Ecosystem-scale Evapotranspiration Research Articles

Increasing contribution of alluvial groundwater to riparian cottonwood forest water use through warm and dry summers

In semi-arid regions, riparian forests access alluvial groundwater to supplement the limited local precipitation. Understanding the sensitivity of riparian forests to variations in weather and climate, and water resources management is essential for guiding riparian conservation measures, especially on rivers impacted by dams and water withdrawal. We anticipated a seasonal trade-off in ecosystem water sources, with increasing relative groundwater use through the summer to compensate for the reduced availability of the shallow soil moisture. To test this hypothesis, we assessed the temporal patterns of alluvial groundwater uptake by a riparian cottonwood (Populus species) forest along a regulated river in southern Alberta, Canada over a three-year interval with relatively warm and dry weather, and low river flows. In support of our hypothesis, groundwater evapotranspiration, as calculated from diurnal fluctuations in the alluvial water table, displayed relative seasonal consistency until the autumnal leaf senescence period. In contrast, the ecosystem-scale evapotranspiration, measured by eddy covariance, gradually receded through the summer as precipitation inputs that fed the shallow soil layers became limited. The consistent uptake rate of alluvial groundwater by cottonwoods enables riparian forests to maintain high rates of water use and productivity in this semi-arid prairie region. We conclude the consistent groundwater use through the warm and dry summer reflected: (1) perennial river flow that replenished the floodplain aquifer; (2) adaptations of cottonwoods to the riparian hydrology and the semi-arid climate; and (3) environmental flow operations that maintained the minimum river and alluvial groundwater levels. Our study provides information on the riparian forest water sources and water use patterns that could help water managers effectively regulate river flows to support healthy downstream riparian ecosystems.

Quantifying the variability in water use efficiency from the canopy to ecosystem scale across main croplands

Current, how to use limited water resources efficiently and improve agricultural water use efficiency, has become one of the greatest challenges for global food security. In this study, multiple site-years of carbon and water flux data across the major crops including maize, winter wheat and soybean, were used to quantify the variability in canopy-scale transpiration (T), ecosystem-scale evapotranspiration (ET) as well as the associated water use efficiencies (WUET and WUEET). On the basis of ET partitioning, the results indicated that the transpiration ratio–T/ET as well as T and ET exhibited an obvious single-peak seasonal pattern across the typical croplands. However, at the early and late growing stages, there existed large discrepancies in T and ET owing to low vegetation coverage, while T and ET were very close during the peak period. Among them, maize exhibited the largest T/ET by 0.50 ± 0.12, followed by soybean of 0.43 ± 0.08 and winter wheat of 0.38 ± 0.09, respectively. Furthermore, the coupling relationships between gross primary productivity (GPP) and water fluxes including T and ET changed from linear to nonlinear. The study also found that the variability in WUET and WUEET were not consistent. Specifically, WUEET showed distinct seasonal characteristic whereas WUET kept constant as a plateau almost throughout the growth period, which reflected the inherent physiological property controlled by plant stomata at the canopy scale. Among these crops, maize exhibited the largest WUET and WUEET (5.30 ± 0.89 and 2.48 ± 1.14 g C kg−1 H2O), followed by winter wheat (4.97 ± 1.52 and 2.35 ± 0.64 g C kg−1 H2O) and soybean (4.88 ± 1.59 and 1.89 ± 0.99 g C kg−1 H2O), respectively.

Parsimony vs predictive and functional performance of three stomatal optimization principles in a big‐leaf framework

Stomatal optimization models can improve estimates of water and carbon fluxes with relatively low complexity, yet there is no consensus on which formulations are most appropriate for ecosystem-scale applications. We implemented three existing analytical equations for stomatal conductance, based on different water penalty functions, in a big-leaf comparison framework, and determined which optimization principles were most consistent with flux tower observations from different biomes. We used information theory to dissect controls of soil water supply and atmospheric demand on evapotranspiration in wet to dry conditions and to quantify missing or inadequate information in model variants. We ranked stomatal optimization principles based on parameter uncertainty, parsimony, predictive accuracy, and functional accuracy of the interactions between soil moisture, vapor pressure deficit, and evapotranspiration. Performance was high for all model variants. Water penalty functions with explicit representation of plant hydraulics did not substantially improve predictive or functional accuracy of ecosystem-scale evapotranspiration estimates, and parameterizations were more uncertain, despite having physiological underpinnings at the plant level. Stomatal optimization based on water use efficiency thus provided more information about ecosystem-scale evapotranspiration compared to those based on xylem vulnerability and proved more useful in improving ecosystem-scale models with less complexity.

Responses of evapotranspiration to droughts across global forests: a systematic assessment

Forest evapotranspiration (ET) is one of the most important factors influencing the terrestrial hydrological cycle and is prone to being influenced by increasing drought events. This highlights the need to understand the interaction between global forest ET and drought. Consequently, we drew 710 sets of ecosystem-scale ET observations from 69 forest sites around the world and then systematically assessed the ET anomalies during droughts across the dominant forest and climate types. Overall, the response of forest ET to drought is non-monotonic. Under severe and extreme droughts with long durations, ET was reduced in most of forests in the world. We attributed the decreased forest ET to both restricted water supply and restricted water consumption by forests; however, lower magnitude and short-term droughts generally increased forest ET, and in some cases, ET even exceeded precipitation during droughts. We attributed this to the increased surface evaporation and the utilization of deep water by deep root systems. Specifically, we find that the positive anomaly of ET under droughts implied the drought paradox, which acts to accelerate the terrestrial hydrological cycle and further amplify the drought. This study as a global synthetic analysis of case studies on the site scale may have great potential for understanding the importance of the drought-modulated forest water cycle and the possibility of increasing drought stress with the effects of the drought paradox.

Ecosystem scale evapotranspiration and CO2 exchange in burned and unburned peatlands: Implications for the ecohydrological resilience of carbon stocks to wildfire

AbstractBoreal peatlands represent a significant global store of soil carbon, which are subject to increasing natural and anthropogenic disturbance. Wildfire is the single largest disturbance to boreal forest and wetlands annually. Critical to the long‐term carbon storage function in peatlands is the (re‐)establishment of a near‐surface water table following wildfire. This has been recently shown to in part be facilitated by post‐fire reductions in water losses via evapotranspiration (ET). However, reduced ET may also have cascade impacts on other ecohydrological processes in recovering peatlands, such as a reduction in carbon sequestration. To investigate the linked cycles of evaporative loss and carbon exchange in burned peatlands, the burned and unburned peatlands in Alberta, Canada, were instrumented with eddy covariance systems to monitor continuous fluxes of energy, carbon dioxide, and water vapour, over two summer seasons (2013 and 2014; 2–3 years post‐burn). The burned site showed significant changes to respiration and productivity and a shift in the partitioning of available energy (significantly larger Bowen ratio; mean values of 1.19 and 1.10 at the burned and unburned sites, respectively), as well as a significant reduction in ET rates. Decreases in respiration did not offset the decrease in primary productivity, and the burned site was significantly less productive than the reference site on a net production basis for the available data period. This provides direct observations of ET and CO2 fluxes at a novel ecosystem scale to show the impacts of fire on short‐term (2–3 years) post‐burn ecosystem ecohydrological function.

Reviews and syntheses: Turning the challenges of partitioning ecosystem evaporation and transpiration into opportunities

Abstract. Evaporation (E) and transpiration (T) respond differently to ongoing changes in climate, atmospheric composition, and land use. It is difficult to partition ecosystem-scale evapotranspiration (ET) measurements into E and T, which makes it difficult to validate satellite data and land surface models. Here, we review current progress in partitioning E and T and provide a prospectus for how to improve theory and observations going forward. Recent advancements in analytical techniques create new opportunities for partitioning E and T at the ecosystem scale, but their assumptions have yet to be fully tested. For example, many approaches to partition E and T rely on the notion that plant canopy conductance and ecosystem water use efficiency exhibit optimal responses to atmospheric vapor pressure deficit (D). We use observations from 240 eddy covariance flux towers to demonstrate that optimal ecosystem response to D is a reasonable assumption, in agreement with recent studies, but more analysis is necessary to determine the conditions for which this assumption holds. Another critical assumption for many partitioning approaches is that ET can be approximated as T during ideal transpiring conditions, which has been challenged by observational studies. We demonstrate that T can exceed 95 % of ET from certain ecosystems, but other ecosystems do not appear to reach this value, which suggests that this assumption is ecosystem-dependent with implications for partitioning. It is important to further improve approaches for partitioning E and T, yet few multi-method comparisons have been undertaken to date. Advances in our understanding of carbon–water coupling at the stomatal, leaf, and canopy level open new perspectives on how to quantify T via its strong coupling with photosynthesis. Photosynthesis can be constrained at the ecosystem and global scales with emerging data sources including solar-induced fluorescence, carbonyl sulfide flux measurements, thermography, and more. Such comparisons would improve our mechanistic understanding of ecosystem water fluxes and provide the observations necessary to validate remote sensing algorithms and land surface models to understand the changing global water cycle.

Partitioning oak woodland evapotranspiration in the rocky mountainous area of North China was disturbed by foreign vapor, as estimated based on non-steady-state 18O isotopic composition

In terrestrial ecosystems, partitioning ecosystem-scale evapotranspiration (ET) between plant transpiration and soil evaporation remains a technical challenge. In this paper, we used a newly-developed laser-based isotope analyzer (OA-ICOS) and the Keeling plot approach to partition ET components of a Quercus variabilis plantation in a lithoid hilly area of north China. The results showed that, on day-of-year (DOY) 254, 257, and 263, the modeled leaf water 18O composition (δsL,b) and observed leaf water 18O composition (δL,b) were in substantial agreement and had a significant linear correlation with coefficient of 0.96, indicating that Keeling plot approach and Graig–Gorden model can be used in portioning ET between plant transpiration and soil evaporation in a terrestrial forest ecosystem. Isotopic partition revealed that the percent contribution of transpiration to total ET increased from the morning, reached maximum values at noon, with maximum values of 91.19%, 86.30%, and 85.37% for DOY 254, 257, and 263, respectively, indicating the transpiration from Q. variabilis Blum contributed the most to the total ET in this forest ecosystem. On DOY 260, the stability stratification was unstable, which resulted from the foreign vapor concentration. The increased vapor concentration led to a 80.83% difference between δsL,b and δL,b. The correlation coefficients between δsL,b and δL,b decreased from 0.96 to 0.43 when dataset on DOY 260 was included, indicating foreign vapor increased the uncertainty in the estimation of δ18O of ET (δET) and δ18O of transpiration (δT) in the forest ecosystem. Path analysis results suggested that water vapor concentration was the major factor influencing the partitioning of ET with isotopic approach in the forest ecosystem. Since the increased water vapor concentration and decreased atmosphere δ18O disturbed the estimation of δE, δT, δET, the isotopic approach cannot be used in partitioning ET under this condition on DOY 260. Therefore, under most circumstances the isotopic approach can be used to partition ET for forest ecosystem in a non-steady state (NSS), while water vapor concentration may cause uncertainties.

Afforestation of Tropical Pasture Only Marginally Affects Ecosystem-Scale Evapotranspiration

Evapotranspiration (ET) from tropical ecosystems is a major constituent of the global land–atmosphere water flux and strongly influences the global hydrological cycle. Most previous studies of ecosystem ET have been conducted predominantly in tropical forests, and only few observations cover other tropical land-use types such as pastures, croplands, savannas or plantations. The objectives of our study were: (1) to estimate daily, monthly, and annual ET budgets in a tropical pasture and an adjacent afforestation site, (2) to assess diurnal and seasonal patterns of ET, (3) to investigate environmental controls of ET, and (4) to evaluate the soil infiltration potential. We performed eddy covariance measurements of ecosystem ET in Sardinilla (Panama) from 2007 to 2009. Daily ET (2.6 ± 1.0 mm day−1) was significantly lower in the pasture compared to the afforestation site (3.0 ± 0.9 mm day−1). The highest ET was observed during the wet–dry transition period in both ecosystems. However, differences in daily ET between sites were relatively small, particularly during the wet season. Radiation was the main environmental control of ET at both sites, however, we observed considerable seasonal variation in the strength of this control, which was stronger during the wet compared to the dry season. In 2008, total annual ET was only slightly higher for the afforestation (1114 mm y−1) than the pasture site (1034 mm y−1). Our results suggest that afforestation of pasture only marginally increases ecosystem-scale ET 6–8 years after establishment. Differences in soil infiltration potentials between our sites seem to explain this pattern.

Comparative water use of native and invasive plants at multiple scales: a global meta‐analysis

Ecohydrology and invasive ecology have become increasingly important in the context of global climate change. This study presents the first in-depth analysis of the water use of invasive and native plants of the same growth form at multiple scales: leaf, plant, and ecosystem. We reanalyzed data for several hundred native and invasive species from over 40 published studies worldwide to glean global trends and to highlight how patterns vary depending on both scale and climate. We analyzed all pairwise combinations of co-occurring native and invasive species for higher comparative resolution of the likelihood of an invasive species using more water than a native species and tested for significance using bootstrap methods. At each scale, we found several-fold differences in water use between specific paired invasive and native species. At the leaf scale, we found a strong tendency for invasive species to have greater stomatal conductance than native species. At the plant scale, however, natives and invasives were equally likely to have the higher sap flow rates. Available data were much fewer for the ecosystem scale; nevertheless, we found that invasive-dominated ecosystems were more likely to have higher sap flow rates per unit ground area than native-dominated ecosystems. Ecosystem-scale evapotranspiration, on the other hand, was equally likely to be greater for systems dominated by invasive and native species of the same growth form. The inherent disconnects in the determination of water use when changing scales from leaf to plant to ecosystem reveal hypotheses for future studies and a critical need for more ecosystem-scale water use measurements in invasive- vs. native-dominated systems. The differences in water use of native and invasive species also depended strongly on climate, with the greater water use of invasives enhanced in hotter, wetter climates at the coarser scales.

Ecosystem scale evapotranspiration and net CO2 exchange from a restored peatland

AbstractAn understanding of the symbiotic water and gas exchange processes at the ecosystem scale is essential to the development of appropriate restoration plans of extracted peatlands. This paper presents ecosystem scale measurements of the atmospheric exchange of water and carbon dioxide (CO2) from a restored vacuum extracted peatland in eastern Québec, utilizing full‐scale micrometeorological measurements of both evaporation and CO2. The results indicate that the adopted restoration practices reduce the loss of water from the peat, but CO2 emissions are ∼25% greater than an adjacent nonrestored comparison site. The blockage of drainage ditches and the existence of a mulch cover at the site keep the moisture conditions more or less constant. Consequently, the CO2 flux, which is predominantly soil respiration, is strongly controlled by peat temperature fluctuations. Copyright © 2001 John Wiley & Sons, Ltd.