance model parameters

Generally, forest transpiration models contain model parameters that cannot be measured independently and therefore are tuned to fit the model results to measurements. Only unique parameter estimates with high accuracy can be used for extrapolation in time or space. However, parameter identification problems may occur as a result of the properties of the data set. Time‐series of environmental conditions, which control the forest transpiration, may contain periods with redundant or coupled information, so called collinearity, and other combinations of conditions may be measured only with difficulty or incompletely. The aim of this study is to select environmental conditions that yield a unique parameter set of a canopy conductance model. The parameter identification method based on localization of information (PIMLI) was used to calculate the information content of every individual artificial transpiration measurement. It is concluded that every measurement has its own information with respect to a parameter. Independent criteria were assessed to localize the environmental conditions, which contain measurements with most information. These measurements were used in separate subdata sets to identify the parameters. The selected measurements do not overlap and the accuracies of the parameter estimates are maximized. Measurements that were not selected do not contain additional information that can be used to further maximize the parameter accuracy. Thereupon, the independent criteria were used to select eddy correlation measurements and parameters were identified with only the selected measurements. It is concluded that, for this forest and data set, PIMLI identifies a unique parameter set with high accuracy, whereas conventional calibrations on subdata sets give non‐unique parameter estimates. Copyright © 2001 John Wiley & Sons, Ltd.     

High precision tracing of soil and sediment movement using fluorescent tracers at hillslope scale

Generating high resolution spatial information on the movement of sediment in response to soil erosion remains a major research challenge. In this paper we present a new tracing method that utilises LED (light emitting diode) light to induce fluorescence in a sand-sized tracer, which is then detected, using a complementary metal oxide semiconductor (CMOS) sensor in a commercial digital camera, at mm-resolution without the need for removal of soil material. First, we detail two complementary, but independent, methods for quantifying the concentration of tracer from images: particle counting and an intensity based method. We show that both methods can produce highly resolved estimates of particle concentrations under laboratory conditions. Secondly, we demonstrate the power of the method for collecting spatial information on soil redistribution by tillage, with mm precision, over an approximately 50 m hillslope and vertically down the soil profile. Our work demonstrates the potential to collect quantitative time-resolved data about soil movement without disturbing the soil surface which is being studied, and with it the possibility to parameterise or evaluate dynamic distributed soil erosion models or to undertake fundamental research focused on particle movement that has been impossible to conduct previously. © 2018 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.     

Improvement of a simplified process‐based model for estimating transpiration under water‐limited conditions

Plant transpiration depends on environmental conditions, and soil water availability is its primary control under water deficit conditions. In this study, we improve a simplified process‐based model (hereafter “BTA”) by including soil water potential (ψ_soil) to explicitly represent the dependence of plant transpiration on root‐zone moisture conditions. The improved model is denoted as the BTA‐ψ model. We assessed the performance of the BTA and BTA‐ψ models in a subtropical monsoon climate and a Mediterranean climate with different levels of water stress. The BTA model performed reasonably in estimating daily and hourly transpiration under sufficient water conditions, but it failed during dry periods. Overall, the BTA‐ψ model provided a significant improvement for estimating transpiration under a wide range of soil moisture conditions. Although both models could estimate transpiration (sap flow) at night, BTA‐ψ was superior to BTA in this regard. Species differences in the calibrated parameters of both models were consistent with leaf‐level photosynthetic measurements on each species, as expected given the physiological basis of these parameters. With a simplified representation of physiological regulation and reasonable performance across a range of soil moisture conditions, the BTA‐ψ model provides a useful alternative to purely empirical models for modelling transpiration.     

Groundwater tracing with post sampling activation analysis

A tracing technique is described which uses low concentrations of a nonradioactive tracer, the bromide ion. Concentrations may be so low that they do not constitute a health or pollution problem when added to segments of the hydrologic cycle. The tracer is detected by post sampling activation analysis techniques. Water samples containing the tracer are made radioactive when irradiated with neutrons. Radiation detectors are used to analyze the radiation emitted by each sample and the unique radiation characteristics of the tracer (⁸⁰Br with a half life of 17.6 min) are sought. No chemical separation techniques are required.Given a “rabbit system” to transport irradiated samples to a Ge(Li) detector 3–5 samples per hour can be analyzed.The technique is safe, and tracers can be detected as low as 20 parts per billion (p.p.b.) over that of background concentrations. The bromide ion does not appear to be lost by precipitation, absorption or adsorption and is biologically stable.A spike concentration of 200 p.p.m. was used in field studies because it is lower than the limit set for drinking water. The technique was tried on sinkhole, conduit and spring systems in a carbonate terrain, in central Pennsylvania. Surface water entering a sinkhole had a background bromine concentration of 0.020 μg/ml and 0.027 μg/ml at the conduit outlet 0.6 miles distant. Although the flow rate into the sinkhole was 50 ft.³/min, no tracer was detected in the spring 16 h after the tracer was added.In a second study ammonium bromide was added in a conduit 500 ft. from its discharge point. Tracer first appeared at the discharge point 1 h after injection (0.11 p.p.m. vs. an average background of 0.03 p.p.m.). The maximum passed at 1 h 30 min and the spread of the spike was estimated to be over 5 h. For a peak maximum of 0.1 p.p.m., only 3 g of ammonium bromide would have been required in the second study. The inlet concentration would have been only 2.8 p.p.m. indicating the sensitivity of the method.Nine of many possible potential field applications are listed for this superior tracing technique.     

Modelling forest transpiration from different perspectives

Forest transpiration models have been developed in different disciplines such as plant physiology, ecology, meteorology, hydrology and soil science. In the present study, three different types of model perspectives for transpiration control are used: leaf cooling, CO₂ assimilation and the combined energy and water balance. All three process‐orientated models are calibrated on measurements in a Douglas fir stand in the Netherlands. The performances of these models are equally good, although they have different complexities, different numbers of calibration parameters (ranging from 1 to 6) and the models are calibrated on different measurements (eddy correlation at canopy level or CO₂ measurements at leaf level). The resemblance of the model results is caused by the calibration procedure and by the high impact of radiation in all three cases. Significant discrepancies become apparent when differences between model responses are examined and when specific (short) periods are selected when input variables are uncoupled. The main differences between the models are caused by another formulation of leaf area index and vapour pressure deficit (VPD). Considerable differences in simulated transpiration occur in the afternoon as a result of the diurnal hysteresis between VPD and radiation. Copyright © 2000 John Wiley & Sons, Ltd.     

Comparison of tree transpiration under wet and dry canopy conditions in a Costa Rican premontane tropical forest

Spatial and temporal variation in wet canopy conditions following precipitation events can influence processes such as transpiration and photosynthesis, which can be further enhanced as upper canopy leaves dry more rapidly than the understory following each event. As part of a larger study aimed at improving land surface modelling of evapotranspiration processes in wet tropical forests, we compared transpiration among trees with exposed and shaded crowns under both wet and dry canopy conditions in central Costa Rica, which has an average 4200 mm annual rainfall. Transpiration was estimated for 5 months using 43 sap flux sensors in eight dominant, ten midstory and eight suppressed trees in a mature forest stand surrounding a 40‐m tower equipped with micrometeorological sensors. Dominant trees were 13% of the plot's trees and contributed around 76% to total transpiration at this site, whereas midstory and suppressed trees contributed 18 and 5%, respectively. After accounting for vapour pressure deficit and solar radiation, leaf wetness was a significant driver of sap flux, reducing it by as much as 28%. Under dry conditions, sap flux rates (J_s) of dominant trees were similar to midstory trees and were almost double that of suppressed trees. On wet days, all trees had similarly low J_s. As expected, semi‐dry conditions (dry upper canopy) led to higher J_s in dominant trees than midstory, which had wetter leaves, but semi‐dry conditions only reduced total stand transpiration slightly and did not change the relative proportion of transpiration from dominant and midstory. Therefore, models that better capture forest stand wet–dry canopy dynamics and individual tree water use strategies are needed to improve accuracy of predictions of water recycling over tropical forests. Copyright © 2016 John Wiley & Sons, Ltd.     

Measurements of transpiration in four tropical rainforest types of north Queensland,Australia

Transpiration of four different rainforest types in north Queensland, Australia, was determined using the heat pulse technique for periods ranging between 391 and 657 days. Despite the complexity of the natural rainforest systems being studied, the relationship between sample tree size and daily water use was found to be strong, thus providing a robust means by which to scale transpiration from individual trees to the entire forest stand. Transpiration was shown to be dependent on solar radiation and atmospheric demand for moisture with little evidence of limitation by soil moisture supply. Total stand transpiration was controlled by forest characteristics such as stem density, size distribution and sapwood area. Annual transpiration for each of the four sites ranged between 353 mm for cloud forest and 591 mm for montane rainforest. In comparison with the international literature, transpiration from Australian rainforests is low; the reasons for this could be related to a combination of differences in forest structure, climatic conditions, canopy wetness duration and tree physiology. Copyright © 2007 John Wiley & Sons, Ltd.     

Responses of canopy transpiration and canopy conductance of peach (Prunus persica) trees to alternate partial root zone drip irrigation

We investigated canopy transpiration and canopy conductance of peach trees under three irrigation patterns: fixed 1/2 partial root zone drip irrigation (FPRDI), alternate 1/2 partial root zone drip irrigation (APRDI) and full root zone drip irrigation (FDI). Canopy transpiration was measured using heat pulse sensors, and canopy conductance was calculated using the Jarvis model and the inversion of the Penman–Monteith equation. Results showed that the transpiration rate and canopy conductance in FPRDI and APRDI were smaller than those in FDI. More significantly, the total irrigation amount was greatly reduced, by 34·7% and 39·6%, respectively for APRDI and FPRDI in the PRDI (partial root zone drip irrigation) treatment period. The daily transpiration was linearly related to the reference evapotranspiration in the three treatments, but daily transpiration of FDI is more than that of APRDI and FPRDI under the same evaporation demand, suggesting a restriction of transpiration water loss in the APRDI and FPRDI trees. FDI needed a higher soil water content to carry the same amount of transpiration as the APRDI and FPRDI trees, suggesting the hydraulic conductance of roots of APRDI and FPRDI trees was enhanced, and the roots had a greater water uptake than in FDI when the average soil water content in the root zone was the same. By a comparison between the transpiration rates predicted by the Penman–Monteith equation and the measured canopy transpiration rates for 60 days during the experimental period, an excellent correlation along the 1:1 line was found for all the treatments (R² > 0·80), proving the reliability of the methodology. Copyright © 2005 John Wiley & Sons, Ltd.     

Spatial patterns of simulated transpiration response to climate variability in a snow dominated mountain ecosystem

Transpiration is an important component of soil water storage and stream‐flow and is linked with ecosystem productivity, species distribution, and ecosystem health. In mountain environments, complex topography creates heterogeneity in key controls on transpiration as well as logistical challenges for collecting representative measurements. In these settings, ecosystem models can be used to account for variation in space and time of the dominant controls on transpiration and provide estimates of transpiration patterns and their sensitivity to climate variability and change. The Regional Hydro‐Ecological Simulation System (RHESSys) model was used to assess elevational differences in sensitivity of transpiration rates to the spatiotemporal variability of climate variables across the Upper Merced River watershed, Yosemite Valley, California, USA. At the basin scale, predicted annual transpiration was lowest in driest and wettest years, and greatest in moderate precipitation years (R² = 0·32 and 0·29, based on polynomial regression of maximum snow depth and annual precipitation, respectively). At finer spatial scales, responsiveness of transpiration rates to climate differed along an elevational gradient. Low elevations (1200–1800 m) showed little interannual variation in transpiration due to topographically controlled high soil moistures along the river corridor. Annual conifer stand transpiration at intermediate elevations (1800–2150 m) responded more strongly to precipitation, resulting in a unimodal relationship between transpiration and precipitation where highest transpiration occurred during moderate precipitation levels, regardless of annual air temperatures. Higher elevations (2150–2600 m) maintained this trend, but air temperature sensitivities were greater. At these elevations, snowfall provides enough moisture for growth, and increased temperatures influenced transpiration. Transpiration at the highest elevations (2600–4000 m) showed strong sensitivity to air temperature, little sensitivity to precipitation. Model results suggest elevational differences in vegetation water use and sensitivity to climate were significant and will likely play a key role in controlling responses and vulnerability of Sierra Nevada ecosystems to climate change. Copyright © 2008 John Wiley & Sons, Ltd.     

Collected Rain Water as Cost-Efficient Source for Aquifer Tracer Testing

Locally collected precipitation water can be actively used as a groundwater tracer solution based on four inherent tracer signals: electrical conductivity, stable isotopic signatures of deuterium [δ²H], oxygen-18 [δ¹⁸O], and heat, which all may strongly differ from the corresponding background values in the tested groundwater. In hydrogeological practice, a tracer test is one of the most important methods for determining subsurface connections or field parameters, such as porosity, dispersivity, diffusion coefficient, groundwater flow velocity, or flow direction. A common problem is the choice of tracer and the corresponding permission by the appropriate authorities. This problem intensifies where tracer tests are conducted in vulnerable conservation or water protection areas (e.g., around drinking water wells). The use of (if required treated) precipitation as an elemental groundwater tracer is a practical solution for this problem, as it does not introduce foreign matters into the aquifer system, which may contribute positively to the permission delivery. Before tracer application, the natural variations of the participating end members' tracer signals have to be evaluated locally. To obtain a sufficient volume of tracer solution, precipitation can be collected as rain using a detached, large-scale rain collector, which will be independent from possibly existing surfaces like roofs or drained areas. The collected precipitation is then stored prior to a tracer experiment.