Evapotranspiration Partitioning Research Articles

Comparing Four Evapotranspiration Partitioning Methods from Eddy Covariance Considering Turbulent Mixing in a Poplar Plantation

Evapotranspiration is a key link in the water cycle of terrestrial ecosystems, and the partitioning of evapotranspiration is a prerequisite for diagnosing vegetation growth and water use strategies. In this study, we used double-layer eddy covariance (DLEC) measurements within and above the canopy of poplar plantations to divide evapotranspiration into transpiration and evaporation during the growing season. We diagnosed the coupling state of airflows in the canopy vertical layer and found that the daytime coupling state at the half-hourly scale can mask nighttime decoupling. Furthermore, we investigated the daytime and nighttime vertical layer airflow coupling states separately and quantified the effects of coupling states on the DLEC of resolved transpiration. The partitioning results of the DLEC method were taken as the standard after the airflow coupling test. Then, the performance and accuracy of evapotranspiration partitioning for the modified relaxed eddy accumulation (MREA), the conditional eddy covariance (CEC), and the flux variance similarity (FVS) with DLEC were compared. Transpiration calculated from MREA showed the best agreement with DLEC, and the other methods showed different degrees of underestimation (1:1 slope = 0.64–0.83). Evaporation calculated from FVS showed the best agreement with DLEC, while CEC and FVS made an overestimation of more than 26% (1:1 slope = 1.26–1.99), but MREA made an underestimation from 5% to 35% (1:1 slope = 0.65–0.95). The correlation coefficients between DLEC and MREA for transpiration were 0.95–0.97 with RMSEs of 15.52–17.04 W m−2, and those between DLEC and FVS for transpiration were 0.73–0.78 with RMSEs of 10–21.26 W m−2 at the daily half-hourly scale. A detailed comparison of the differences between DLEC and evapotranspiration partitioning methods from high-frequency eddy covariance data under the condition of canopy vertical layer airflow mixing provides knowledge about the consistency of results for evapotranspiration partitioning in poplar plantation forests.

Estimating and partitioning evapotranspiration in a film mulched cropland with shallow groundwater by the improved dual source model

Evapotranspiration (ET) is a key link in terrestrial water cycle and influenced by complex environmental factors in cropland. Accurate assessment of crop ET over heterogeneous underlying surfaces remains challenging. In this study, the dual source Shuttleworth-Wallace (SW) model was modified to estimate λET (latent heat corresponding to ET) through incorporating the effects of film mulching and shallow groundwater. λET and latent heat from transpiration (λT) were measured in a maize field in northwest China during 2017 and 2018 to assess the performance of the original (SW) and modified (SWM) models. Results indicate that the overestimation of latent heat from soil evaporation (λE) due to ignoring film mulching could be obviously mitigated by SWM. Besides, considering capillary rise from shallow groundwater in SWM improved not only λET estimates but also the accuracy of canopy resistance parameter. The SWM model outperformed the SW model that the slopes of fitting lines for λET and λT increased from 0.60 to 0.79 to 0.77–0.99 at daily scale and Nash-Sutcliffe Efficiency coefficient exceeded 0.86. Relative error and RMSE were decreased to below 0.15 and 19.88 W m−2, respectively. The estimations of λET and λT by SWM were also improved at hourly scale. Seasonal variation and partition of λET between two years were comparable and average T/ET for the entire growing season was about 78 %-84 %, which was largest at the heading stage. Path analysis on the responses of λET to the environmental factors recognized the greatest direct effect of crop height or leaf area index, and identified the significantly direct control of shallow groundwater on λT in this area. The study presented an improved dual source model to better quantify λET, which would benefit water resources management for film mulched croplands with shallow groundwater.

Partitioning evapotranspiration of Camellia oleifera during the growing season based on the Penman-Monteith model combined with the micro-lysimeter and stable isotope methods

Frequent seasonal droughts impact on crop growth and yield in the hilly areas of southern China. A good knowledge of its evapotranspiration partitioning of the main economic oil crop Camellia Oleifera is particularly important to increase water productivity and improve water resource management. The isotope-based method was applied to partition ET in the hilly Camellia Oleifera Forest of southern China. The isotopic composition of evapotranspiration vapor (δET), transpiration vapor (δT) and evaporation vapor (δE) were calculated by the Keeling plot method, the Craig-Gordon model (C-G model), and isotopic steady-state assumptions, respectively. The conventional Penman-Monteith model and the micro-lysimeter method (PM-L) were used to compare and assess the ET partitioning results (Ft). It showed that the Ft values calculated by the two methods were in good agreement. The isotopic composition of soil water at the evaporating front (δS) ahead and after DOY151 was sampled at 5 and 10 cm depth, respectively, where the correlation coefficients (R2) of the Ft values calculated by δ18O and δ2H were 0.84 and 0.66. Ft Calculated by δ18O was more accurate than δ2H because of the higher correlation coefficient of δ18O in the keeling plot regression and in the linear fit with PM-L. The isotopic composition of soil evaporation vapor（δE）calculated by dynamic εk (kinetic fractionation effects) values can be more accurate to quantify the ET partitioning results (with the R2 of 0.90). The mean values of Ft were 0.79 and 0.82 by the PM-L method and the isotope-based method respectively over the whole growing season, indicating the isotope method is robust in calculating ET partitioning for Camellia oleifera in the region. We also found that the seasonal variation of Ft is mainly controlled by temperature (T), soil water content (SWC), and leaf area index (LAI), all of which can be effectively expressed as power functions. The results provide potential assistance for water management in Camellia oleifera woodlands.

Revisiting the hydrological legacy of revegetation on China's Loess Plateau using Eagleson's ecohydrological perspective

Revegetation has resulted in a trend of increasing vegetation greenness on the Chinese Loess Plateau. However, it remains unclear whether the regional vegetation coverage exceeds hydroclimatic limitations in the context of revegetation, and the hydrological effects of greening are controversial. Eagleson's optimality hypothesis can explain some of the hydrological effects on the Loess Plateau. Here, building on previous research, the geospatial vegetation states were estimated for pre- and post-revegetation periods on the Loess Plateau from 1982 to 2015 using Eagleson's ecological optimality theory. Additionally, a drought composite analysis approach was utilized to investigate the hydrological effects related to drought (including sensitivity and partitioning) under various vegetation states. It was found that revegetation increased the proportion of catchments in the equilibrium state and decreased the proportion in the disturbed state, owing to a wetter climate compared with the pre-revegetation period. Root-zone soil drought, driven by precipitation (P) deficit, asymmetrically triggered hydrological effects for both the pre- and post-revegetation periods, with reduced runoff (Q) for both periods and a decrease in evapotranspiration (ET) during the pre-revegetation period but an increase in ET during the post-revegetation period. Moreover, catchments in an equilibrium state exhibited lower sensitivity between ET and P, and more stable partitioning of ET with regards to P, compared with those in a disturbed state. These results underscore the theoretical framework that an equilibrium state is crucial for maintaining ecosystem ET. Our results highlight the necessity of considering the hydrologic regulation of vegetation states when assessing the hydrological effects of vegetation change.

Evapotranspiration partitioning through water stable isotopic measurements in a subtropical coniferous forest

AbstractEvapotranspiration (ET) partitioning distinguishes the soil evaporation (E) and plant transpiration (T) components and is crucial for understanding the land‐atmosphere interactions and ecosystem water budget. However, the mechanism and controls of ET partitioning for subtropical forests in heterogeneous environments remain poorly understood. Here, we present δ18O and δ2H of about 1,527 isotope samples including atmospheric water, soil and plant water during different seasons in 2 years of 2020–2021 from a coniferous forest across Southeast China. We used the isotopic mass balance of ecosystem water pools, the Craig‐Gordon model and the Keeling‐Plot method to partition T from ET (T/ET) and quantify the controls on T/ET. Results indicated that the uncertainty in the T/ET was principally from the soil water evaporation (δE) value, about 20–30 cm was found to be a reasonable evaporating front depth for estimating δE in this coniferous forest. T/ET presented a “U” shape diurnal pattern and varied from 66.7% to 89.9%. Isotope‐based T/ET in autumn with high temperatures and little rain was higher than those in the summer and winter seasons. Relative humidity (or vapour pressure deficit) dominated the diurnal T/ET variations (relative contributions of > 40%) in summer and autumn, while air temperature and soil water content were the main controls in winter. Our study also showed that δ18O‐derived T/ET was consistent with that of δ2H, although δ2H was found to be more stable in ET partitioning, the dual stable isotope approach should be employed in future studies for the uncertainties brought by samplings or measurements. The agreement between the isotope‐based T/ET and ET partitioning approach that uses eddy covariance and sap flux data was stronger at midday. These isotope‐inferred ET partitioning can inform land surface models and provide more insights into water management in subtropical forests.

Coupled DSSAT and HYDRUS-1D Simulation of the Farmland–Crop Water Cycling Process in the Dengkouyangshui Irrigation District

(1) Background: Effective water management in agricultural systems poses a significant challenge, particularly in the Dengkouyangshui irrigation district. Inefficiencies and insufficient detail in water usage across crop growth stages have resulted in suboptimal water cycling. Recent infrastructure improvements and technological interventions necessitate a reevaluation of water usage, especially concerning changes in irrigation and seepage dynamics. (2) Methods: This study addresses these concerns by employing an integrated modeling approach that combines the DSSAT with the HYDRUS-1D soil hydrology model to simulate complex interactions among soil, crop growth, and irrigation practices within the district. Observational data were used to calibrate and validate the integrated model, including soil moisture, LAI, and crop yields from the 2022 and 2023 agricultural seasons. (3) Results: The simulation results strongly align with the empirical data, highlighting the ability of the model to capture the intricate dynamics of soil–water–atmosphere–plant interactions. (4) Conclusions: The soil’s retention and moisture-holding characteristics exhibited resilience during periods without water supplementation, with measurable declines in soil moisture at various depths, indicating the soil’s capacity to support crops in water-limited conditions. This study delineates water consumption by maize crops throughout their growth cycle, providing insights into evapotranspiration partitioning and quantifying seepage losses. An in-depth analysis of water balances at different growth stages informs irrigation strategies, suggesting optimal volumes to enhance efficiency during critical crop development phases. This integrative modeling approach is valuable for providing actionable data to optimize the water cycling process and improve agricultural sustainability in the Dengkouyangshui irrigation district.

Using Median Point in Keeling Plot to Reduce the Uncertainty of the Isotopic Composition of Evapotranspiration

Abstract The isotopic composition of evapotranspiration δET is a crucial parameter in isotope-based evapotranspiration (ET) partitioning and moisture recycling studies. The Keeling plot method is the most prevalent method to calculate δET, though it contains large extrapolated uncertainties from the least squares regression. Traditional Keeling regression uses the mean point of individual measurements. Here, a modified Keeling plot framework was proposed using the median point of individual measurements. We tested the δET uncertainty using the mean point [σET (mean)] and median point [σET (median)]. Multiple resolutions of input and output data from six independent sites were used to test the performance of the two methods. The σET (mean) would be greater than σET (median) when the mean value of inverse vapor concentration () is greater than the median value of inverse vapor concentration []. When applying the filter of r2 > 0.8, around 70% of σET (mean) was greater than σET (median). This phenomenon might be due to the normality of the vapor concentration Cυ producing the asymmetric distribution of 1/Cυ. The median method could perform significantly better than the mean method when inputting high-resolution measurements (e.g., 1 Hz) and when the water vapor concentration Cυ is relatively low. Compared to the mean method, applying the median method could on average reduce 6.88% of ET partitioning uncertainties and could on average reduce 9.00% of moisture recycling uncertainties. This study provided a new insight of the Keeling plot method and emphasized handling model output uncertainty from multiple perspectives instead of only from input parameters.

Environmental Controls on Evapotranspiration and Its Components in a Qinghai Spruce Forest in the Qilian Mountains.

Qinghai spruce forests, found in the Qilian mountains, are a typical type of water conservation forest and play an important role in regulating the regional water balance and quantifying the changes and controlling factors for evapotranspiration (ET) and its components, namely, transpiration (T), evaporation (Es) and canopy interceptions (Ei), of the Qinghai spruce, which may provide rich information for improving water resource management. In this study, we partitioned ET based on the assumption that total ET equals the sum of T, Es and Ei, and then we analyzed the environmental controls on ET, T and Es. The results show that, during the main growing seasons of the Qinghai spruce (from May to September) in the Qilian mountains, the total ET values were 353.7 and 325.1 mm in 2019 and 2020, respectively. The monthly dynamics in the daily variations in T/ET and Es/ET showed that T/ET increased until July and gradually decreased afterwards, while Es/ET showed opposite trends and was mainly controlled by the amount of precipitation. Among all the ET components, T always occupied the largest part, while the contribution of Es to ET was minimal. Meanwhile, Ei must be considered when partitioning ET, as it accounts for a certain percentage (greater than one-third) of the total ET values. Combining Pearson's correlation analysis and the boosted regression trees method, we concluded that net radiation (Rn), soil temperature (Ts) and soil water content (SWC) were the main controlling factors for ET. T was mainly determined by the radiation and soil hydrothermic factors (Rn, photosynthetic active radiation (PAR) and TS30), while Es was mostly controlled by the vapor pressure deficit (VPD), atmospheric precipitation (Pa), throughfall (Pt) and air temperature (Ta). Our study may provide further theoretical support to improve our understanding of the responses of ET and its components to surrounding environments.

Evapotranspiration partitioning by integrating eddy covariance, micro-lysimeter and unmanned aerial vehicle observations: A case study in the North China Plain

Partitioning of evapotranspiration (ET) into soil evaporation (E) and plant transpiration (T) poses a significant challenge. This study proposed a novel approach that combines eddy covariance, micro-lysimeter, and high-resolution unmanned aerial vehicle (UAV) images within the flux footprint to optimize the canopy and soil resistances of the Shuttleworth-Wallace model (S-W) using a multi-objective optimization scheme. A two-year experiment was conducted at the Yucheng and Luancheng sites to investigate the effectiveness of the proposed method for the summer maize-wheat winter system in the North China Plain. Our results showed that the S-W model with multi-objective optimization by using high-resolution UAV images within the flux footprint significantly improved the estimation accuracy of ET components compared with the traditional one- and two-objective optimization schemes. The estimation of ET, T, and E of our method had an average root mean square error of 0.67 mm day−1, 0.66 mm day−1, and 0.28 mm day−1, respectively. The optimized S-W model using high-resolution UAV data within the flux footprint produced better ET partitioning than that optimized using Moderate Resolution Imaging Spectroradiometer (MODIS) data. We then applied the proposed method to estimate ET and ET components in the North China Plain. This study highlighted the importance of utilizing multi-source data (especially high-resolution UAV images) and multi-objective optimization for accurate ET partitioning.

Multi-temporal variations in evapotranspiration partitioning and its controlling factors of a xerophytic shrub ecosystem

Partitioning evapotranspiration (ET) into its components of canopy interception (Ic), transpiration (T) and soil evaporation (E) enhances understanding of hydrological cycle and plant water use strategy in drylands. However, variations in all three ET components and their fractions across multi-temporal scales (daily, monthly, annual), and their controlling factors remain poorly understood for shrub ecosystems. In this study, we aimed to address these issues by measuring daily rainfall partitioning, sap flow, soil evaporation and soil moisture in a xerophytic shrub (Salix psammophila) ecosystem in northern China during 2019–2021. We found T dominated ET with a mean ratio of T to ET ranging 61.7 %–71.1 %, which was consistently higher than the mean ratio of Ic and E to ET ranging 12.1 %-41.5 % and 24.5 %–32.2 %, respectively, from daily to annual scales (p < 0.05). There was no significant difference in mean ratio of T to ET among different temporal scales (p > 0.05), while mean ratios of Ic and E to ET were significantly higher at daily scale than those at monthly and annual scales (p < 0.05). Compared with monthly and annual scales, ET components and their ratios at daily scale were more responsive to bio-/abiotic factors. ET components were more susceptible to be affected by the influencing factors when compared to their ratios, regardless of the temporal scales. Specifically, daily ET components and their ratios were mainly controlled by precipitation and solar radiation. In addition, soil moisture also explained 10.2 % and 10.9 % of the variations of daily T and ET, respectively. At monthly scale, however, variations in T and ET were mainly controlled by LAI with explained variance of 26.6 % and 32.4 %, respectively. Our results highlighted the significant differences in ET partitioning and their different responses to bio- and abiotic factors at various temporal scales of the shrub ecosystems in drylands.

Dynamics of evapotranspiration from concurrent above- and below-canopy flux measurements in a montane Sierra Nevada forest

Evapotranspiration (ET) from the land surface to the atmosphere is a major component of Earth's water cycle, and comprises both transpiration (T) of xylem water from plants and evaporation (E) of water from soils and vegetation surfaces. These two component fluxes respond differently to changes in temperature, water availability and atmospheric CO2 concentrations. Concurrent eddy covariance (EC) measurements above and below forest canopies provide a promising approach to partition ET into E and T. However, below-canopy EC measurements are rare, and questions remain regarding their spatial variability, canopy coupling, and temporal dynamics. To address these challenges, we measured and partitioned ET over more than three years, using concurrent above- and below-canopy EC towers in a montane forest at Sagehen Creek in California's Sierra Nevada mountains. This is the establishing study for the AmeriFlux site US-SHC. The main environmental control for ET was available energy; other important controls were canopy & soil temperature, soil moisture, vapor pressure deficit, and wind speed. Below-canopy measurements at two locations within the above-canopy footprint were similar to one another, suggesting low spatial heterogeneity in understory ET near the creek at our Sagehen site. We observed a total forest ET of 606 ± 50 mm yr−1 with 275 ± 17 mm yr−1 measured in the understory (all mean ± SD) during the water years 2018–2020. Interannual variability in ET and T was small despite large variability in precipitation totals; thus the P–ET water balance was mainly driven by variations in water supply. Partitioning the components of total forest ET at Sagehen with concurrent EC measurements showed that on average, 67–74% of ET originated from T (47% from trees and 20–27% from understory vegetation), while 26–33% were from E (mostly from the understory). Our results demonstrate the potential of concurrent above- and below-canopy EC measurements for ET partitioning.

Using foliar δ13C from high‐Andean plants (Silala River basin) as a measure of potential evapotranspiration through water use efficiency

AbstractThe water‐dependent nature of arid ecosystems is closely related to the coupling between energy input through photosynthesis and the loss of water through transpiration (Tr), which can be expressed as water use efficiency (WUE). The relationship, however, between environmental factors and plant physiology in controlling evapotranspiration is not well understood in high‐altitude arid environments. Here, we review the use of carbon isotope fractionation (δ13C) to indirectly track fluctuations in WUE and the use of the portable chamber method to partition landscape actual evapotranspiration (ETa) into Tr and bare soil evaporation (Ebs) in the alluvial deposits of the Silala River, a high elevation watershed located in northern Chile. Landscape ETa was also measured with Eddy covariance (EC) systems in the basin's riparian wetland and alluvial hillslope deposits. Carbon isotope results were consistent with what is known from the literature regarding these high‐elevation ecosystems. WUE, as estimated by carbon isotope discrimination values, decreased in summer (the wet season), and increased in winter at all sites. These results were consistent with the EC measurements. Changes in WUE were much greater in the valley wetlands than along the hillslopes, most likely due to a large drop in available soil moisture along the valley bottom during the dry season. Portable chamber results obtained during summer and winter field campaigns showed that at the landscape scale, hillslope ETa was mainly dominated by bare soil evaporation; and ETa partitioning into Tr and Ebs had a seasonal change.This article is categorized under: Engineering Water &gt; Methods Science of Water &gt; Hydrological Processes

Evapotranspiration Partitioning and Estimation Based on Crop Coefficients of Winter Wheat Cropland in the Guanzhong Plain, China

Accurate estimation and effective portioning of actual evapotranspiration (ETa) into soil evaporation (E) and plant transpiration (T) are important for increasing water use efficiency (WUE) and optimizing irrigation schedules in croplands. In this study, E/T partitioning was performed on ETa rates measured using the eddy covariance (EC) technique in three winter wheat growing seasons from October 2020 to June 2023. The variation in the crop coefficients (Kc, α, and KHc) were quantified by combining the ETa and reference evapotranspiration rates using the Penman–Monteith, Priestley–Taylor, and Hargreaves equations. In addition, the application of models based on the modified crop coefficient (Kc, α, and KHc) was proposed to estimate the ETa rates. According to the obtained results, the average cumulative ETa, T, and E rates in the three winter wheat growth seasons were 471.4, 265.2, and 206.3 mm, respectively. The average T/ETa ratio ranged from 0.16 to 0.72 at the different winter wheat growth stages. Vapor pressure deficit (VPD) affected the ETa rates at a threshold of 1.27 KPa. The average Kc, α, and KHc values in the middle stage were 1.34, 1.54, and 1.21, respectively. The measured ETa rates and ETa rates estimated using the adjusted Kc, α, and KHc showed regression slope coefficients of 0.96, 0.99, and 0.96, and coefficients of determination (R2) of 0.92, 0.93, and 0.90, respectively. Therefore, the Priestley–Taylor-equation-based adjusted crop coefficient is recommended. The adjusted crop-coefficient-based models can be used as valuable tools for local policymakers to effectively improve water use.

Quantitative contribution of cryogenic vacuum extraction and radial water transport to xylem-source water deuterium offsets

The positions and magnitudes of deuterium offsets between bulk xylem and corresponding source waters are under debate and quantifying them is essential for isotope-based ecohydrological investigations. In this study, stable isotopes (δ2H, δ18O, and δ13C), iteration method, and rehydration experiments were combined to quantitatively determine the magnitude of cryogenic vacuum extraction (CVE)- and radial water transport (RWT)-induced deuterium offsets using one riparian tree species Salix babylonica L. A modified potential water source line (MPWL) was proposed to identify the total δ2H offsets between bulk xylem and source waters. The relationships between δ2H offsets induced by CVE or RWT and plant water content, leaf δ13C values, soil water content (SWC), and the depth to the water table (WTD) were investigated. Results showed that the bulk xylem waters in different tissue positions of S. babylonica showed −7.0 ‰ to −4.0 ‰ deuterium depletion relative to MPWL at four different sites (p < 0.01). The isotopic compositions of sap water coincided well with MPWL on the dual-isotope plot at the four sites. The CVE- and RWT-induced δ2H offsets accounted for 75.1 % and 24.9 % of the total δ2H offsets, respectively. The CVE-induced δ2H offsets were significantly negatively correlated with plant water content. In comparison, the RWT-induced δ2H offsets were negatively related to plant leaf δ13C values, trunk water content, and SWC, but positively correlated with WTD. This study provides a quantitative contribution of two major sources of deuterium offsets. The results provide critical insights into isotope-based plant water source identification and evapotranspiration partitioning.

Partitioning of Evapotranspiration and Crop Coefficients of Lentil Under Conserved Soil Moisture Conditions

Partitioning of Evapotranspiration and Crop Coefficients of Lentil Under Conserved Soil Moisture Conditions

Water Consumption Structure and Root Water Absorption Source of an Oasis Cotton Field in an Arid Area of China

This research, conducted at the National Field Science Observation and Research Station of the Aksu Farmland Ecosystem in Xinjiang, was performed to partition evapotranspiration components, identify the main water absorption depth, and quantify the contribution of soil water at different depths during different growing stages of cotton by combining hydrogen and oxygen stable isotopes and the MixSIAR model. The results indicated that evapotranspiration in the seeding stage, bud stage, flowering and boll stage, boll opening stage, and harvesting stage were 88 mm, 137 mm, 542 mm, 214 mm, and 118 mm, respectively, and the corresponding transpiration accounted for 51%, 82%, 88%, 85%, and 72% of evapotranspiration. With the development of cotton roots, the water absorption depth gradually increased, and the main absorption depths in the late bud stage, mid flowering and boll stage, late flowering and boll stage, boll opening stage, and harvesting stage were 0–20 cm, 40–60 cm, 60–80 cm, 80–100 cm, and 0–20 cm, respectively, with corresponding contributions of 35.4%, 40.9%, 27.7%, 29.9%, and 22.5%. Our results can provide a theoretical foundation for the accurate irrigation management of cotton fields.

Evapotranspiration partitioning based on underlying conductance in a complex tree-grass orchard ecosystem in the humid area of southern China

Partitioning of evapotranspiration (ET) into its components (vegetation transpiration (T), soil evaporation (ESoil) and evaporation from intercepted water (Ei)) in agroforestry systems can provide valuable assessments of plant water use. However, it is still challenging to quantify the contributions of different vegetation elements to T, especially when the underlying surface is complex, consisting of trees and heterogeneous grass. Based on the water-carbon coupling theory and flux conservation hypothesis, an ET partitioning method was proposed to partition total ET into T from the tree (TTree) and understory grass (TGrass), and ESoil in a humid region tree-grass orchard ecosystem during three growing seasons (from mid-March to end-October) of 2018–2020. The results demonstrated that the partitioned TTree and understory evapotranspiration (ETun=TGrrass+ESoil) agreed well with the values measured by the heat-pulse sap flow sensors (R2 of 0.744, and RMSE of 0.379 mm d–1) and micro-lysimeters (R2 of 0.650 and RMSE of 0.545 mm d–1). The derived underlying transpiration (TGs=TTree+TGrass) was strongly consistent with Zhou's underlying water use efficiency approach (R2 of 0.939 and RMSE of 0.382 mm d–1). The estimated TGs was 536, 433 and 383 mm during the three growing seasons, resulting in a higher TGs/ET ratio of 0.74 ± 0.03 (±standard deviation). Among them, TGrass contributed to 0.32 ± 0.05 ET, while the contribution was 0.42 ± 0.02 for TTree. Esoil and residual evaporation (including Ei and estimated error) accounted for a small ET proportion, with the value of 0.13 ± 0.02 and 0.14 ± 0.01, respectively. The partitioned ET components (TTree/ET, TGs/ET and ESoil/ET) were well controlled by variations of atmospheric evaporation demand and vegetation factors, but were little affected by soil water content. Increased tree leaf area limited TGrass due to shading effects, however, TGrass tended to contribute more transpiration water under a higher atmospheric evaporation environment. The findings of this study enhance the knowledge about the contributions of individual ET components on total ET in a complex tree-grass ecosystem, and provide further insights on plant-plant interactions in terms of water use strategies and orchard water management.

Characterizing uncertainty in process-based hydraulic modeling, exemplified in a semiarid Inner Mongolia steppe

Assessing root sources of three uncertainties – parameterization of soil hydraulic characteristics, boundary conditions, and estimation of source/sink terms – is a significant challenge in soil water transport modeling. This study aims to evaluate the uncertainty of three each widely-used parameter estimation methods affecting plot-scale water dynamics. The study employs HYDRUS, a process-based hydrologic model, to incorporate these uncertainties and compare model predictions to measured values in a semiarid Inner Mongolia steppe, China. Soil hydraulic parameters are determined using two direct methods (laboratory-derived approach and evaporation method) and one indirect method (neural network). While each hydraulic parameter method generally simulates soil moisture dynamics, the evaporation method performed better, especially under dry conditions. This suggests that measuring the intensity properties, such as unsaturated hydraulic conductivity, with the evaporation method is crucial for reasonable soil moisture simulation. The study also demonstrates the impact of different applied boundary conditions on simulated soil moisture, specifically the partitioning of reference FAO evapotranspiration via one direct method (soil fraction cover) and two indirect methods (leaf area index and crop height). The partitioning via soil fraction cover reflected a better simulation. Additionally, the study compares the uncertainties of root water uptake function with root growth parameters and constant root depth referenced to grass and pasture, and finds no significant difference among them. Comparing three sources of uncertainty in predicting soil moisture, the study concludes that the input soil hydraulic parameter is more sensitive than evapotranspiration partitioning or representation of root water uptake function. Our study highlights that measuring soil intensity properties can better reflect the effects of land use change, such as compaction, on field water transports.

Management practices regulate the response of canopy and ecosystem water use efficiency in cropland ecosystems

ContextWith rising global temperature and frequent drought events, water resource supply is facing to great challenge. Understanding the effects of different management practices on cropland water use efficiency (WUE) is crucial to promote effective allocation of water resources and food security. ObjectiveThe objective of this study was to reveal the contrasting effects of different farm management practices including continuous and rotational, tillage and no-till, irrigated and rainfed on cropland water use efficiency from canopy (WUET) to ecosystem (WUEET) scales, respectively. MethodsOn the basis of 26 site-years eddy covariance-based carbon and water flux observations, this study used the underlying WUE (uWUE) model to partition evapotranspiration (ET) across maize and soybean ecosystems. Three control groups were used to compare the effects of different management practices on the coupling relationships between terrestrial carbon and water cycles, respectively. ResultsCompared with the rotational maize-soybean site, both WUET and WUEET at the continuous maize site were larger, which was mainly ascribed to stronger gross primary productivity (GPP) for the C4 crop. But over the maize-planting years, both WUET and WUEET at the rotational site were higher than the continuous site owing to the nitrogen fixation by rhizobia for preceding year, especially for WUET. Under conventional tillage and no-till management, multi-year mean WUEET and WUET were contrasting. WUEET under no-till was apparently larger than WUEET under till, because of similar GPP but relatively lower ET. Meanwhile, more water was consumed by crop transpiration (T) under no-till, which led to lower WUET in comparison with WUET under till. In comparison with the rainfed farming, GPP, ET and T all increased apparently after irrigation. However, both WUET and WUEET at the irrigated site were not improved owing to more water lost with relatively fewer GPP increase. ConclusionsThe study suggested that the crop rotation system can improve both WUEET and WUET apparently. Meanwhile, fewer water resource was consumed by cropland under no-till management. Finally, irrigation farming improved the cropland GPP but reduced WUEET and WUET. ImplicationsOur results provide valuable management implications for understanding the mechanisms that can enhance water use efficiency and result in significant water savings and high yield.

Ecosystem Evapotranspiration Partitioning and Its Spatial–Temporal Variation Based on Eddy Covariance Observation and Machine Learning Method

Partitioning evapotranspiration (ET) into vegetation transpiration (T) and soil evaporation (E) is challenging, but it is key to improving the understanding of plant water use and changes in terrestrial ecosystems. Considering that the transpiration of vegetation at night is minimal and can be negligible, we established a machine learning model (i.e., extreme gradient boosting algorithm (XGBoost)) for soil evaporation estimation based on night-time evapotranspiration observation data from eddy covariance towers, remote sensing data, and meteorological reanalysis data. Daytime T was consequently calculated as the difference between the total evapotranspiration and predicted daytime soil evaporation. The soil evaporation estimation model was validated based on the remaining night-time ET data (i.e., model test dataset), the non-growing season ET data of the natural ecosystem, and ET data during the fallow periods of croplands. The validation results showed that XGBoost had a better performance in E estimation, with the average overall accuracy of NSE 0.657, R 0.806, and RMSE 11.344 W/m2. The average annual T/ET of the examined ten ecosystems was 0.50 ± 0.08, with the highest value in deciduous broadleaf forests (0.68 ± 0.11), followed by mixed forests (0.61 ± 0.04), and the lowest in croplands (0.40 ± 0.08). We further examined the impact of the leaf area index (LAI) and vapor pressure deficit (VPD) on the variation in T/ET. Overall, at the interannual scale, LAI contributed 28% to the T/ET variation, while VPD had a small (5%) influence. On a seasonal scale, LAI also exerted a stronger impact (1~90%) on T/ET compared to VPD (1~77%). Our study suggests that the XGBoost machine learning model has good performance in ET partitioning, and this method is mainly data-driven without prior knowledge, which may provide a simple and valuable method in global ET partitioning and T/ET estimation.