Simulation of evapotranspiration and carbon dioxide flux in the wheat‐maize rotation croplands of the North China Plain using the Simple Biosphere Model

The North China Plain, which is critical for food production in China, is encountering serious water shortage due to heavy agricultural water requirement. The accurate modelling of carbon dioxide flux and evapotranspiration (ET) in croplands is thus essential for yield prediction and water resources management. The land surface model is powerful in simulating energy and carbon dioxide fluxes between land and atmosphere. Some key processes in the Simple Biosphere Model (Version 2, SiB2) were parameterized based on the observations. The simulated fluxes were tested against the eddy covariance flux measurements over two typical winter wheat/maize double cropping fields. A simple diagnostic parameterisation of soil respiration, not included in SiB2, was added and calibrated using the observations to model the carbon budget. The Ball‐Berry stomatal conductance model was calibrated using observed leaf gas exchange rate, showing that the original SiB2 could significantly underpredict the ET in the wheat field. SiB2 significantly underpredicted soil resistance at the Weishan site, leading to overpredict the ET. Overall, there was a close agreement between the simulated and observed latent heat fluxes and net CO₂ exchange using the re‐parameterized SiB2. These findings are important when the model is used for the regional simulation in the North China Plain. Copyright © 2011 John Wiley & Sons, Ltd.     

Simulating CO2 flux of three different ecosystems in ChinaFLUX based on artificial neural networks

The nonlinearity of the relationship between CO₂ flux and other micrometeorological variables flux parameters limits the applicability of carbon flux models to accurately estimate the flux dynamics. However, the need for carbon dioxide (CO₂) estimations covering larger areas and the limitations of the point eddy covariance technique to address this requirement necessitates the modeling of CO₂ flux from other micrometeorological variables. Artificial neural networks (ANN) are used because of their power to fit highly nonlinear relations between input and output variables without explaining the nature of the phenomena. This paper applied a multilayer perception ANN technique with error back propagation algorithm to simulate CO₂ flux on three different ecosystems (forest, grassland and cropland) in ChinaFLUX. Energy flux (net radiation, latent heat, sensible heat and soil heat flux) and temperature (air and soil) and soil moisture were used to train the ANN and predict the CO₂ flux. Diurnal half-hourly fluxes data of observations from June to August in 2003 were divided into training, validating and testing. Results of the CO₂ flux simulation show that the technique can successfully predict the observed values with R ² value between 0.75 and 0.866. It is also found that the soil moisture could not improve the simulative accuracy without water stress. The analysis of the contribution of input variables in ANN shows that the ANN is not a black box model, it can tell us about the controlling parameters of NEE in different ecosystems and micrometeorological environment. The results indicate the ANN is not only a reliable, efficient technique to estimate regional or global CO₂ flux from point measurements and understand the spatiotemporal budget of the CO₂ fluxes, but also can identify the relations between the CO₂ flux and micrometeorological variables.

     

A simple method using climatic variables to estimate canopy temperature,sensible and latent heat fluxes in a winter wheat field on the North China Plain

Estimation of evapotranspiration from a crop field is of great importance for detecting crop water status and proper irrigation scheduling. The Penman–Monteith equation is widely viewed as the best method to estimate evapotranspiration but it requires canopy resistance, which is very difficult to determine in practice. This paper presents a simple method simplified from the Penman–Monteith equation for estimating canopy temperature (T_c). The proposed method is a biophysically‐sound extended version of that proposed by Todorovic. The estimated canopy temperature is used to calculate sensible heat flux, and then latent heat flux is calculated as the residual of the surface energy balance. An eddy covariance (EC) system and an infrared thermometer (IRT) were installed in an irrigated winter wheat field on the North China Plain in 2004 and 2005, to measure T_c, and sensible and latent heat fluxes were used to test the modified Todorovic model (MTD). The results indicate that the original Todorovic model (TD) severely underestimates T_c and sensible heat flux, and hence severely overestimates the latent heat flux. However, the MTD model has good capability for estimating T_c, and gives acceptable results for latent heat flux at both half‐hourly and daily scales. The MTD model results also agreed well with the evapotranspiration calculated from the measured T_c. Copyright © 2008 John Wiley & Sons, Ltd.     

Water, heat fluxes and water use efficiency measurement and modeling above a farmland in the North China Plain

Net radiation (Rn), water vapor flux (LE), sensible heat flux (Hs) and soil heat flux (G)were measured above a summer maize field with the eddy-covariance technique, simulation and analysis of water, heat fluxes and crop water use efficiency were made with the RZ-SHAW model at the same time in this study. The results revealed significant diurnal and seasonal variability of water vapor flux for summer maize. Most part of Rn was consumed by the evapotranspiration of the summer maize. The proportion of water vapor flux to net radiation ((LE/Rn) increased with the crop development and peaked around milk-filling stage with a value of 60%, a slightly lower than that obtained by the RZ-SHAW model. Daily evapotranspiration estimated by the model agreed with the results measured with the eddy-covariance technique, indices of agreement (IA) for hourly water vapor fluxes simulated and measured were above 0.75, root mean square errors (RMSE) were no more than 1.0. Diurnal patterns of Hs showed the shape of inverted "U" shifted to the forenoon with a maximum value around 11:30 (Beijing time), while LE exhibited an inverted "V" with a maximum value at around 13:00, about an hour later than Hs. Diurnal change of CO2showed an asymmetrical "V" curve and its maximal rates occurred at about 11:30. Variations of water use efficiency during the phonological stages of the summer maize showed a rapid increase with the photosynthetic photon flux density (PPFD) after sunrise, a state of equilibrium around 10:00 followed a decrease. Maximum values of water use efficiency were 24.3, and its average value ranged from 7.6 to 10.3 g kg-1.     

A comparison between simulated and measured CO2 and water flux in a subtropical coniferous forest

Using data from eddy covariance measurements in a subtropical coniferous forest, a test and evaluation have been made for the model of Carbon Exchange in the Vegetation-Soil-Atmosphere (CEVSA) that simulates energy transfers and water, carbon and nitrogen cycles based on ecophysiological processes. In the present study, improvement was made in the model in calculating LAI, carbon allocation among plant organs, litter fall, decomposition and evapotranspiration. The simulated seasonal variations in carbon and water vapor flux were consistent with the measurements. The model explained 90% and 86% of the measured variations in evapotranspiration and soil water content. However, the modeled evapotranspiration and soil water content were lower than the measured systematically, because the model assumed that water was lost as runoff if it was beyond the soil saturation water content, but the soil at the flux site with abundant rainfall is often above water saturated. The improved model reproduced 79% and 88% of the measured variations in gross primary production (GPP) and ecosystem respiration (R _e), but only 31% of the variations in measured net ecosystem exchange (NEP) despite the fact that the modeled annual NEP was close to the observation. The modeled NEP was generally lower in winter and higher in summer than the observations. The simulated responses of photosynthesis and respiration to water vapor deficit at high temperatures were different from measurements. The results suggested that the improved model underestimated ecosystem photosynthesis and respiration in extremely condition. The present study shows that CEVSA can simulate the seasonal pattern and magnitude of CO₂ and water vapor fluxes, but further improvement in simulating photosynthesis and respiration at extreme temperatures and water deficit is required.

     

Spatio‐temporal variability of water and energy fluxes – a case study for a mesoscale catchment in pre‐alpine environment

Water and energy fluxes at and between the land surface, the subsurface and the atmosphere are inextricably linked over all spatio‐temporal scales. Our research focuses on the joint analysis of both water and energy fluxes in a pre‐alpine catchment (55 km²) in southern Germany, which is part of the Terrestrial Environmental Observatories (TERENO). We use a novel three‐dimensional, physically based and distributed modelling approach to reproduce both observed streamflow as an integral measure for water fluxes and heat flux and soil temperature measurements at an observation location over a period of 2 years. While heat fluxes are often used for comparison of the simulations of one‐dimensional land surface models, they are rarely used for additional validation of physically based and distributed hydrological modelling approaches. The spatio‐temporal variability of the water and energy balance components and their partitioning for dominant land use types of the study region are investigated. The model shows good performance for simulating daily streamflow (Nash–Sutcliffe efficiency > 0.75). Albeit only streamflow measurements are used for calibration, the simulations of hourly heat fluxes and soil temperatures at the observation site also show a good performance, particularly during summer. A limitation of the model is the simulation of temperature‐driven heat fluxes during winter, when the soil is covered by snow. An analysis of the simulated spatial fields reveals heat flux patterns that reflect the distribution of the land use and soil types of the catchment. The water and energy partitioning is characterized by a strong seasonal cycle and shows clear differences between the selected land use types. Copyright © 2016 The Authors Hydrological Processes Published by John Wiley & Sons Ltd.     

Integrating a model with remote sensing observations by a data assimilation approach to improve the model simulation accuracy of carbon flux and evapotranspiration at two flux sites

Model simulation and in situ observations are often used to research water and carbon cycles in terrestrial ecosystems, but each of these methods has its own advantages and limitations. Combining these two methods could improve the accuracy of quantifying the dynamics of the water and carbon fluxes of an ecosystem. Data assimilation is an effective means of integrating modeling with in situ observation. In this study, the ensemble Kalman filter(En KF) and the unscented Kalman filter(UKF) algorithms were used to assimilate remotely sensed leaf area index(LAI) data with the Biome-BGC model to simulate water and carbon fluxes at the Harvard Forest Environmental Monitoring Site(EMS) and the Dinghushan site. After MODIS LAI data from 2000–2004 were assimilated into the improved Biome-BGC model using the En KF algorithm at the Harvard Forest site, the R2 between the simulated and observed results for NEE and evapotranspiration increased by 7.8% and 4.7%, respectively. In addition, the sum of the absolute error(SAE) and the root mean square error(RMSE) of NEE decreased by an average of 21.9% and 26.3%, and the SAE and RMSE of evapotranspiration decreased by 24.5% and 25.5%, respectively. MODIS LAI data of 2003 were assimilated into the Biome-BGC model for the Dinghushan site, and the R2 values between the simulated and observed results for NEE and evapotranspiration were increased by 6.7% and 17.3%, respectively. In addition, the SAE values of NEE and ET were decreased by 11.3% and 30.7%, respectively, and the RMSE values of NEE and ET decreased by 10.1% and 30.9%, respectively. These results demonstrate that the accuracy of carbon and water flux simulations can be effectively improved when remotely sensed LAI data are properly integrated with ecosystem models through a data assimilation approach.     

Diurnal cycle of surface energy fluxes in high mountain terrain: High-resolution fully coupled atmosphere-hydrology modelling and impact of lateral flow

Water and energy fluxes are inextricably interlinked within the interface of the land surface and the atmosphere. In the regional earth system models, the lower boundary parameterization of land surface neglects lateral hydrological processes, which may inadequately depict the surface water and energy fluxes variations, thus affecting the simulated atmospheric system through land-atmosphere feedbacks. Therefore, the main objective of this study is to evaluate the hydrologically enhanced regional climate modelling in order to represent the diurnal cycle of surface energy fluxes in high spatial and temporal resolution. In this study, the Weather Research and Forecasting model (WRF) and coupled WRF Hydrological modelling system (WRF-Hydro) are applied in a high alpine catchment in Northeastern Tibetan Plateau, the headwater area of the Heihe River. By evaluating and intercomparing model results by both models, the role of lateral flow processes on the surface energy fluxes dynamics is investigated. The model evaluations suggest that both WRF and coupled WRF-Hydro reasonably represent the diurnal variations of the near-surface meteorological fields, surface energy fluxes and hourly partitioning of available energy. By incorporating additional lateral flow processes, the coupled WRF-Hydro simulates higher surface soil moisture over the mountainous area, resulting in increased latent heat flux and decreased sensible heat flux of around 20–50 W/m² in their diurnal peak values during summertime, although the net radiation and ground heat fluxes remain almost unchanged. The simulation results show that the diurnal cycle of surface energy fluxes follows the local terrain and vegetation features. This highlights the importance of consideration of lateral flow processes over areas with heterogeneous terrain and land surfaces.     

Long-term analysis of measured and simulated evapotranspiration and soil water content

In our study, we analysed a period from 2003 to 2012 with micrometeorological data measured at a boundary-layer field site operated by the Lindenberg Meteorological Observatory – Richard-Aßmann-Observatory of the German Meteorological Service (DWD). Amongst others, these data consist of real evapotranspiration (ETr) rates measured by eddy covariance and soil water contents determined by time domain reflectometry. Measured ETr and soil water contents were compared with those simulated by a simple soil–vegetation–atmosphere transfer (SVAT) scheme consisting of the FAO56 Penman-Monteith equation and the soil water flux model Hydrus-1D. We applied this SVAT scheme using uncompensatory and compensatory root water uptake (RWU). Soil water contents and ETr rates calculated using uncompensatory RWU showed an acceptable fit to the measured ones. In comparison, the use of compensatory RWU resulted in lower model performance due to higher deviations between measured and simulated soil moisture values and ETr rates during dry summer periods.     

Modelling non-stationary water ages in a tropical rainforest: A preliminary spatially distributed assessment

Pristine tropical forests play a critical role in regional and global climate systems. For a better understanding of the eco-hydrology of tropical “evergreen” vegetation, it is essential to know the partitioning of water into transpiration and evaporation, runoff and associated water ages. For this purpose, we evaluated how topography and vegetation influence water flux and age dynamics at high temporal (hourly) and spatial (10 m) resolution using the Spatially Distributed Tracer-Aided Rainfall-Runoff model for the tropics (STARRtropics). The model was applied in a tropical rainforest catchment (3.2 km²) where data were collected biweekly to monthly and during intensive monitoring campaigns from January 2013 to July 2018. The STARRtropics model was further developed, incorporating an isotope mass balance for evapotranspiration partitioning into transpiration and evaporation. Results exhibited a rapid streamflow response to rainfall inputs (water and isotopes) with limited mixing and a largely time-invariant baseflow isotope composition. Simulated soil water storage showed a transient response to rainfall inputs with a seasonal component directly resembling the streamflow dynamics which was independently evaluated using soil water content measurements. High transpiration fluxes (max 7 mm/day) were linked to lower slope gradients, deeper soils and greater leaf area index. Overall water partitioning resulted in 65% of the actual evapotranspiration being driven by vegetation with high transpiration rates over the drier months compared to the wet season. Time scales of water age were highly variable, ranging from hours to a few years. Stream water ages were conceptualized as a mixture of younger soil water and slightly older, deeper soil water and shallow groundwater with a maximum age of roughly 2 years during drought conditions (722 days). The simulated soil water ages ranged from hours to 162 days and for shallow groundwater up to 1,200 days. Despite the model assumptions, experimental challenges and data limitation, this preliminary spatially distributed model study enhances knowledge about the water ages and overall young water dominance in a tropical rainforest with little influence of deeper and older groundwater.     

Modelling hourly evapotranspiration and soil water content at the grass-covered boundary-layer field site Falkenberg,Germany

Abstract

The study analyses a 2-year period of hourly rates of real evapotranspiration (ETr) derived from eddy covariance measurements and soil water contents at depths from 8 to 90 cm, monitored by time domain reflectometry probes at the grass-covered boundary-layer field site Falkenberg of the Lindenberg Meteorological Observatory – Richard-Aßmann-Observatory, operated by the German Meteorological Service (DWD). The ETr rates and soil water contents were compared with the results of a modelling approach consisting of the Penman-Monteith equation and the soil water balance model Hydrus-1D using a noncompensatory and a compensatory root-water uptake model. After optimization of soil hydraulic parameters by inverse modelling, using measured soil water contents as the objective function, simulated and measured model outputs showed good agreement for soil water contents above 90 cm depth and for ETr rates simulated by our modelling approaches using noncompensatory root-water uptake. The application of a compensatory root-water uptake model led to a decrease in the simulation quality for the total investigation period.

Editor Z.W. Kundzewicz

Citation Wegehenkel, M. and Beyrich, F., 2014. Modelling of hourly evapotranspiration and soil water content at the grass-covered boundary-layer field site Falkenberg, Germany. Hydrological Sciences Journal, 59 (2), 376–394.     

Energy flux partitioning and evapotranspiration in a sub‐alpine spruce forest ecosystem

In this study, we examined the year 2011 characteristics of energy flux partitioning and evapotranspiration of a sub‐alpine spruce forest underlain by permafrost on the Qinghai–Tibet Plateau (QPT). Energy balance closure on a half‐hourly basis was H + λE = 0.81 × (R_n ? G ? S) + 3.48 (W m^?2) (r² = 0.83, n = 14938), where H, λE, R_n, G and S are the sensible heat, latent heat, net radiation, soil heat and air‐column heat storage fluxes, respectively. Maximum H was higher than maximum λE, and H dominated the energy budget at midday during the whole year, even in summer time. However, the rainfall events significantly affected energy flux partitioning and evapotranspiration. The mean value of evaporative fraction (Λ = λE/(λE + H)) during the growth period on zero precipitation days and non‐zero precipitation days was 0.40 and 0.61, respectively. The mean daily evapotranspiration of this sub‐alpine forest during summer time was 2.56 mm day^?1. The annual evapotranspiration and sublimation was 417 ± 8 mm year^?1, which was very similar to the annual precipitation of 428 mm. Sublimation accounted for 7.1% (30 ± 2 mm year^?1) of annual evapotranspiration and sublimation, indicating that the sublimation is not negligible in the annual water balance in sub‐alpine forests on the QPT. The low values of the Priestley–Taylor coefficient (α) and the very low value of the decoupling coefficient (Ω) during most of the growing season suggested low soil water content and conservative water loss in this sub‐alpine forest. Copyright © 2013 John Wiley & Sons, Ltd.     

Modelling plant canopy effects on annual variability of evapotranspiration and heat fluxes for a semi‐arid grassland on the southern periphery of the Eurasian cryosphere in Mongolia

The ability to predict vegetation cover effects on thermal/water regimes can enhance our understanding of canopy controls on evapotranspiration. The Simultaneous Heat and Water (SHAW) model is a detailed process model of heat and water movement in a snow–residue–soil system. This paper describes provisions added to the SHAW model for vegetation cover and simulation of heat and water transfer through the soil–plant–air continuum. The model was applied to four full years (May 2003–April 2007) of data collected on sparse grassland at Nalaikh in north‐eastern Mongolia. Simulated soil temperature and radiation components agreed reasonably well with measured values. The absolute differences between simulated and measured soil temperatures were larger at both the surface layer and deeper layer, but relatively smaller in the layer from 0·8 to 2·4 m. Radiation components were mimicked by the SHAW model with model efficiency (ME) reaching 0·93–0·72. Latent and sensible heat fluxes were simulated well with MEs of 0·93 and 0·87, respectively. The vegetation control on evapotranspiration was investigated by sensitivity experiments of model performance with changing leaf area index (LAI) values but constant of other variables. The results suggest that annual evapotranspiration ranged from 16 to ? 22% in response to extremes of doubled and zero LAI. Copyright © 2010 John Wiley & Sons, Ltd.     

Hydrometeorological behaviour of pine and larch forests in eastern Siberia

Seasonal changes in the water and energy exchanges over a pine forest in eastern Siberia were investigated and compared with published data from a nearby larch forest. Continuous observations (April to August 2000) were made of the eddy‐correlation sensible heat flux and latent heat flux above the canopy. The energy balance was almost closed, although the sum of the turbulent fluxes sometimes exceeded the available energy flux (R_n ? G) when the latent heat flux was large; this was related to the wind direction. We examined the seasonal variation in energy balance components at this site. The seasonal variation and magnitude of the sensible heat flux (H) was similar to that of the latent heat flux (λE), with maximum values occurring in mid‐June. Consequently, the Bowen ratio was around 1·0 on many days during the study period. On some clear days just after rainfall, λE was very large and the sum of H and λE exceeded R_n ? G. The evapotranspiration rate above the dry canopy from May to August was 2·2 mm day^?1. The contributions of understory evapotranspiration (E_u) and overstory transpiration (E_o) to the evapotranspiration of the entire ecosystem (E_t) were both from 25 to 50% throughout the period analysed. These results suggest that E_u plays a very important role in the water cycle at this site. From snowmelt through the tree growth season (23 April to 19 August 2000), the total incoming water, comprised of the sum of precipitation and the water equivalent of the snow at the beginning of the melt season, was 228 mm. Total evapotranspiration from the forest, including interception loss and evaporation from the soil when the canopy was wet, was 208–254 mm. The difference between the incoming and outgoing amounts in the water balance was from +20 to ?26 mm. The water and energy exchanges of the pine and larch forest differed in that λE and H increased slowly in the pine forest, whereas λE increased rapidly in the larch forest and H decreased sharply after the melting season. Consequently, the shape of the Bowen ratio curves at the two sites differed over the period analysed, as a result of the differences in the species in each forest and in soil thawing. Copyright © 2003 John Wiley & Sons, Ltd.     

CO2 absorption of sandy soil induced by rainfall pulses in a desert ecosystem

Soil CO₂ flux is strongly influenced by precipitation in many ecosystem types, yet knowledge of the effects of precipitation on soil CO₂ flux in semi‐arid desert ecosystems remains insufficient, particularly for sandy soils. To address this, we investigated the response of sandy soil CO₂ flux to rainfall pulses in a desert ecosystem in northern China during August–September 2011. Significant changes (P < 0.05) were found in diel patterns of soil CO₂ flux induced by small (2.1 mm), moderate (12.4 mm) and large (19.7 mm) precipitation events. Further analysis indicated that rainfall pulses modified the response of soil CO₂ flux to soil temperature, including hysteresis between soil CO₂ flux and soil temperature, with F_s higher when T_s was increasing than when T_s was decreasing, and the linear relationship between them. Moreover, our results showed that rainfall could result in absorption of atmospheric CO₂ by soil, possibly owing to mass flow of CO₂ induced by a gradient of gas pressure between atmosphere and soil. After each precipitation event, soil CO₂ flux recovered exponentially to pre‐rainfall levels with time, with the recovery times exhibiting a positive correlation with precipitation amount. On the basis of the amounts of precipitation that occurred at our site during the measurement period (August–September), the accumulated rain‐induced carbon absorption evaluated for rainy days was 1.068 g C m^?2; this corresponds approximately to 0.5–2.1% of the net primary production of a typical desert ecosystem. Thus, our results suggest that rainfall pulses can strongly influence carbon fluxes in desert ecosystems. Copyright © 2014 John Wiley & Sons, Ltd.     

Evaluating the dual‐boundary forcing concept in subsurface–land surface interactions of the hydrological cycle

Subsurface and land surface processes (e.g. groundwater flow, evapotranspiration) of the hydrological cycle are connected via complex feedback mechanisms, which are difficult to analyze and quantify. In this study, the dual‐boundary forcing concept that reveals space–time coherence between groundwater dynamics and land surface processes is evaluated. The underlying hypothesis is that a simplified representation of groundwater dynamics may alter the variability of land surface processes, which may eventually affect the prognostic capability of a numerical model. A coupled subsurface–land surface model ParFlow.CLM is applied over the Rur catchment, Germany, and the mass and energy fluxes of the coupled water and energy cycles are simulated over three consecutive years considering three different lower boundary conditions (dynamic, constant, and free‐drainage) based on groundwater dynamics to substantiate the aforementioned hypothesis. Continuous wavelet transform technique is applied to analyze scale‐dependent variability of the simulated mass and energy fluxes. The results show clear differences in temporal variability of latent heat flux simulated by the model configurations with different lower boundary conditions at monthly to multi‐month time scales (~32–91 days) especially under soil moisture limited conditions. The results also suggest that temporal variability of latent heat flux is affected at even smaller time scales (~1–3 days) if a simple gravity drainage lower boundary condition is considered in the coupled model. This study demonstrates the importance of a physically consistent representation of groundwater dynamics in a numerical model, which may be important to consider in local weather prediction models and water resources assessments, e.g. drought prediction. Copyright © 2015 John Wiley & Sons, Ltd.     

Response of water and energy exchange to the environmental variable in a desert‐oasis wetland of Northwest China

A case study on a desert‐oasis wetland ecosystem in the arid region of Northwest China measured the seasonal and interannual variation in energy partitioning and evapotranspiration to analyse the response of water and energy exchange on soil moisture, groundwater, and environmental variables. Energy partitioning showed a clear seasonal and interannual variability, and the process of water and energy exchange differed significantly in the monthly and interannual scales. The net radiation was 7.31 MJ m^?2· day^?1, and sensible heat flux accounted for 50.42% of net radiation in energy fluxes, 40.56% for latent heat flux, and 9.02% for ground heat flux. The parameters in energy fluxes were best described by a unimodal curve, whereas sensible heat flux followed a bimodal curve. Variations in the daily evapotranspiration and crop evapotranspiration also exhibited a single peak curve with annual values of 569.84 and 644.47 mm, respectively. Canopy conductance averaged 20.77 ± 13.75 mm s^?1 and varied from 0.16 to 83.96 mm s^?1 during the two hydrological years. The variation in water and energy exchange reflected environmental conditions and depended primarily on vapour pressure deficit, net radiation, soil moisture, and water depth. Although the effects of precipitation on evapotranspiration showed that the response of this ecosystem to climate changes was not obvious, the variation of air temperatures had a strong influence on evapotranspiration, resulting in a significant increase in evapotranspiration (R = 0.730; P < 0.01). Copyright © 2013 John Wiley & Sons, Ltd.     

Eddy covariance imaging of diffuse volcanic CO<Subscript>2</Subscript> emissions at Mammoth Mountain,CA, USA

Use of eddy covariance (EC) techniques to map the spatial distribution of diffuse volcanic CO₂ fluxes and quantify CO₂ emission rate was tested at the Horseshoe Lake tree-kill area on Mammoth Mountain, California, USA. EC measurements of CO₂ flux were made during September–October 2010 and ranged from 85 to 1,766 g m⁻² day⁻¹. Comparative maps of soil CO₂ flux were simulated and CO₂ emission rates estimated from three accumulation chamber (AC) CO₂ flux surveys. Least-squares inversion of measured eddy covariance CO₂ fluxes and corresponding modeled source weight functions recovered 58–77% of the CO₂ emission rates estimated based on simulated AC soil CO₂ fluxes. Spatial distributions of modeled surface CO₂ fluxes based on EC and AC observations showed moderate to good correspondence (R ² = 0.36 to 0.70). Results provide a framework for automated monitoring of volcanic CO₂ emissions over relatively large areas.     

Treatment of anchor pixels in the METRIC model for improved estimation of sensible and latent heat fluxes

Abstract

Reliable estimation of sensible heat flux (H) is important in energy balance models for quantifying evapotranspiration (ET). This study was conducted to evaluate the value of adding the Priestley-Taylor (PT) equation to the METRIC (Mapping Evapotranspiration at high Resolution with Internalized Calibration) model. METRIC was used to estimate energy fluxes for 10 Landsat images from the 2005, 2006 and 2007 crop growing seasons in south-central Nebraska, USA, where each image owing to recent rainfall exhibited high residual moisture content even at the hot pixel. The METRIC model performed satisfactorily for net radiation (R_n ) and soil heat flux (G) estimation with a root mean square error (RMSE) of 52 and 24 W m^-2, respectively. A RMSE of 122 W m^-2 for H indicated the limitation of the METRIC model in estimating H for high residual moisture content of the hot pixel (Alfalfa reference ET fraction, ET _r F > 0.15). The modified METRIC model (wet METRIC or wMETRIC) incorporating the PT equation was applied to calculate H at the anchor pixels (hot and cold) for high residual moisture content of the hot pixel. The α coefficient of the PT equation was locally calibrated using hourly meteorological data from an automatic weather station and R_n and G data from a Bowen ratio flux tower. The mean α coefficient value was 1.14. The wMETRIC model reduced the RMSE of H from 122 to 64 W m^-2 and that of latent heat flux, LE, from 163 to 106 W m^-2. The RMSE of daily ET decreased from 1.7 to 1.1 mm d^-1 with wMETRIC. The results indicate that treatment of anchor pixels for high residual moisture content with the PT approach gives improved estimation of H, LE and daily ET. It is recommended that the wMETRIC model be used for estimating ET if the hot pixel has high residual moisture (i.e. reference ET fraction > 0.15).

Citation Singh, R. K. & Irmak, A. (2011) Treatment of anchor pixels in the METRIC model for improved estimation of sensible and latent heat fluxes. Hydrol. Sci. J. 56(5), 895–906.     

Water flux estimates from a Belgian Scots pine stand: a comparison of different approaches

Four distinct approaches, that vary markedly in the spatial and temporal resolution of their measurement and process-level outputs, are used to investigate the daily and seasonal water vapour exchange in a 70-year-old Belgian Scots pine forest. Transpiration, canopy interception, soil evaporation and evapotranspiration are simulated, using a stand-level process model (SECRETS) and a soil water balance model (WAVE). Simulated transpiration was compared with up-scaled sap flow measurements and simulated evapotranspiration to eddy covariance measurements.

Reasonable agreement in the temporal trends and in the annual water balance between the two models was observed, however daily and weekly predictions often diverged. Most notably, WAVE estimated very low, to no transpiration during late autumn, winter and early spring when incident radiation fell below 50 W m⁻² while SECRETS simulated low (0.1–0.4 mm day⁻¹) fluxes during the same period. Both models exhibited similar daily trends in simulated transpiration when compared with sap flow estimates, although simulations from SECRETS were more closely aligned. In contrast, WAVE over-estimated transpiration during periods of no rainfall and under-estimated transpiration during rainfall. Yearly, total evapotranspiration simulated by the models were similar, i.e. 658 mm (1997) and 632 mm (1998) for WAVE and 567 mm (1997) and 619 mm (1998) for SECRETS.

Maximum weekly-average evapotranspiration for WAVE exceeded 5 mm day⁻¹, while SECRETS never exceeded 4 mm day⁻¹. Both models, in general, simulated higher evapotranspiration than that measured with the eddy covariance technique. An impact of the soil water content in the direct relationship between the models and the eddy covariance measurements was found.

The results suggest that: (1) different model formulations can reproduce similar results depending on the scale at which outputs are resolved, (2) SECRETS estimates of transpiration were well correlated with the empirical measurements, and (3) neither model fitted favourably to the eddy covariance technique.