Response of water vapour D-excess to land-atmosphere interactions in a semi-arid environment

Abstract. The stable isotopic composition of water vapour provides information about moisture sources and processes that is difficult to obtain with traditional measurement techniques. Recently, it has been proposed that the D-excess (dv = δ2H − 8 × δ18O) of water vapour can provide a diagnostic tracer of continental moisture recycling. However, D-excess exhibits a diurnal cycle that has been observed across a variety of ecosystems and may be influenced by a range of processes beyond regional scale moisture recycling, including local evaporation (ET) fluxes. There is a lack of measurements of D-excess in evaporation (ET) fluxes, which has made it difficult to assess how ET fluxes modify the D-excess in water vapour (dv). With this in mind, we employed a chamber based approach to directly measure D-excess in ET (dET) fluxes. We show that ET fluxes imposed a negative forcing on the ambient vapour and could not explain the higher daytime dv values. The low dET observed here was sourced from a soil water pool that had undergone an extended drying period, leading to low D-excess of the soil moisture. A strong correlation between daytime dv and locally measured relative humidity was consistent with an oceanic moisture source, suggesting that remote hydrological processes were the major contributor to daytime dv variability. During the early evening, ET fluxes into a shallow nocturnal inversion layer caused a lowering of the dv values near the surface. In addition, transient mixing of vapour with a higher D-excess from above the nocturnal inversion modified these values, causing large within night variability. These results indicate dET can generally be expected to show large spatial and temporal variability and to depend on the soil moisture state. For long periods between rain events, common in semi-arid environments, ET would be expected to impose negative forcing on the surface dv. The variability of D-excess in ET fluxes therefore needs to be considered when using dv to study moisture recycling and during extended dry periods may act as a tracer of the relative humidity of the oceanic moisture source.

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Response of water vapour D-excess to land–atmosphere interactions in a semi-arid environment

Hydrology and Earth System Sciences ◽

10.5194/hess-21-533-2017 ◽

2017 ◽

Vol 21 (1) ◽

pp. 533-548 ◽

Cited By ~ 6

Author(s):

Stephen D. Parkes ◽

Matthew F. McCabe ◽

Alan D. Griffiths ◽

Lixin Wang ◽

Scott Chambers ◽

...

Keyword(s):

Water Vapour ◽

Regional Scale ◽

Measurement Techniques ◽

Inversion Layer ◽

Arid Environment ◽

Large Variability ◽

Moisture Recycling ◽

Stable Isotopic ◽

Nocturnal Inversion ◽

Transient Mixing

Abstract. The stable isotopic composition of water vapour provides information about moisture sources and processes difficult to obtain with traditional measurement techniques. Recently, it has been proposed that the D-excess of water vapour (dv  =  δ2H − 8  ×  δ18O) can provide a diagnostic tracer of continental moisture recycling. However, D-excess exhibits a diurnal cycle that has been observed across a variety of ecosystems and may be influenced by a range of processes beyond regional-scale moisture recycling, including local evaporation (ET) fluxes. There is a lack of measurements of D-excess in evaporation (ET) fluxes, which has made it difficult to assess how ET fluxes modify the D-excess in water vapour (dv). With this in mind, we employed a chamber-based approach to directly measure D-excess in ET (dET) fluxes. We show that ET fluxes imposed a negative forcing on the ambient vapour and could not explain the higher daytime dv values. The low dET observed here was sourced from a soil water pool that had undergone an extended drying period, leading to low D-excess in the soil moisture pool. A strong correlation between daytime dv and locally measured relative humidity was consistent with an oceanic moisture source, suggesting that remote hydrological processes were the major contributor to daytime dv variability. During the early evening, ET fluxes into a shallow nocturnal inversion layer caused a lowering of dv values near the surface. In addition, transient mixing of vapour with a higher D-excess from above the nocturnal inversion modified these values, causing large variability during the night. These results indicate d

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Deuterium excess as a proxy for continental moisture recycling and plant transpiration

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-13-29721-2013 ◽

2013 ◽

Vol 13 (11) ◽

pp. 29721-29784 ◽

Cited By ~ 6

Author(s):

F. Aemisegger ◽

S. Pfahl ◽

H. Sodemann ◽

I. Lehner ◽

S. I. Seneviratne ◽

...

Keyword(s):

Relative Humidity ◽

Water Vapour ◽

Monitoring Site ◽

Deuterium Excess ◽

Plant Transpiration ◽

Surface Evaporation ◽

Moisture Source ◽

Moisture Recycling ◽

Continental Scale ◽

Measurement Site

Abstract. Studying the evaporation process and its link to the atmospheric circulation is central for a better understanding of the feedbacks between the surface water components and the atmosphere. Stable water isotopes are ideal tools to investigate surface evaporation as they are naturally available tracers of water phase changes in the atmosphere. The strength of isotope fractionation processes depends on environmental conditions such as relative humidity and temperature. In this study, we use five months of deuterium excess (d) measurements at the hourly to daily timescale from a cavity ring-down laser spectrometer to characterise the evaporation source of low-level continental water vapour at the long-term hydrometeorological monitoring site Rietholzbach in northeastern Switzerland. To reconstruct the phase change history of the air masses in which we measure the d signature and to diagnose its area of surface evaporation we apply an established Lagrangian moisture source diagnostic. With the help of a correlation analysis we investigate the strength of the relation between d measurements and the moisture source conditions. Temporal episodes with a duration of a few days of strong anticorrelation between d and relative humidity as well as temperature are identified. The role of plant transpiration, the large-scale advection of remotely evaporated moisture, the local boundary layer dynamics at the measurement site and recent precipitation at the site of evaporation are discussed as reasons for the existence of these modes of strong anticorrelation between d and moisture source conditions. The relation between d in atmospheric water vapour at the measurement site and the relative humidity conditions at the location of evaporation exhibits distinct characteristics for land surface evaporation and ocean evaporation. We show that the importance of continental moisture recycling and the contribution of plant transpiration to the continental evaporation flux can be deduced from the d-relative humidity relation at the seasonal timescale as well as for individual events. The slope of the relation between d and the diagnosed moisture source relative humidity provides a novel framework to estimate the transpiration fraction of land evapotranspiration at the local to continental scale. Over the whole analysis period (August to December 2011) a transpiration fraction of the evapotranspiration flux over the continental part of the moisture source region of 63% is found albeit with a large event-to-event variability (0% to 99%) for continental Europe. During days of strong local moisture recycling a higher overall transpiration fraction of 82% (varying between 65% and 94%) is found. Such Lagrangian estimates of the transpiration part of continental evaporation could potentially be useful for the verification of model estimates of this important land-atmosphere coupling parameter.

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Regional scale spatial and temporal variability of soil moisture in a prairie region

Hydrological Processes ◽

10.1002/hyp.10954 ◽

2016 ◽

Vol 30 (20) ◽

pp. 3639-3649 ◽

Cited By ~ 5

Author(s):

Travis T. Burns ◽

Aaron A. Berg ◽

Jaclyn Cockburn ◽

Erica Tetlock

Keyword(s):

Soil Moisture ◽

Temporal Variability ◽

Regional Scale ◽

Spatial And Temporal Variability

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A simple ecohydrological model captures essentials of seasonal leaf dynamics in semi-arid tropical grasslands

Biogeosciences Discussions ◽

10.5194/bgd-6-8661-2009 ◽

2009 ◽

Vol 6 (5) ◽

pp. 8661-8690

Author(s):

P. Choler ◽

W. Sea ◽

P. Briggs ◽

M. Raupach ◽

R. Leuning

Keyword(s):

Soil Moisture ◽

Plant Growth ◽

Linear Models ◽

Nonlinear Models ◽

Plant Cover ◽

Spatial And Temporal Variability ◽

Tropical Grasslands ◽

Low Dimensional ◽

Semi Arid ◽

Leaf Dynamics

Abstract. Modelling leaf phenology in water-controlled ecosystems remains a difficult task because of high spatial and temporal variability in the interaction of plant growth and soil moisture. Here, we move beyond widely used linear models to examine the performance of low-dimensional, nonlinear ecohydrological models that couple the dynamics of plant cover and soil moisture. The study area encompasses 400 000 km2 of semi-arid perennial tropical grasslands, dominated by C4 grasses, in the Northern Territory and Queensland (Australia). We prepared 8 yr time series (2001–2008) of climatic variables and estimates of fractional vegetation cover derived from MODIS Normalized Difference Vegetation Index (NDVI) for 400 randomly chosen sites, of which 25% were used for model calibration and 75% for model validation. We found that the mean absolute error of linear and nonlinear models did not markedly differ. However, nonlinear models presented key advantages: (1) they exhibited far less systematic error than their linear counterparts; (2) their error magnitude was consistent throughout a precipitation gradient while the performance of linear models deteriorated at the driest sites, and (3) they better captured the sharp transitions in leaf cover that are observed under high seasonality of precipitation. Our results showed that low-dimensional models including feedbacks between soil water balance and plant growth adequately predict leaf dynamics in semi-arid perennial grasslands. Because these models attempt to capture fundamental ecohydrological processes, they should be the favoured approach for prognostic models of phenology.

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A simple ecohydrological model captures essentials of seasonal leaf dynamics in semi-arid tropical grasslands

Biogeosciences ◽

10.5194/bg-7-907-2010 ◽

2010 ◽

Vol 7 (3) ◽

pp. 907-920 ◽

Cited By ~ 34

Author(s):

P. Choler ◽

W. Sea ◽

P. Briggs ◽

M. Raupach ◽

R. Leuning

Keyword(s):

Soil Moisture ◽

Plant Growth ◽

Linear Models ◽

Nonlinear Models ◽

Plant Cover ◽

Spatial And Temporal Variability ◽

Tropical Grasslands ◽

Low Dimensional ◽

Semi Arid ◽

Leaf Dynamics

Abstract. Modelling leaf phenology in water-controlled ecosystems remains a difficult task because of high spatial and temporal variability in the interaction of plant growth and soil moisture. Here, we move beyond widely used linear models to examine the performance of low-dimensional, nonlinear ecohydrological models that couple the dynamics of plant cover and soil moisture. The study area encompasses 400 000 km2 of semi-arid perennial tropical grasslands, dominated by C4 grasses, in the Northern Territory and Queensland (Australia). We prepared 8-year time series (2001–2008) of climatic variables and estimates of fractional vegetation cover derived from MODIS Normalized Difference Vegetation Index (NDVI) for 400 randomly chosen sites, of which 25% were used for model calibration and 75% for model validation. We found that the mean absolute error of linear and nonlinear models did not markedly differ. However, nonlinear models presented key advantages: (1) they exhibited far less systematic error than their linear counterparts; (2) their error magnitude was consistent throughout a precipitation gradient while the performance of linear models deteriorated at the driest sites, and (3) they better captured the sharp transitions in leaf cover that are observed under high seasonality of precipitation. Our results showed that low-dimensional models including feedbacks between soil water balance and plant growth adequately predict leaf dynamics in semi-arid perennial grasslands. Because these models attempt to capture fundamental ecohydrological processes, they should be the favoured approach for prognostic models of phenology.

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Deuterium excess as a proxy for continental moisture recycling and plant transpiration

Atmospheric Chemistry and Physics ◽

10.5194/acp-14-4029-2014 ◽

2014 ◽

Vol 14 (8) ◽

pp. 4029-4054 ◽

Cited By ~ 78

Author(s):

F. Aemisegger ◽

S. Pfahl ◽

H. Sodemann ◽

I. Lehner ◽

S. I. Seneviratne ◽

...

Keyword(s):

Boundary Layer ◽

Relative Humidity ◽

Deuterium Excess ◽

Parameter Uncertainties ◽

Fractionation Factor ◽

Data Set ◽

Plant Transpiration ◽

Moisture Source ◽

Moisture Recycling ◽

Boundary Layer Dynamics

Abstract. Studying the evaporation process and its link to the atmospheric circulation is central for a better understanding of the feedbacks between the surface water components and the atmosphere. In this study, we use 5 months of deuterium excess (d) measurements at the hourly to daily timescale from a cavity ring-down laser spectrometer to characterise the evaporation source of low-level continental water vapour at the long-term hydrometeorological monitoring site Rietholzbach in northeastern Switzerland. To reconstruct the phase change history of the air masses in which we measure the d signature and to diagnose its area of surface evaporation we apply a Lagrangian moisture source diagnostic. With the help of a correlation analysis we investigate the strength of the relation between d measurements and the moisture source conditions. Temporal episodes with a duration of a few days of strong anticorrelation between d and relative humidity as well as temperature are identified. The role of plant transpiration, the large-scale advection of remotely evaporated moisture, the local boundary layer dynamics at the measurement site and recent precipitation at the site of evaporation are discussed as reasons for the existence of these modes of strong anticorrelation between d and moisture source conditions. We show that the importance of continental moisture recycling and the contribution of plant transpiration to the continental evaporation flux may be deduced from the d–relative humidity relation at the seasonal timescale as well as for individual events. The methodology and uncertainties associated with these estimates of the transpiration fraction of evapotranspiration are presented and the proposed novel framework is applied to individual events from our data set. Over the whole analysis period (August to December 2011) a transpiration fraction of the evapotranspiration flux over the continental part of the moisture source region of 62% is found albeit with a large event-to-event variability (0% to 89%) for continental Europe. During days of strong local moisture recycling a higher overall transpiration fraction of 76% (varying between 65% and 86%) is found. These estimates are affected by uncertainties in the assumptions involved in our method as well as by parameter uncertainties. An average uncertainty of 11% results from the strong dependency of the transpiration estimates on the choice of the non-equilibrium fractionation factor. Other uncertainty sources like the influence of boundary layer dynamics are probably large but more difficult to quantify. Nevertheless, such Lagrangian estimates of the transpiration part of continental evaporation could potentially be useful for the verification of model estimates of this important land–atmosphere coupling parameter.

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Root Reinforcement in Slope Stability Models: A Review

Geosciences ◽

10.3390/geosciences11050212 ◽

2021 ◽

Vol 11 (5) ◽

pp. 212

Author(s):

Elena Benedetta Masi ◽

Samuele Segoni ◽

Veronica Tofani

Keyword(s):

Soil Moisture ◽

Slope Stability ◽

Regional Scale ◽

Moisture Gradient ◽

Future Research ◽

Spatial And Temporal Variability ◽

Soil Behavior ◽

Root Reinforcement ◽

Soil Moisture Gradient ◽

Root Strength

The influence of vegetation on mechanical and hydrological soil behavior represents a significant factor to be considered in shallow landslides modelling. Among the multiple effects exerted by vegetation, root reinforcement is widely recognized as one of the most relevant for slope stability. Lately, the literature has been greatly enriched by novel research on this phenomenon. To investigate which aspects have been most treated, which results have been obtained and which aspects require further attention, we reviewed papers published during the period of 2015–2020 dealing with root reinforcement. This paper—after introducing main effects of vegetation on slope stability, recalling studies of reference—provides a synthesis of the main contributions to the subtopics: (i) approaches for estimating root reinforcement distribution at a regional scale; (ii) new slope stability models, including root reinforcement and (iii) the influence of particular plant species, forest management, forest structure, wildfires and soil moisture gradient on root reinforcement. Including root reinforcement in slope stability analysis has resulted a topic receiving growing attention, particularly in Europe; in addition, research interests are also emerging in Asia. Despite recent advances, including root reinforcement into regional models still represents a research challenge, because of its high spatial and temporal variability: only a few applications are reported about areas of hundreds of square kilometers. The most promising and necessary future research directions include the study of soil moisture gradient and wildfire controls on the root strength, as these aspects have not been fully integrated into slope stability modelling.

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Control on millennial scale events (H-events) inferred from triple oxygen isotope ratios of speleothems from a Northeast Indian cave

10.5194/egusphere-egu2020-7701 ◽

2020 ◽

Author(s):

Sasadhar Mahata ◽

Pengzhen Duan ◽

Lijuan Sha ◽

Jonathan Baker ◽

Gayatri Kathayat ◽

...

Keyword(s):

Relative Humidity ◽

Oxygen Isotope ◽

Asian Summer Monsoon ◽

Source Region ◽

Precipitation Amount ◽

Large Temperature ◽

Tropical Ocean ◽

Moisture Source ◽

Secondary Sources ◽

Moisture Recycling

The carbonate 17O anomaly (&#916;17O) has recently been developed as a geochemical proxy for estimating the relative humidity of moisture at the source point of evaporation, which can be a vital tool in paleoclimate research. Speleothem &#916;17O variability in particular may provide a quantitative constraint on moisture regimes at millennial and orbital timescales&#8212;far longer than can be addressed by analyzing 17O in other materials, such as tree rings. Modern observations and calibration studies have established a robust negative correlation between the &#916;17O (17O excess) of rainfall and relative humidity, so that &#916;17O is enhanced during arid conditions at the moisture source. Herein, we report novel triple oxygen isotope data across the Last Glacial in speleothems collected from Cherrapunji Cave, northeastern India. Triple oxygen isotope measurements were obtained by an O2-CO2 Pt-catalyzed oxygen-isotope equilibration method. Preliminary results suggest lower &#916;17O in speleothems during cold periods (e.g. Heinrich Events), which would imply higher relative humidity over the oceanic moisture source. Importantly, higher source relative humidity does not necessarily imply changes in precipitation amount at the cave site. While it is possible that a substantial geographic shift in the moisture source region (or additional contributions from secondary sources) could obfuscate the &#916;17O signal, we argue that this explanation is unlikely for our study site. Alternatively, we cannot exclude the effects of excessive moisture recycling in the tropical ocean, which can enhance the 17O anomaly in cloud vapor (particularly if combined with large temperature swings) and thereby alter speleothem &#916;17O. To refine our interpretation of the &#916;17O signal in Cerrapunji Cave samples, we investigate multiple cold periods, as well as coeval samples from the Asian Summer Monsoon region. Finally, we compare our results with novel data from westernmost Asia, where temperature variations during cold events are more likely to be accompanied by large shifts in predominant moisture source, due to the migration of wintertime westerlies.

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High-resolution soil moisture mapping through the use of Cosmic-Ray Neutron Sensor and Sentinel-1 data for temperate and semi-arid environments

10.5194/egusphere-egu21-9688 ◽

2021 ◽

Author(s):

Hami Said ◽

Modou Mbaye ◽

Lee Kheng Heng ◽

Emil Fulajtar ◽

Georg Weltin ◽

...

Keyword(s):

Water Management ◽

Remote Sensing ◽

Soil Moisture ◽

Regional Scale ◽

Cosmic Ray ◽

Microwave Remote Sensing ◽

Nuclear Technology ◽

Agricultural Water ◽

Agricultural Water Management ◽

Semi Arid

Global climate change has a major impact on the availability of water in agriculture. Sustainable agricultural productivity to ensure food security requires good agricultural water management.Soil moisture is one of the important variables in irrigation management, and there are many different techniques for estimating it at different scales, from point to landscape scales.Cosmic-Ray Neutron Sensor (CRNS) technology has the capability to estimate field-scale soil moisture (SM) in large areas of up to 20 to 30 ha and has demonstrated its ability to support agricultural water management and hydrology studies. However, measurement of soil moisture on a global or regional scale can only be achieved from satellite remote sensing.Recently, active microwave remote sensing Synthetic Aperture Radar (SAR) imaging from Sentinel-1 shows great potential for high spatial resolution soil moisture monitoring and can be the basis for producing soil moisture maps. However, these maps can be only used after calibration. Such calibration can be done through traditional, point soil moisture sampling or measurement, which is time-consuming and costly. CRNS technology can be used for calibration and validation remote sensing imagery predictions at field and area-wide level.In this study a conversion model to retrieve soil moisture from Sentinel-1 (SAR) was developed using the VV (vertical-vertical) polarization, which is highly sensitive to soil moisture, and then calibrated and validated using CRNS data from temperate (Austria) and semi-arid (Kuwait) Environments. This study is a major step in the monitoring of soil moisture at high spatial and temporal resolution by combining remote sensing and the CRNS based nuclear technology. The preliminary results show the great potential of using nuclear technology such as CRNS for remote sensing calibration of Sentinel-1 (SAR).

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Drying and Wetting Trends and Vegetation Covariations in the Drylands of China

Water ◽

10.3390/w12040933 ◽

2020 ◽

Vol 12 (4) ◽

pp. 933 ◽

Cited By ~ 2

Author(s):

Chuanzhuang Liang ◽

Tiexi Chen ◽

Han Dolman ◽

Tingting Shi ◽

Xueqiong Wei ◽

...

Keyword(s):

Soil Moisture ◽

Arid Region ◽

Vegetation Index ◽

Root Zone ◽

Regional Scale ◽

Climatic Research Unit ◽

Drought Indices ◽

Drought Severity ◽

Semi Arid Region ◽

Semi Arid

The semi-arid and arid drylands of China, which are located in the inland region of Eurasia, have experienced rapid climate change. Some regions in particular, have shown upward trends in the observational records of precipitation. However, there is more to drying and wetting than just changes in precipitation which still have large uncertainties. Coherent results, however, can be obtained, at the regional scale, with the use of multiple indices as shown in the recent literature. We divided the drylands of China into three sub-regions, i.e., a semi-arid (SA), an eastern-arid (EA) and a western-arid (WA) region. Precipitation from the China Meteorological Administration (CMA) and Climatic Research Unit (CRU), statistical and physical drought indices, including the Standardized Precipitation Evapotranspiration Index (SPEI), the Palmer Drought Severity Index (PDSI), self-calibrating PDSI (sc_PDSI), Root zone soil moisture (Root_sm) and Surface soil moisture (Surf_sm) from Global Land Evaporation Amsterdam Model (GLEAM), and Normalized Difference Vegetation Index (NDVI) were used to identify temporal and spatial patterns in drying and wetting. Data were selected from 1982–2012, in line with the availability of the remotely sensed vegetation data. Results show that the drylands of China exhibits a pattern of wetting in the west and drying in the east. The semi-arid region in the east is becoming drier and the drought area is increasing, with the values of CMA_P, CRU_P, PDSI, sc_PDSI, SPEI-01,SPEI-06, SPEI-12, Root_sm, Surf_sm at −1.064 mm yr−1, −0.834 mm yr−1, −0.050 yr−1 (p < 0.1), −0.174 yr−1 (p < 0.1), −0.014 yr−1, −0.06, −0.021 (p < 0.1), −0.257×10−3 m3 m−3 yr−1, −0.024×10−3 m3 m−3 yr−1, respectively. The arid region generally exhibits a wetting trend, while the area in drought declines only in the western arid region, but not in the eastern arid part. In the semi-arid region, growing season (May to September) NDVI is significantly correlated (p < 0.1) with eight out of nine indicators. We show in this study that the semi-arid region needs more study to protect the vegetation ecosystem and the water resources.

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