Variations of crop coefficient and its influencing factors in an arid advective cropland of northwest China

Understanding the variation and magnitude of crop coefficient (K_c) is important for accurate determination of crop evapotranspiration and water use. In this study, we calculated K_c in an irrigated maize field with ground mulching by eddy covariance evapotranspiration measurements during the whole growing periods in 2009 and 2010 in an arid region of northwest China. A semi‐empirical practical approach for estimating K_c was proposed by introducing the dynamic fraction of canopy cover and incorporating the effect of leaf senescence as a function of days after sowing. The contribution of arid advection of sensible heat resulting from irrigation to K_c and the response of K_c to canopy conductance (G_c) were investigated. The averaged values of daily K_c were lower than typical values obtained previously without mulching due to decreasing effect of mulching on K_c, with 0.82 and 0.80 for the 2 years, respectively. The maximum average K_c occurred at the heading stage, with 1.21 and 1.04 for the 2 years, respectively. The difference of K_c was attributed to the difference of leaf area index. The semi‐empirical practical approach could well estimate the variations of K_c, thus could be a robust and useful tool for the practical users and water managers. The contributions to daily K_c from the arid advection were 4.4–28.0% of the measured K_c. The G_c had stronger control on daily K_c at the early and later stages than at the middle stage. When G_c, leaf area index and relative soil extractable water were lower than the respective threshold values of 20 mm s^?1, 3.0 m² m^?2 and 0.5, the daily K_c increased significantly with the increase of the three factors, and almost remained constant when the three factors were beyond the threshold values. These results are helpful for quantifying contributions of individual factors to K_c, and subsequently improving water management practices according to K_c. Copyright © 2014 John Wiley & Sons, Ltd.     

Evaluation of actual evapotranspiration measured by large-scale weighing lysimeters in a humid alpine meadow,northeastern Qinghai-Tibetan Plateau

The accurate estimation of evapotranspiration (ET) is essential for assessing water availability and requirements of regional-scale terrestrial ecosystems, and for understanding the hydrological cycle in alpine ecosystems. In this study, two large-scale weighing lysimeters were employed to estimate the magnitude and dynamics of actual evapotranspiration in a humid alpine Kobresia meadow from January 2018 to December 2019 on the northeastern Qinghai-Tibetan Plateau (QTP). The results showed that daily ET_a averaged 2.24 ± 0.10 mm day ⁻¹ throughout the study period, with values of 3.89 ± 0.14 and 0.81 ± 0.06 mm day⁻¹ during the growing season and non-growing season, respectively. The cumulative ET_a during the study period was 937.39 mm, exceeding precipitation (684.20 mm) received at the site during the same period by 37%, suggesting that almost all precipitation in the lysimeters was returned to the atmosphere by evapotranspiration. Furthermore, the cumulative ET_a (805.04 mm) was almost equal to the maximum potential evapotranspiration estimated by the FAO-56 reference evapotranspiration (ET₀) (801.94 mm) during the growing season, but the cumulative ET_a (132.25 mm) was 113.72% less than the minimum equilibrium ET_eq) (282.86 mm) during the non-growing season due to the limited surface moisture in frozen soil. The crop coefficient (K_c) also showed a distinct seasonal pattern, with a monthly average of 1.01 during the growing season. Structural equation model (SEM) and boosted regression tree (BRT) show that net radiation and air temperature were the most important factors affecting daily ET_a during the whole study period and growing season, but that non-growing season ET_a was dominated by soil water content and net radiation. The daily K_c was dominated by net radiation. Furthermore, both ET_a and K_c were also affected by aboveground biomass.     

Estimation of urban subtropical bahiagrass (Paspalum notatum) evapotranspiration using crop coefficients and the eddy covariance method

Eddy covariance (EC) and micro‐meteorological data were collected from May 2010 to January 2013 from urban, non‐irrigated bahiagrass (Paspalum notatum) in subtropical south Florida. The objectives were to determine monthly crop coefficients (K_c) for non‐irrigated bahiagrass by using EC evapotranspiration (ET) data and the Food and Agriculture Organization 56 Penman–Monteith reference evapotranspiration equation; compare crop ET (ET_c) calculated with new K_c values to ET_c obtained using K_c values available in the literature; and compare results and methodologies for statistical differences. New K_c values ranged from 0.62 to 0.92 and were different from K_c values found in the scientific literature for bahiagrass. Resulting ET_c calculated using literature K_c values were significantly different from EC ET data, whereas ET_c using the new K_c values was not. Specifically, literature K_c values were temporally biased to miscalculate the timing of convergence between potential and actual ET, assuming that our new K_c values calculated with EC methods were most accurate. As a consequence, ET_c calculated using the literature K_c values was either too large or too small. However, one set of literature K_c values from a similar climate and water table depth were closer to our new K_c values, indicating that climate should be considered when selecting urban non‐irrigated K_c from the literature to estimate ET. Results also indicated that more than 1 year of EC ET data was needed when establishing monthly K_c values because of annual variability in factors controlling ET, such as water availability. The new K_c values reported herein could be used as an estimate for urban non‐irrigated bahiagrass within similar climates. Copyright © 2013 John Wiley & Sons, Ltd.     

Comparison of radial basis function networks and empirical equations for converting from pan evaporation to reference evapotranspiration

Evapotranspiration (ET) is one of the major processes in the hydrological cycle, and its reliable estimation is essential to water resources management. Numerous equations have been developed for estimating ET, most of which are complex and require numerous items of weather data. In many areas, the necessary data are lacking, and simpler techniques are required. Evaporation pans are used throughout the world because of the simplicity of technique, low cost, and ease of application. In this study, the radial basis function (RBF) network is applied for pan evaporation to evapotranspiration conversions. The adaptive pan‐based RBF network was trained using daily Policoro data from 15 May 1981 to 23 December 1983. The RBF network obtained, Christiansen, FAO‐24 pan, and FAO‐56 Penman–Monteith equations were verified in comparison with lysimeter measurements of grass evapotranspiration using daily Policoro data from 25 February to 18 December 1984. Based on summary statistics, the RBF network ranked first with the lowest RMSE value (0·433 mm day^?1). The RBF network obtained on the basis of the daily data from Policoro, Italy and pan‐based equations were further tested using mean monthly data collected in Novi Sad, Serbia, and Kimberly, Idaho, USA. The overall results favoured use of the RBF network for pan evaporation to evapotranspiration conversions. The use of the RBF network is very simple and does not require any knowledge of ANNs. Users require only code (RBF network), E_pan data and corresponding R_a data. Copyright © 2009 John Wiley & Sons, Ltd.     

Estimating evapotranspiration for a temperate salt marsh,Newcastle, Australia

Evapotranspiration was studied at a salt marsh site in the Hunter River estuary, NSW, Australia, during 1996–8. Estimates of actual evapotranspiration (E_a) were obtained for three sites using the eddy correlation method. These values were compared with results obtained with the Penman and Penman–Monteith equations, and with pan evaporation. The Penman–Monteith method was found to be most reliable in estimating daily and hourly evapotranspiration. Surface resistance values averaging 12 s m^?1 were derived from the eddy correlation estimates. Recent tidal flooding and rainfall were found to decrease surface resistance and increase E_a/E_p ratios. Estimates of evapotranspiration obtained using the Penman–Monteith method were shown to be sensitive to changes in surface resistance, canopy height and the method used to estimate net radiation from incoming solar radiation. These results underline the importance of accurately estimating such parameters based on site‐specific data rather than relying on empirical equations, which are derived primarily for crops and forests. Copyright © 2001 John Wiley & Sons, Ltd.     

A meteorological-based crop coefficient model for estimation of daily evapotranspiration

Estimation of daily evapotranspiration (ET) over cloudy regions highly desires models which rely on meteorological data only. Notwithstanding, the conventional crop coefficient (K_c) method requires detailed knowledge of geo/biophysical properties of the coupled land-vegetation system, precipitation, and soil moisture. Six Eddy Covariance (EC) towers in Iowa, California and New Hampshire of the USA (covering corn, soybeans, prairie, and deciduous forest) were selected. Investigation on 6 years (2007–2012) 15-min micrometeorological records of these sites revealed that there is an indubitable strong interaction between relative humidity (RH), reference ET (ET_o), and actual ET at different timescales. This allowed to bypass the need for the non-meteorological inputs and express K_c as a second-order polynomial function of RH and ET_o, the ambient regression evapotranspiration model (AREM). The coefficients of the empirical function are crop-specific and may require calibration over different soil types. The mean absolute percentage error (MAPE) of the regression against daily EC observations was 17% during the growing season, and 32% throughout the year with root mean square error (RMSE) of 0.74 mm day⁻¹ and coefficient of determination of 0.71. The model was fully operational (MAPE of 34% and RMSE of 0.82 mm day⁻¹) over the four Iowan sites based on inputs from local weather stations and NLDAS-2 forcing data of NASA. AREM was capable of capturing the dynamics of ET at 15-min and daily timescales irrespective of varying complexities associated with biophysical, geophysical and climatological states.     

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.     

Eddy covariance‐based evapotranspiration for a subtropical wetland

The eddy covariance (EC) method was used in a 30‐month study to quantify evapotranspiration (ET) and vegetation coefficient (K_CW) for a wetland on a ranch in subtropical south Florida. To evaluate the errors in ET estimates, the EC‐based ET (ET_C‐EC) and the Food and Agricultural Organization (FAO) Penman–Monteith (PM) based ET (ET_C‐PM) estimates (with literature crop coefficient, K_C) were compared with each other. The ET_C‐EC and FAO‐PM reference ET were used to develop K_CW. Regression models were developed to estimate K_CW using climatic and hydrologic variables. Annual and daily ET_C‐EC values were 1152 and 3.27 mm, respectively. The FAO‐PM model underestimated ET by 25% with ET_C‐EC being statistically higher than ET_C‐PM. The K_CW varied from 0.79 (December) to 1.06 (November). The mean K_CW for the dry (November–April) season (0.95) was much higher than values reported for wetlands in literature; whereas for the wet (May–October) season, K_CW (0.97) was closer to literature values. Higher than expected K_CW values during the dry season were due to higher temperature, lower humidity and perennial wetland vegetation. Regression analyses showed that factors affecting the K_CW were different during the dry (soil moisture, temperature and relative humidity) and wet (net radiation, inundation and wind speed) seasons. Separate regression models for the dry and wet seasons were developed. Evapotranspiration and K_CW from this study, one of the first for the agricultural wetlands in subtropical environment, will help improve the ET estimates for similar wetlands. Copyright © 2013 John Wiley & Sons, Ltd.     

Parameterizing canopy resistance using mechanistic and semi‐empirical estimates of hourly evapotranspiration: critical evaluation for irrigated crops in the Mediterranean

In this paper two models are presented for calculating the hourly evapotranspiration λE (W m^?2) using the Penman–Monteith equation. These models were tested on four irrigated crops (grass, soya bean, sweet sorghum and vineyard), with heights between 0·1 and 2·2 m at the adult growth stage. In the first model (Katerji N, Perrier A. 1983. Modélisation de l'évapotranspiration réelle ETR d'une parcelle de luzerne : rôle d'un coefficient cultural. Agronomie 3(6): 513–521, KP model), the canopy resistance r_c is parameterized by a semi‐empirical approach. In the second model (Todorovic M. 1999. Single‐layer evapotranspiration model with variable canopy resistance. Journal of Irrigation and Drainage Engineering—ASCE 125: 235–245, TD model), the resistance r_c is parameterized by a mechanistic model. These two approaches are critically analysed with respect to the underlying hypotheses and the limitations of their practical application. In the case of the KP model, the mean slope between measured and calculated values of λE was 1·01 ± 0·6 and the relative correlation coefficients r² ranged between 0·8 and 0·93. The observed differences in slopes, between 0·96 and 1·07, were not associated with the crop height. This model seemed to be applicable to all the crops examined. In the case of the TD model, the observed slope between measured and calculated values of λE for the grass canopy was 0·79. For the other crops, it varied between 1·24 and 1·34. In all the situations examined, the values of r² ranged between 0·73 and 0·92. The TD model underestimated λE in the case of grass and overestimated it in the cases of the other three crops. The under‐ or overestimation of λE in the TD model were due: (i) to some inaccuracies in the theory of this model, (ii) to not taking into account the effect of aerodynamic resistance r_a in the canopy resistance modelling. Therefore, the values of r_c were under‐ or overestimated in consequence of mismatching the crop height. The high value of air vapour pressure deficit also contributed to the overestimation of λE, mainly for the tallest crop. The results clarify aspects of the scientific controversy in the literature about the mechanistic and semi‐empirical approaches for estimating λE. From the practical point of view the results also present ways for identifying the most appropriate approach for the experimental situations encountered. Copyright © 2010 John Wiley & Sons, Ltd.     

Reedbed evapotranspiration rates in England

Measurements of evapotranspiration from contrasting stands of Phragmites australis [ET(Reed)] were obtained over several years at three sites within England between 1994 and 1998. A comparison with published ET(Reed) values showed the UK rates to be considerably lower than those measured in continental Europe. From the ET(Reed) data monthly crop coefficients [K_c(Reed)] have been developed, which when used with Reference Crop Evapotranspiration will enable reedbed water use to be determined. Due to the differences in study site characteristics, values for K_c varied widely, and it was not considered appropriate to combine results to produce a single set of monthly mean values. However, suggestions are made regarding the application of the K_c(Reed) values to wetland design. Copyright © 2001 John Wiley & Sons, Ltd.     

Evaporation of grass under non-restricted soil moisture conditions

Abstract

The behaviour of various formulas for evapotranspiration of grass in Nonrestricted soil water conditions is considered. These are the expressions based on the Penman formula, i.e. “old” Penman, Penman-Monteith, Thorn-Oliver and the version recommended more recently by the FAO. Moreover, the Priestley-Taylor and the Makkink formulas are considered, which are radiation-based. Comparisons are made between daily mean values estimated with these formulas and direct measurements. The latter were collected over grass in the period 1979–1982 in the catchment area of the Hupselse Beek (The Netherlands). It was found that if all required input data were measured, the Priestley-Taylor and the “old” Penman formula yielded the best results. The assumption that soil heat flux can be neglected introduces a systematic and a random error of roughly 5%. The empirical estimates for net radiation from sunshine duration, temperature and humidity appear to perform rather poorly. These estimates improved significantly if solar radiation was measured directly. The empirical expression proposed by Slob (unpublished) that requires incoming solar radiation only as input, provided better results than the other more complicated expressions. Moreover, this study reveals that evaporation of unstressed grass is primarily determined by the available energy, i.e. good evaporation estimates can be obtained by using simply λE = 0.86(R_n ? G). The Makkink method appears to be attractive for practical applications. These findings support the use of Makkink's formula for routine calculations of crop-reference evapotranspiration as has been done by the Royal Netherlands Meteorological Institute since 1987.     

Effectiveness of introducing crop coefficient and leaf area index to enhance evapotranspiration simulations in hydrologic models

Evapotranspiration (ET) is an essential component of the hydrological cycle and plays a critical role in water resource management. However, ET is often overlooked in order to transform rainfall to runoff for better streamflow simulation. Hydrological models are commonly used to estimate areal actual evapotranspiration (AET) after calibration against observed discharge. However, classical approaches are often inadequate to appropriately simulate other hydrologic components. Hence, it is important to introduce natural heterogeneity to enhance hydrological processes and reduce water balance errors. In this study, the effectiveness of introducing a constant crop coefficient (K_c), flux tower‐based K_c, and leaf area index (LAI) to three hydrological models (Three‐Parametric Hydrologic Model [TPHM], Génie Rural à 4 paramètres Journalier [GR4J], and Catchment hydrologic cycle Assessment Tool [CAT]) is assessed for the simulation of daily streamflow and AET in a mountainous mixed forest watershed (8.54 km²) in South Korea. The results show that the streamflow simulations after introduction of K_c and LAI to the original models are quite similar. However, the effectiveness of the AET estimation was significantly enhanced after introduction of the flux tower‐based K_c and LAI. The information criterion computed to compare the models reveals that the flux tower‐based K_c‐simulated AET demonstrated better agreement with the observed AET. The Pearson's correlation coefficients (R²) of the TPHM (8%), GR4J (55%), and CAT (55%) models also showed improvements that were greater than the constant based K_c simulation. Similarly, the limitations of the three models with respect to capturing seasonal variation as well as high and low flows were enhanced after the introduction of the flux tower‐based K_c, which adequately reproduced hydrological processes with minimum water balance errors and bias. A regression analysis revealed the potential of estimating K_c as a linear function of LAI (R² = 0.84). The results of this study indicate that introduction of K_c and LAI to the conceptual rainfall–runoff models can be considered an effective approach to reduce water balance errors and uncertainties in hydrological models and improve the reliability of climate change studies and water resource management.     

A simple temperature method for the estimation of evapotranspiration

Accurate estimation of evapotranspiration (ET) is essential in water resources management and hydrological practices. Estimation of ET in areas, where adequate meteorological data are not available, is one of the challenges faced by water resource managers. Hence, a simplified approach, which is less data intensive, is crucial. The FAO‐56 Penman–Monteith (FAO‐56 PM) is a sole global standard method, but it requires numerous weather data for the estimation of reference ET. A new simple temperature method is developed, which uses only maximum temperature data to estimate ET. Ten class I weather stations data were collected from the National Meteorological Agency of Ethiopia. This method was compared with the global standard PM method, the observed Piche evaporimeter data, and the well‐known Hargreaves (HAR) temperature method. The coefficient of determination (R²) of the new method was as high as 0.74, 0.75, and 0.91, when compared with that of PM reference evapotranspiration (ET_o), Piche evaporimeter data, and HAR methods, respectively. The annual average R² over the ten stations when compared with PM, Piche, and HAR methods were 0.65, 0.67, and 0.84, respectively. The Nash–Sutcliff efficiency of the new method compared with that of PM was as high as 0.67. The method was able to estimate daily ET with an average root mean square error and an average absolute mean error of 0.59 and 0.47 mm, respectively, from the PM ET_o method. The method was also tested in dry and wet seasons and found to perform well in both seasons. The average R² of the new method with the HAR method was 0.82 and 0.84 in dry and wet seasons, respectively. During validation, the average R² and Nash–Sutcliff values when compared with Piche evaporation were 0.67 and 0.51, respectively. The method could be used for the estimation of daily ET_o where there are insufficient data. Copyright © 2013 John Wiley & Sons, Ltd.     

Spatio-temporal variations of reference crop evapotranspiration and pan evaporation in the West Songnen Plain of China

Abstract

The spatio-temporal variations of reference crop evapotranspiration (ET_ref) reflect the combined effects of meteorological variables, primarily wind speed, relative humidity, net radiation and air temperature. This study investigated the spatial distribution and temporal trends of ET_ref (calculated by the FAO-56 Penman-Monteith equation), pan evaporation (E_pan) and pan coefficient (K_p) in a 140?×?10³ km² semi-humid to semi-arid area in China. The results show that: (i) although the spatial distributions of ET_ref and E_pan are roughly similar and their spatial correlation is high over the growing season, K_p varied considerably in space due to high humidity in the east of the region and low humidity in the southwest; (ii) the monthly variations of ET_ref and E_pan are similar to that of net radiation and opposite to that of relative humidity, while the monthly variation of K_p is similar to that of relative humidity and opposite to that of wind speed, and the long-term trend is slightly increasing for ET_ref and E_pan, while significantly (10% significance level) increasing for K_p; and (iii) generally, the time series of ET_ref and E_pan from 1951 to 2001 could be divided into three phases due to variations of meteorological variables.

Citation Liang, L.-Q., Li, L.-J. & Liu, Q. (2011) Spatio-temporal variations of reference crop evapotranspiration and pan evaporation in the West Songnen Plain of China. Hydrol. Sci. J. 56(7), 1300–1313.     

Estimation of evapotranspiration and its components from an apple orchard in northwest China using sap flow and water balance methods

Field experiments were conducted to investigate the effects of leaf area index and soil moisture content on evapotranspiration and its components within an apple orchard in northwest China for 2 years. Evapotranspiration in the non‐rainfall period was estimated using two approaches: the soil water balance method based on tube‐type time‐domain reflection measurements, and sap flow plus micro‐lysimeter methods. The two methods were in good agreement, with differences usually less than 10%. The components of evapotranspiration varied with canopy development. During spring and autumn, soil evaporation was dominating as result of low leaf area index. In summer, plant transpiration became significant, with an average transpiration to evapotranspiration ratio of 0·87. The crop coefficient K_c showed a strong linear dependence on leaf area index. The water stress coefficient K_s was around 1·0 when soil moisture was above 23% and started to decrease linearly after that. This study demonstrates that prediction of evapotranspiration in apple orchards can be made using the Food and Agriculture Organization's crop coefficient method from commonly available meteorological data in the area. Copyright © 2006 John Wiley & Sons, Ltd.     

Comparison of the Penman‐Monteith method and regional calibration of the Hargreaves equation for actual evapotranspiration using SWAT-simulated results in the Seolma-cheon basin,South Korea

ABSTRACT

The Hargreaves method provides reference evapotranspiration (ET_o) estimates when only air temperature data are available, although it requires previous local calibration for an acceptable performance. This method was evaluated using the data from 71 meteorological stations in the Seolma-cheon basin (8.48 km²), South Korea, comparing daily estimates against those from the Penman‐Monteith (PM) method, which was used as the standard. To estimate reference ET_o more exactly, considering the climatological characteristics in South Korea, parameter regionalization of the Hargreaves equation is carried out. First, the modified Hargreaves equation is presented after an analysis of the relationship between solar radiation and temperature. Second, parameter (K_ET) optimization of the regional calibration of the Hargreaves equation (RCH) is performed using the PM method and the modified equation at 71 meteorological stations. Next, an application was carried out to evaluate the evapotranspiration methods (PM, original Hargreaves and RCH) in the SWAT (Soil and Water Assessment Tool) model by comparing these with the measured actual evapotranspiration (AET) in the basin. The SWAT model was calibrated using 3 years (2007–2009) of daily streamflow at the watershed outlet and 3 years (2007–2009) of daily AET measured at a mixed forest. The model was validated with 3 years (2010‐2012) of streamflow and AET. RCH will contribute to a better understanding of evapotranspiration of an ungauged watershed in areas where meteorological information is scarce.

EDITOR D. Koutsoyiannis ASSOCIATE EDITOR Not assigned     

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.     

Performance of the White method for estimating groundwater evapotranspiration under conditions of deep and fluctuating groundwater

The White method is a simple but the most frequently applied approach to estimate groundwater evapotranspiration (ET_g) from groundwater level diurnal signals. Because of a lack of direct measurements of ET_g, it is difficult to evaluate the performance of the White method, particularly in field environments with variable groundwater fluctuations. A 2‐year field observation in a hyper‐arid riparian tamarisk (Tamarix spp.) stand with deep groundwater depth in the lower Tarim River basin of China was conducted to measure the surface evapotranspiration (ET_s) and groundwater table. The performance of the White method and the influences of the variable groundwater fluctuations on the determinations of the specific yield (S_y) and recharge rate of groundwater (r) in the White method were investigated. The results showed that the readily available S_y determined by Loheide's method was feasible but must be finely determined based on the soil textures in the layers in which the groundwater level fluctuated. A constant S_y value for a defined porous medium could be assumed regardless of the discharge or recharge processes of groundwater. The time span of 0000 h to 0600 h for r estimation for the White method worked best than other time spans. A 2‐day moving average of r values further enhance ET_g estimation. Slight effects of environmental or anthropogenic disturbances on the diurnal fluctuations of groundwater level did not influence the ET_g estimations by the White method. Our results provide valuable references to the application of the White method for estimating daily ET_g in desert riparian forests with deep groundwater depth. Copyright © 2015 John Wiley & Sons, Ltd.     

Field-aligned conductance values estimated from Maxwellian and kappa distributions in quiet and disturbed events using Freja electron data

We study the question of what difference it makes for the derived field-aligned conductance (K) values if one uses Maxwellian or kappa distributions for the fitting of low-orbiting satellite electron flux spectra in the auroral region. This question has arisen because sometimes a high-energy tail is seen in the spectra. In principle, the kappa fits should always be better, because the kappa distribution is a generalization of the Maxwellian. However, the physical meaning of the parameters appearing in the Maxwellian is clearer. It therefore makes sense to study under which circumstances it is appropriate to use a Maxwellian. We use Freja electron data (TESP and MATE) from two events. One of the events represents quiet magnetospheric conditions (stable arc) and the other represents disturbed conditions (surge). In these Freja events, at least, using kappa rather than Maxwellian fitting gives a better fit to the observed distribution, but the difference in K values is not large (usually less than 20%). The difference can be of either sign. However, sometimes even the kappa distribution does not provide a good fit, and one needs a more complicated distribution such as two Maxwellians. We investigate the relative contributions of the two Maxwellians to the total field-aligned conductance value in these cases. We find that the contribution of the high-energy population is insignificant (usually much less than 20%). This is because K is proportional to n/E_c E_c, where n is the source plasma density and E_c is the characteristic energy.     

A framework for sourcing of evaporation between saturated and unsaturated zone in bare soil condition

ABSTRACT

Sourcing subsurface evaporation (E_ss) into groundwater (E_g) and unsaturated zone (E_u) components has received little scientific attention so far, despite its importance in water management and agriculture. We propose a novel sourcing framework, with its implementation in dedicated post-processing software called SOURCE (used along with the HYDRUS1D model), to study evaporation sourcing dynamics, define quantitatively “shallow” and “deep” water table conditions and test the applicability of water table fluctuation (WTF) and “bucket” methods for estimation of E_g and E_u separately.

For the “shallow” and “deep” water table we propose E_g?>?0.95E_ss and E_g = 0 criteria, respectively. Assessment of the WTF method allowed sourcing of very small fluxes otherwise neglected by standard hydrological methods. Sourcing with SOURCE software was more accurate than the standard “bucket” method mainly because of greater flexibility in spatio-temporal discretization. This study emphasized the dry condition relevance of groundwater evaporation which should be analysed by applying coupled flow of heat, vapour and liquid water.

Editor D. Koutsoyiannis; Associate editor S. Kanae