Evapotranspiration Partitioning Research Articles

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.

Spatial patterns and recent temporal trends in global transpiration modelled using eco-evolutionary optimality

Transpiration from vegetation accounts for about two thirds of land evapotranspiration (ET), and exerts important effects on of global water, energy, and carbon cycles. Resistance-based ET partitioning models using remote sensing data are one of the main methods to estimate global land transpiration, overcoming the limitation by the sparse distribution and short observation periods of site-level measurements. However, the uncertainties of estimated transpiration for these models mainly come from the resistance parameterization based on specific empirical parameters across different plant functional types (PFT). A model based on eco-evolutionary optimization (P model) has recently been proposed to simulate stomatal conductance without the need of calibrated parameters. Here, we calculated global long-term (1982–2018) monthly transpiration with the Penman-Monteith (PM) equation using canopy conductance estimated by the P model (PM-P) and Ball-Berry-Leuning model (PM-BBL). Using the observations of SAPFLUXNET and FLUXNET sites as reference, the performance of PM-P was comparable with that of PM-BBL and Global Land Evaporation Amsterdam model (GLEAM). Multi-year mean and trends in growing season transpiration estimated by GLEAM and the PM-P model revealed a similar spatial distribution globally. Both GLEAM and the PM-P model showed a widespread increasing trend of growing season transpiration over 72.06%∼80.38% of global land, especially for some main greening hotspots with >3.0 mm/year. The good performance of the P model indicated that it could avoid the uncertainties emerging from the resistance parameterization with too many empirical parameters and had the potential to accurately estimate global transpiration.

Asymmetric response of Amazon forest water and energy fluxes to wet and dry hydrological extremes reveals onset of a local drought-induced tipping point.

Understanding the effects of intensification of Amazon basin hydrological cycling-manifest as increasingly frequent floods and droughts-on water and energy cycles of tropical forests is essential to meeting the challenge of predicting ecosystem responses to climate change, including forest "tipping points". Here, we investigated the impacts of hydrological extremes on forest function using 12+ years of observations (between 2001-2020) of water and energy fluxes from eddy covariance, along with associated ecological dynamics from biometry, at the Tapajós National Forest. Measurements encompass the strong 2015-2016 El Niño drought and La Niña 2008-2009 wet events. We found that the forest responded strongly to El Niño-Southern Oscillation (ENSO): Drought reduced water availability for evapotranspiration (ET) leading to large increases in sensible heat fluxes (H). Partitioning ET by an approach that assumes transpiration (T) is proportional to photosynthesis, we found that water stress-induced reductions in canopy conductance (Gs ) drove T declines partly compensated by higher evaporation (E). By contrast, the abnormally wet La Niña period gave higher T and lower E, with little change in seasonal ET. Both El Niño-Southern Oscillation (ENSO) events resulted in changes in forest structure, manifested as lower wet-season leaf area index. However, only during El Niño 2015-2016, we observed a breakdown in the strong meteorological control of transpiration fluxes (via energy availability and atmospheric demand) because of slowing vegetation functions (via shutdown of Gs and significant leaf shedding). Drought-reduced T and Gs , higher H and E, amplified by feedbacks with higher temperatures and vapor pressure deficits, signaled that forest function had crossed a threshold, from which it recovered slowly, with delay, post-drought. Identifying such tipping point onsets (beyond which future irreversible processes may occur) at local scale is crucial for predicting basin-scale threshold-crossing changes in forest energy and water cycling, leading to slow-down in forest function, potentially resulting in Amazon forests shifting into alternate degraded states.

Partitioning evapotranspiration using 18O abundance to understand water use efficiency in maize produced using ridge-furrow mulching system

The ridge-furrow mulching system (RFMS) is an efficient farming strategy that has been used widely for improving crop productivity in dryland agriculture. However, the physical mechanism that allows RFMS and its optimal ridge/furrow ratio to drive the efficient utilization of water in fields are unclear. Thus, we conducted a field research for two years with spring maize based on a long-term field experiment in a semiarid region of Northwestern China to quantify evaporation and transpiration, and their relationship with maize growth, yield and water use efficiency (WUE) under RFMS. Three planting patterns of ridge/furrow ratio were tested: (1) RF40, RFMS with 60 cm wide ridges and 40 cm wide furrows; (2) RF60, RFMS with 60 cm wide ridges and 60 cm wide furrows; (3) FP, traditional flat planting (control). Stable oxygen isotope (18O) analysis was used to partition the evapotranspiration (ET) into evaporation (E) and transpiration (T). The results showed that RFMS significantly increased the soil water content (SWC) in the early and late growing season, but not in the middle period (60–120 days after sowing, DAS) when dry matter accumulated rapidly and the highest transpiration occurred. RFMS significantly decreased the nonproductive evaporation, thereby increased the physiologically significant maize transpiration by 25.0–35.7% during the whole growing season. Therefore, the transpiration fraction of ET (T/ET) was 73% and 70% under RF40 and RF60 during the whole growing season, which were significantly higher than FP. Compared to FP, the maize yield and WUE increased significantly by 55.1% and 52.2% under RF40 and by 49.5% and 47.6% under RF60. We found RFMS drives optimization of ET fractions by increased transpiration but decreased evaporation and then improved WUE of maize, higher ridge/furrow ratio (RF40) create uneven line space and higher mulching area in the field, thus drives optimization of ET components and increased yield and WUE the most. Clarification of this physical mechanism improves our understanding of the optimization of maize production under RFMS, thereby helping to optimize the ridge/furrow ratio in semiarid regions.

Cactus–sorghum intercropping combined with management interventions of planting density, row orientation and nitrogen fertilisation can optimise water use in dry regions

Some strategies can optimise the use of water in crops under deficit, either by increasing yield or by reducing actual crop evapotranspiration (ET), to promote the sustainable intensification of production systems. The objective was to evaluate how the spacing, planting orientation, nitrogen fertilisation and intercropping strategies impact the dynamics of water in the soil, ET partitioning, and water use indicators for forage cactus and cactus–sorghum intercropping. Four experiments were conducted between 2018 and 2020 in the Brazilian semi-arid region. In the first two sites (I and II), the cladodes of the intercropped forage cactus and sorghum were spaced at 0.10, 0.20, 0.30, 0.40 and 0.50 m with rows-oriented east–west and north–south. In site III, the intercropped rows were spaced at 1.00, 1.25, 1.50 and 1.75 m. Site IV, which contained the forage cactus crop exclusively, was treated with four nitrogen levels (50, 150, 300 and 450 kg N ha−1). The management interventions improved water use more by increasing dry matter than by reducing ET in the cropping system. Intercropping promoted the greatest increase in water productivity (130 %). Increasing N doses in the forage cactus-only crop reduced ET by up to 39 % but increased deep drainage losses by up to 365 %. The most promising management practices for optimising water resources were as follows: spacing of 0.10 m between cactus plants in the intercropping trial under east–west row orientation, as it promoted greater water use efficiency (76 %); spacing of 0.30 m in the north–south orientation; and row spacing of 1.50 m, as it improved water productivity (6.89 kg m−3). Thus, interventions in management should be adopted to optimise water use in intercropping systems with forage cactus, aiming at sustainable intensification in dry environments.

Dependence of evapotranspiration validity on shallow groundwater in arid area-a three years field observation experiment

Groundwater resources strongly support crop water requirement and economic yield improvement in semi-arid and arid areas. Water resources management require more investigation in the response of evapotranspiration (ET) partitioning and water use efficiency to groundwater change. Observational data collected over a three-year period, including water and carbon flux, vegetable sap flow, meteorological factors, soil water content, and groundwater depth, were employed in this study. Additionally, an improved Shuttleworth-Wallace model was used for partitioning plant transpiration and bare soil evaporation (E) in a sunflower cropland with shallow groundwater. The ET of the sunflower was founded to be 353.5 ± 15.0 mm, and approximately 48.3 ± 7.4% derived from the groundwater evaporation. E amounted approximately 145.6 ± 6.1 mm over the growth season, and around 47.2% of which was lost during the seeding stage. Furthermore, its dynamics depended on crop growth and groundwater table depth (WTD). These large amounts of non-productive soil E had a negative impact on the water-carbon coupling. The presence of E had resulted in a lower ecosystem water use efficiency (WUEe), measured at 1.02 ± 0.33 g C kg−1 H2O, compared to canopy water use efficiency (WUEc) of 1.68 ± 0.41 g C kg−1 H2O. These results demonstrated that regulating WTD could improve WUEe by increasing WUEc and reducing soil E. To achieve the higher WUEe, an optimum WTD was range of 1.5–2.0 m for border-irrigated sunflower field grown on sandy loam, identified by an analysis of normalized evaporation difference at the varying WTD. This finding highlights the significance of maintaining specified WTD, providing valuable insights into the complex interplay between crop evapotranspiration components and shallow groundwater. Moreover, it offers guidance for precision agriculture irrigation practices.

Partitioning Evapotranspiration in a Cotton Field under Mulched Drip Irrigation Based on the Water-Carbon Fluxes Coupling in an Arid Region in Northwestern China

Measuring evapotranspiration (ET) components in cotton fields under mulched drip irrigation is needed to improve water use efficiency and promote the development of water-saving agriculture. In this study, an Eddy Covariance (EC) system was used to observe the water-carbon fluxes of cotton fields under mulched drip irrigation in an arid region during two years (2021–2022). The Underlying Water Use Efficiency (uWUE) method was used to partition the ET into transpiration (T) and evaporation (E) in order to reveal the changing characteristics of ET and its components in cotton fields under mulched drip irrigation and analyze the effects of environmental factors on each component. The results showed that the diurnal variation of ET was the same as gross primary productivity (GPP), and their course of change showed a bimodal curve at budding, blooming, and boll stages. The relationship of T at different growth stages was the same as ET, which is blooming and boll stage &gt; budding stage &gt; boll opening stage &gt; seedling stage. ET and its components were mainly affected by temperature (Tair) and net radiation (Rn). This study can provide a theoretical and practical basis for the application of uWUE in cotton fields under mulched drip irrigation and a scientific basis for the rational allocation of water resources and the formulation of a scientific water-saving irrigation system for farmland in an arid region.

The Influence of the Osmotic Potential on Evapotranspiration

Climate change is expected to affect the quality of soil and water, resulting in a significant impact on soil water balance in various regions around the world. Soil water potential plays a significant role in influencing evapotranspiration (ET), which is a crucial component of the soil water balance. The matric potential and the osmotic potential are the main components of the soil water potential. The osmotic potential is particularly important in dry soils, salt-affected soils, coastal lands, or when low-quality water is utilized for irrigation. Despite its importance, the impact of osmotic potential on ET has not been well-studied compared to other factors. Therefore, we conducted a study to investigate the impact of osmotic potential on ET from small-scale lab lysimeters planted with grass and equipped with scales and data loggers connected to computers. To create different osmotic potential levels, we irrigated the lysimeters with two different water qualities: distilled water and 4.79 dS.m−1 NaCl solution. The lysimeters were kept in well-watered conditions, and daily ET was monitored. Our results indicate a strong correlation between osmotic potential and ET. After three months of applying the treatments, the lysimeters with lower osmotic potential had a 39% reduction in cumulative ET compared to those irrigated with distilled water. Moreover, the osmotic stress affected plant health, leading to a notable decrease in the leaf area index and exerting a significant influence on evapotranspiration partitioning components, including transpiration and evaporation.

Evapotranspiration partitioning and agricultural drought quantification with an optical trapezoidal framework

The Optical Trapezoidal method is a recently proposed method using solely optical remote sensing data with promising potential in hydroclimatic applications; however, its application in evapotranspiration (ET) partitioning is still questionable. This study therefore improved an Optical Trapezoidal-based Evaporative Fraction (OPTREF) model reliant on the physical association between soil surface properties and shortwave infrared transformed reflectance (STR). The proposed model was parameterized using trapezoidal shapes formed by STR and Normalized Difference Vegetation Index (NDVI) retrieved from Sentinel-2 to compute contribution of transpiration to ET (T/ET) in the Central Valley region of California, United States. Results indicated that the OPTREF model efficiently captured regional plant characteristics and seasonal variation of T/ET for three typical land cover types in study region (cropland, grassland, and savanna), particularly T/ET discrepancies rising from alternative cropping systems in cropland. Importantly, predicted T/ET values were in good agreement with previous modelling and empirical reports; for example, T/ET ranges in [0.37-0.75] for cropland, [0.30-0.82] for grassland, and [0.39-0.77] for savanna. In addition, applicability of the OPTREF approach was investigated in tracking agricultural drought using evaporative drought index (EDI), together with analysis of physiological response of T/ET to drought. The OPTREF-based EDI consistently reflected the same drought patterns indicated by records of vegetation health index and drought area percentages data from NOAA. Also, our findings uncovered that under extreme drought, the T/ET ratio is likely to increase and might be approximately equal to 1.0 in areas covered by drought-tolerant plants. Overall, this OPTREF approach offers novelty, simplicity, and versatility to satellite-based remote sensing of ET partitioning and identifying agricultural drought, especially in the context of rapid climate variation.

Comparison of Evapotranspiration Partitioning and Dual Crop Coefficients of Direct-Seeded and Transplanted Rice in the Poyang Lake Basin, China

Direct-seeded rice (DSR) has received much attention because of its advantages in having low labor costs compared to the traditional transplanted rice (TPR). Investigating the differences in evapotranspiration (ET) partitioning and crop coefficients (Kc) between DSR and TPR is essential in understanding how agricultural water demand is affected by crop rotation. In this study, the water fluxes of two-year (2017–2018) growing seasons were collected from a pair of eddy covariance (EC) towers for DSR and TPR in the Poyang Lake Basin, Southern China. This study aims to compare the seasonal characteristics of the ET components (evaporation, E, and transpiration, T) and dual crop coefficients (basal crop coefficient, Ks·Kcb, and soil/water crop coefficient, Ke) of DSR with those of TPR. The ET values for the 2017 and 2018 growing seasons were 374 mm and 436 mm for the DSR, respectively, and 309 mm and 342 mm for the TPR. The seasonal T/ET values in 2017 and 2018 were 0.40 and 0.46 for the DSR, respectively, and 0.49 and 0.52 for the TPR, indicating that the higher ET values for the DSR can be mostly attributed to E. The DSR had overall higher Kc values than the TPR because of free water evaporation during the initial stage and a higher plant density. Our results enrich the Kc dataset for DSR and have great implications for the sustainable irrigation of the Poyang Lake Basin in the future.