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Combining the FAO-56 method and the complementary principle to partition the evapotranspiration of typical plantations and grasslands in the Chinese Loess Plateau
Numerous studies have evaluated the application of Remote Sensing (RS) techniques for mapping actual evapotranspiration (ETa) using Vegetation-Index-based (VI-based) and surface energy balance methods (SEB). SEB models computationally require a large effort for application. VI-based methods are fast and easy to apply and could therefore potentially be applied at high resolution; however, the accuracy of VI-based methods in comparison to SEB-based models remains unclear. We tested the ETa computed with the modified 2-band Enhanced Vegetation Index (METEVI2) implemented in the Google Earth Engine – for mapping croplands’ water use dynamics in the Lower Colorado River Basin. We compared METEVI2 with the well-established RS-based products of OpenET (Ensemble, eeMETRIC, SSEBop, SIMS, PT_JPL, DisALEXI and geeSEBAL). METEVI2 was then evaluated with measured ETa from four wheat fields (2017–2018). Results indicated that the monthly ETa variations for METEVI2 and OpenET models were comparable, though of varying magnitudes. On average, METEVI2 had the lowest difference rate from the average observed ETa with 17 mm underestimation, while SIMS had the highest difference rate (82 mm). Findings show that METEVI2 is a cost-effective ETa mapping tool in drylands to track crop water use. Future studies should test METEVI2’s applicability to croplands in more humid regions.
Providing adequate water for crops is a key consideration for agricultural resiliency. Climate change is expected to exert substantial pressure on crop water supplies through increased rainfall variability and increased evaporative demand. The temporal and spatial variability of these climate change impacts, in combination with many feedbacks, at times operating at different scales, makes identifying specific, yet broadly applicable “one‐size‐fits‐all” soil management solutions challenging. Adaptations that work well in one location may represent a poor management choice in another. Considering the local combinations of climate, soils, and crop production systems is fundamental for identifying appropriate strategies. Nonetheless, some broadly applicable soil management pathways for increasing soil water availability can be identified: (i) maximizing rainfall capture, (ii) maximizing soil water storage capacity (intensity × volume), and (iii) suppressing unproductive evaporative water losses, particularly during intervals when leaf area of the primary crop is low. Selecting climate, soil, and production‐system‐appropriate tillage and surface cover management practices is among the most relevant considerations for improving soil water availability. Maintaining surface cover favors evaporation suppression and increased capture of precipitation. Tillage choices affect internal soil water storage through impacts on soil properties that influence storage intensity and by altering the volume of soil available for root growth, as well as through effects on surface cover. Promoted management practices aimed at improving soil water availability for increased agricultural resiliency must be practically feasible and cost‐effective in order to support their broad adoption, and such constraints should be an important consideration in their development.
Water residence time (WRT) is an important lake attribute that influences almost all in‐lake nutrient processes. However, information on WRT is often missing or outdated, especially in regions lacking lake inflow or outflow observations. Recent advances in global precipitation and evapotranspiration (ET) products and calibration‐free ET models provide the possibility of estimating lake WRT at large scales using estimated runoff from the water balance over the lake catchment. Here, we tested four global ET products and two calibration‐free ET models for estimating the mean annual lake outflow and WRT of 67 globally distributed lakes using precipitation inputs from four global precipitation products. The results show that the WRT estimated using the water balance method and global ET estimates generally correlates well with reference WRT (R = 0.90 ∼ 0.99) when using reliable precipitation inputs and is slightly more accurate than the estimates from the existing global WRT dataset and methods (R = 0.79 ∼ 0.94). Despite the large relative error (RE) in the estimates of the WRT, the RE of the phosphorus retention derived from the estimated WRT is acceptable (RE < ±30%). The simplicity and versatility of this approach underline its potential applications to large‐scale studies that require a fair estimation of WRT.
The complementary relationship (CR) between actual and potential evaporation has undergone rapid development over the past decades. Commonly, the evaluation of CR models does not comprehensively cover spatial and temporal aspects of model performance; thus, the spatial and temporal accuracy and parameter sensitivity of different CR models remain relatively unknown, especially in ungauged basins. We examine the spatial and temporal performance and parameter sensitivity of four CR models with fixed parameters at the monthly scale over the source region of the Yellow River in China. Additionally, two CR models with distributed parameters were selected as comparisons. Because of the lack of evaporation spatiotemporal “true” values, the corrected water‐balance‐derived evaporation was used as the temporal reference, and the ensemble mean of multiple evaporation products was used as the spatial reference. We find that trade‐offs between the spatial and temporal accuracy of CR models with fixed parameters for basin evaporation should be considered in the application. Although four CR models with fixed parameters exhibited different spatial and temporal performances, their overall performance was generally similar. The parameters of CR models had consistent spatial and temporal sensitivities, and the sensitivity of parameter αe was stronger than that of other parameters. This study demonstrates that CR models with distributed parameters have a promising future in estimating basin or regional evaporation. An integrated spatial and temporal evaluation framework can better distinguish the performance of CR models. The methodologies can be used as a guideline for evaluating and calibrating CR models that match an application's needs.
The linear form of the nondimensional complementary relationship (CR) follows from an isenthalpic process of evaporation under a constant surface available energy and unchanging wind. Mixing of external moisture into the boundary layer (BL) alters the dry‐end second‐type boundary condition yielding a polynomial that can be further generalized into a three‐parameter (Priestley‐Taylor α, a, b) power function (PF3), capable of responding to the level of such admixing. With the help of FLUXNET data and setting a = 2 for a possible recapture of the linear and/or polynomial versions of the CR, it is demonstrated that the resulting two‐parameter PF (i.e., PF2) excels among the CR‐based two‐parameter models considered in this study. PF2 is then employed with a globally set constant value of α = 1.1 and 0.5° monthly data across Australia, while calibrating b against the multiyear water‐balance evaporation rate on a cell‐by‐cell basis. The resulting bi‐modal histogram peaks first near b = 2 (recapturing the polynomial CR) when moisture admixing is significant, and then at b → 1 (yielding the linear CR) when mixing effects are negligible. Unlike the linear or polynomial CR versions, PF2 can respond to the general efficiency of external moisture admixing through its parameter b, making it applicable even near sudden discontinuities in surface moisture. A new duality emerges with the PF2: while α accounts for the effect of entrainment of free tropospheric drier air into the BL on the resulting wet‐environment evaporation rate, b does so for moisture on the drying‐environment evaporation rates.
The potential evapotranspiration is considered as an important element of the hydrological cycle, which plays an important role in agricultural studies, management plans of irrigation and drainage networks, and hydraulic structures. Estimating the potential evapotranspiration reference of particular climatic regions at different time scales, which is one of the most important atmospheric parameters, is of a particular importance in the optimal use of resources. The time series analysis method, GARCH model, is applied in order to investigate changes and estimate the potential evapotranspiration. In the present study, the efficiency of GARCH series model related to processes of modeling and estimating potential evapotranspiration, which is estimated by FAO Penman–Monteith and Hargreaves methods, was investigated. Also, future values of potential evapotranspiration are modelled and estimated at the synoptic station of Tabriz. Results showed that Time Series is considered as a precise tool to estimate evapotranspiration values. It was found that GARCH (1.1) time series has better results for FAO Penman–Monteith and Hargreaves methods compared to other models; also, it simulates the process of time series changes with less error. Observed and predicted evapotranspiration charts of both methods indicated that observational evapotranspiration was highly close to the lower limit of estimated evapotranspiration. Therefore, applying lower limit estimation as a prediction value was suggested.
Actual evapotranspiration (ETa) is important since it is an important link to water, energy, and carbon cycles. Approximately 96% of the Qinghai-Tibet Plateau (QTP) is underlain by frozen ground, however, the ground observations of ETa are particularly sparse–which is especially true in the permafrost regions–leading to great challenge for the accurate estimation of ETa. Due to the impacts of freeze-thaw cycles and permafrost degradation on the regional ET process, it is therefore urgent and important to find a reasonable approach for ETa estimation in the regions. The complementary relationship (CR) approach is a potential method since it needs only routine meteorological variables to estimate ETa. The CR approach, including the modified advection-aridity model by Kahler (K2006), polynomial generalized complementary function by Brutsaert (B2015) and its improved versions by Szilagyi (S2017) and Crago (C2018), and sigmoid generalized complementary function by Han (H2018) in the present study, were assessed against in situ measured ETa at four observation sites in the frozen ground regions. The results indicate that five CR-based models are generally capable of simulating variations in ETa, whether default and calibrated parameter values are employed during the warm season compared with those of the cold season. On a daily basis, the C2018 model performed better than other CR-based models, as indicated by the highest Nash-Sutcliffe efficiency (NSE) and lowest root mean square error (RMSE) values at each site. On a monthly basis, no model uniformly performed best in a specific month. On an annual basis, CR-based models estimating ETa with biases ranging from −94.2 to 28.3 mm year−1, and the H2018 model overall performed best with the smallest bias within 15 mm year−1. Parameter sensitivity analysis demonstrated the relatively small influence of each parameter varying within regular fluctuation magnitude on the accuracy of the corresponding model.
Soil hydrological properties play a key role in soil hydrological processes. However, the effect of long-term vegetation restoration on soil hydrological properties and the corresponding influencing mechanisms remains poorly understood. Here, three soil hydrological properties including saturated water-holding capacity (SWHC), field capacity (FC) and saturated hydraulic conductivity (Ks), as well as several basic soil properties in the Zhifanggou watershed of the Loess plateau were investigated. The variations in SWHC, FC and Ks under different vegetation restoration types and their dominant influencing factors were analyzed. Moreover, we collected available Ks data from peer-reviewed publications to determine the land use with the largest Ks across the entire Loess Plateau. The results showed that SWHC FC and Ks were increased after 20 years of vegetation restoration. The higher Ks was found in shrubland and forest in the whole Loess Plateau. Compared with cropland, Ks in shrubland was increased by 87.10% at 0–20 cm, 48.89% at 20–40 cm, and 18.37% at 40–100 cm, respectively, indicating that the impact of revegetation on Ks were most obvious in the upper soil layer. Bulk density (BD), total porosity (TP), capillary porosity (CP), noncapillary porosity (NCP) and soil organic matter (SOM) had a significant effect on SWHC, FC and Ks for different land-use types (P < 0.01). Soil porosity (i.e., TP, CP and NCP) and BD, soil chemical properties (i.e., SOM and pH), and soil particle composition explained 93.8%, 59.2%, and 13.4% of the total variance in soil hydrological properties (i.e., SWHC, FC and Ks), respectively. This indicates that soil porosity and BD are the dominant factors affecting soil hydrological properties. Moreover, soil particle composition played an important role in regulating Ks, with the contribution of 38.6%. The established pedotransfer function (PTF) of Ks using BD, clay and silt content had a better performance than two existing PTFs. This research provides a more systematic and comprehensive understanding of the soil hydrological effect of vegetation restoration in the Loess Plateau.
Background
Water migration and use are important processes in trees. However, it is possible to overestimate transpiration by equating the water absorbed through the plant roots to that diffused back to the atmosphere through stomatal transpiration. Therefore, it is necessary to quantify the water transpired and stored in plants.
Method
The δ ² H/δ ¹⁸ O technique and heat ratio method were used to explore the water usage of coniferous and broad-leaved tree species, including the proportions of water used for transpiration and water storage.
Results
Platycladus orientalis and Quercus variabilis had strong plasticity in their water usage from different sources. Platycladus orientalis primarily used groundwater (30.5%) and the 60–100-cm soil layer (21.6%) throughout the experimental period and was sensitive to precipitation, absorbing water from the 0–20-cm layer (26.6%) during the rainy season. Quercus variabilis absorbed water from all sources (15.7%–36.5%) except from the 40–60-cm soil layer during the dry season. In addition, it did not change its water source but increased its groundwater uptake during the rainy season. The annual mean water fluxes of P. orientalis and Q. variabilis were 374.69 and 469.50 mm·year − 1 , with 93.49% and 93.91% of the water used for transpiration, respectively. However, nocturnal sap flow in P. orientalis and Q. variabilis was mainly used for water storage in the trunk rather than transpiration, which effectively alleviated drought stress and facilitated the transport of nutrients.
Conclusions
The water stored in both species comprised 6%–7% of the total water fluxes and, therefore, should be considered in water balance models.
Soil moisture is a limiting factor of ecosystem development in the semi-arid Loess Plateau. Characterizing the soil moisture response to its dominant controlling factors, such as land use and topography, and quantifying the soil-water carrying capacity for revegetation is of great significance for vegetation restoration in this region. In this study, soil moisture was monitored to a depth of 2 m in three land use types (native grassland, introduced grassland, and forestland), two landforms (hillslope and gully), and two slope aspects (sunny and shady) in the Nanxiaohegou watershed of the Loess Plateau, Northwest China. The MIKE SHE model was then applied to simulate the soil moisture dynamics under different conditions and determine the optimal plant coverage. Results showed that the average soil moisture was higher in native grassland than in introduced grassland and Platycladus orientalis forestland for a given topographic condition; however, a high soil moisture content was found in Robinia pseudoacacia forestland, with a value that was even higher than the native grassland of a sunny slope. The divergent results in the two forestlands were likely attributed to the differences in plant coverage. Gully regions and shady slopes usually had higher soil moisture, while lower soil moisture was usually distributed on the hillslope and sunny slope. Furthermore, the mean absolute relative error and Nash-Sutcliffe efficiency coefficient of the MIKE SHE model ranged between 2.8%–7.8% and 0.550–0.902, respectively, indicating that the model could effectively simulate the soil moisture dynamics. The optimal plant coverage was thus determined for hillslope P. orientalis by the model, corresponding to a leaf area index (LAI) value of 1.92. Therefore, for sustainable revegetation on the Loess Plateau, selecting suitable land use types (natural vegetation), controlling the planting density/LAI, and selecting proper planting locations (gully and shady slope regions) should be considered by local policy makers to avoid the over-consumption of soil water resources.
The Budyko framework, widely regarded as a simple and convenient tool to synthesize catchment water balance, is often employed with the atmospheric evaporative potential (Ep) that responds to water availability over a land surface. In this study, we demonstrated how the responsiveness of Ep to soil moisture deficiency affects outcomes from a conventional Budyko equation. We combined a two‐parameter Budyko equation with the state‐of‐the‐art complementary relationship (CR) of evaporation (E), and analytically showed that the two‐parameter Budyko equation corrects Ep to the wet environment E (Ew) of the CR. Using the Budyko equation combined with the CR, we assessed runoff sensitivity to climatic and land surface changes. Results showed that the CR could become a constraint for calibrating the implicit parameter of the Budyko equation. When compared to the Turc‐Mezentsev equation with Ep, the shape parameters of the two‐parameter Budyko equation increased to regenerate an ensemble of global E data sets. Correcting Ep to Ew via the Budyko equation with CR reduced runoff elasticities to land property changes, suggesting that climatic changes are more important to changes in runoff than a prior sensitivity assessment would suggest. This study also suggests that the two‐parameter Budyko equation isolates the effect of the Ep adjustment from the shape parameter, allowing it to more properly account for surface energy availability.
While large-scale terrestrial evapotranspiration (ET) information is essential for our understanding of the Earth's water and energy cycles, substantial differences exist in current global ET products due partly to uncertainties in soil- and vegetation-related parameters and/or precipitation forcing. Here a calibration-free complementary relationship (CR) model, driven purely by routine meteorological forcing (air and dew-point temperature, wind speed, and net radiation), mainly from ERA5, was employed to estimate global ET rates during 1982–2016. Modeled ET agrees favorably with (a) monthly eddy-covariance measurements of 129 global FLUXNET sites, and; (b) water-balance-derived ETof 52 basins at the multiyear mean and annual scales. Additional evaluations demonstrate that the CR is
very competitive, in comparison with other 12 widely used global ET products. The 35-years mean global land ET rate from the CR is 500 ± 6 mm yr−1 (72.3 ± 0.9 × 103 km3 yr−1) with more than 70% of the land
area exhibiting increasing annual ET rates over the study period. Globally, CR ET significantly increased at a rate of 0.31 mm yr−1 during 1982–2016, suggesting a 2.2% increase in global land ET over last 35 years. Model inter-comparisons indicate that global annual CR ET values and their trend are close to the median of not only the 12 ET products chosen but also that of 20 CMIP6 models. Since this calibration-free CR model requires no precipitation (except in sea-shore deserts for a subsequent ET correction), vegetation or
soil data, it could be incorporated into complex hydrological and/or climate models, thereby facilitating large-scale hydrological and climate simulations.
The deviations of the Priestley‐Taylor (PT) coefficient from a fixed value around 1.26 indicate a nonlinear dependence of wet surface evaporation (E) on the equilibrium evaporation (Erad, which is the radiation term in Penman potential evaporation [EPen]). The linear PT equation with a fixed coefficient underestimates E for small Erad but overestimates E for large Erad, whereas the Penman equation with calibrated linear wind function has the opposite bias. In this study, the sigmoid generalized complementary (SGC) equation by Han and Tian (2018a), https://doi.org/10.1029/2017wr021755 was applied to estimate the wet surface evaporation by setting its asymmetric parameter to infinity. The SGC equation with one fixed parameter captures the nonlinear dependence of E on Erad over wet surfaces by including the aerodynamic component of EPen with a reference wind function, and amends the shortages of the linear PT and Penman equation. By using datasets over open water surfaces of lakes and ocean, wetlands, and paddy fields, the validation results indicate that the wet surface SGC equation performed better than the linear PT and Penman equations on evaporation estimation. The success of the wet surface SGC equation has implications for the extension of the complementary principle to consider varying aerodynamic conditions over wet surfaces.
Proper parameterization of the parameter (α e ) that governs the wet environment evaporation is critical for the regional estimation of evapotranspiration (ET) using the generalized complementary relationship (GCR) model. Here, we proposed a global parameterization for the GCR model. We found that the GCR model is sensitive to the parameter α e , which varies spatially with the climate aridity index (AI, the ratio between the apparent potential ET and the precipitation) across 60 sites that span a large variety in climate types worldwide. We found that α e and the AI are generally more strongly correlated in drier climates (AI > 2) where water supply instead of energy supply is the limiting factor for actual ET. The strong correlation between α e and AI can be partly explained by 1) the usage of the air temperature measurements in the non-potential conditions instead of potential conditions, and 2) the insensitivity of the actual ET to the apparent potential ET in the drier climate. Temporally, the parameter α e exhibits seasonal courses at monthly scales and decreases with increasing of vapor pressure deficit (VPD) in a hysteresis loop. Incorporation of the seasonal course and hysteresis significantly improved the model performances at most of the sites. The global parameterization we established can help the GCR model to be a more useful tool for regional and global ET estimations.
The complementary principle has been widely used to estimate evaporation under different conditions. However, it remains unclear at which timescale the complementary principle performs best. In this study, evaporation estimations were conducted at 88 eddy covariance (EC) monitoring sites at multiple timescales (daily, weekly, monthly, and yearly) by using sigmoid and polynomial generalized complementary functions. The results indicate that the generalized complementary functions exhibit the highest skill in estimating evaporation at the monthly scale. The uncertainty analysis shows that this conclusion is not affected by ecosystem type or energy balance closure method. Through comparisons at multiple timescales, we found that the slight difference between the two generalized complementary functions only exists when the independent variable (x) in the functions approaches 1. The results differ for the two models at daily and weekly scales. However, such differences vanish at monthly and annual timescales, with few high x values occurring. This study demonstrates the applicability of generalized complementary functions across multiple timescales and provides a reference for choosing a suitable time step for evaporation estimations in relevant studies.
The simultaneous thermodynamic pathways (i.e., isenthalps) of the air at the measurement height and at the vegetated land surface under isobaric and adiabatic wetting/drying cycles of the environment make it possible to define the actual evaporation rate with the help of three (one measured and two derived) vapor pressure (and corresponding temperature) terms. From the first-order approximation about the constancy of the relative average speed which the two isenthalps are travelled at during drying out of the environment, a non-dimensional, linear form of the complementary relationship (CR) of evaporation naturally emerges, but now expressed by vapor pressures (and temperatures, respectively). Without an artificially low Priestley-Taylor parameter value this linear CR would overestimate the evaporation rates because the surface warms faster than the constant relative speed assumption permits. With the appropriate estimation of the wet-surface temperature and employment of realistic boundary conditions, the latter leading to a nonlinear CR, land evaporation rates can be estimated fairly accurately with minimal input variables (air temperature, humidity, wind speed and net surface radiation) and without any information of land surface properties. Not only actual but three potential evaporation rates can also be defined by linking the temperature/vapor pressure coordinates of the air and the surface isenthalps, thus reproducing certain existing formulations of the CR as well as re-creating an existing hybrid (containing, both non-dimensional vapor pressure and evaporation terms) version of it.
The Gully Consolidation and Highland Protection (GCHP) project was implemented across the Loess Plateau. Understanding the impacts of the GCHP requires applications of a distributed hydrologic model. The Soil and Water Assessment Tool (SWAT) and Geography Information System (GIS) were used for evaluating the GCHP project’s impact on runoff and sediments in the catchment. The correlation coefficient value, the relative error and the Nash–Sutcliffe index are 0.83, –2.30% and 0.89 for the calibration period of our SWAT model in Yanwachuan (YWC) gully by using SWAT Calibration and Uncertainty Programs (SWAT-CUP) software. The distributed hydrological model was used to simulate five scenarios of a typical watershed YWC gully in Longdong Loess Plateau, and analyzed the response of runoff, sediment, and evapotranspiration under different designed scenarios of GCHP project. The results show that gully head landfill (the loess soil were directly transferred and dumped from other places) can reduce the runoff and sediment. The simulated annual average runoff is about 2.5% lower after landfill than that in the natural state, but the evapotranspiration is about 2.4% higher. The grassland plays a more important role in water conservation than forest vegetation in the YWC watershed. The results also show that the SWAT model is useful for the study of the hydrological response of GCHP at the watershed scale. The results of this study are providing scientific information to predict and prevent soil erosion.
Most large-scale evapotranspiration (ET) estimation methods require detailed information of land use, land cover, and/or soil type on top of various atmospheric measurements. The complementary relationship of evaporation (CR) takes advantage of the inherent dynamic feedback mechanisms found in the soil-vegetation-atmosphere interface for its estimation of ET rates without the need of such biogeophysical data. ET estimates over the conterminous United States by a new, globally calibrated, static scaling (GCR-stat) of the generalized complementary relationship (GCR) of evaporation were compared to similar estimates of an existing, calibration-free version (GCR-dyn) of the GCR that employs a temporally varying dynamic scaling. Simplified annual water balances of 327 medium and 18 large watersheds served as ground-truth ET values. With long-term monthly mean forcing, GCR-stat (also utilizing precipitation measurements) outperforms GCR-dyn as the latter cannot fully take advantage of its dynamic scaling with such data of reduced temporal variability. However, in a continuous monthly simulation, GCR-dyn is on a par with GCR-stat, and especially excels in reproducing long-term tendencies in annual catchment ET rates even though it does not require precipitation information. The same GCR-dyn estimates were also compared to similar estimates of eight other popular ET products and they generally outperform all of them. For this reason, a dynamic scaling of the GCR is recommended over a static one for modeling long-term behavior of terrestrial ET.
The complementary principle, which was first proposed by Bouchet (1963), illustrates a complementary relationship among the actual evaporation, the potential evaporation and the apparent potential evaporation. It has generated increasing attention for estimating evaporation by using only routinely observed meteorological variables (radiation, wind speed, air temperature and humidity) without complex surface property parameters. However, this principle still poses great challenges because of the underlying uncertainties in estimating its critical parameter, namely, asymmetric parameter b. In this study, we adopted a sigmoid generalized complementary function and utilized the eddy covariance (EC) data from 217 sites around the world to determine b values in different ecosystems and their correlation with environmental factors. We found b has a mean value of 6.01 ± 0.08. The asymmetric parameter b is small in dry regions (i.e., the desert ecosystem, 0.42 ± 0.02) and increases as the land surface wetness improves. The ecosystem mean air temperature and vapor pressure deficit have negative correlations with b (Pearson correlation coefficients are −0.57 and −0.52, respectively), and the mean soil water content has a positive correlation with b (0.69). Besides, the sigmoid function has a favorable capability in estimating evaporation no matter based on the site-specific b values or the ecosystem mean b values. The ecosystem mean b values given in the current study also perform acceptably in the independent verifications, indicating these values can be applied extendedly for regional and global studies.
The complementary principle is an important methodology for estimating actual evaporation by using routinely observed meteorological variables. This review summaries its 56-year development, focusing on how related studies have shifted from adopting a symmetric linear complementary relationship (CR) to employing generalized nonlinear functions. The original CR denotes that the actual evaporation (E) and “apparent” potential evaporation (Epa) depart from the potential evaporation (Ep0) complementarily when the land surface dries from a completely wet environment with constant available energy. The CR was then extended to an asymmetric linear relationship, and the linear nature was retained through properly formulating Epa and/or Ep0. Recently, the linear CR was generalized to a sigmoid function and a polynomial function. The sigmoid function does not involve the formulations of Epa and Ep0 but uses the Penman (1948) potential evaporation and its radiation component as inputs, whereas the polynomial function inherits Ep0 and Epa as inputs and requires proper formulations for application. The generalized complementary principle has a more rigorous physical base and offers a great potential in advancing evaporation estimation. Future studies may cover several topics, including the boundary conditions in wet environments, the parameterization and application over different regions of the world, and integration with other approaches for further development.
Declining ecosystem services have prompted numerous studies aiming at developing more sustainable management practices for vegetation restoration. Advances in functional ecology indicate that the sustainable management of afforestation ecosystems should be performed based on plant functional traits, which provides pivotal knowledge for long-term sustainable vegetation restoration. Currently, the mechanism of how plant functional traits affect long term ecosystem services in restored areas is still unclear. This study investigates plant functional traits and the associated ecosystem services from artificial forestlands (Robinia pseudoacacia, Caragana korshinskii) and natural grasslands following different durations of vegetation restoration (10, 20, 30 and 40 years) in the Danangou watershed, a loess hilly-gully region in the Loess Plateau, China. The results showed that 1) the water conservation services of artificial forestlands first decreased and then increased over time, whereas the soil conservation service had an opposite trend; in turn, natural grassland led to a consistent increase in soil conservation and carbon sequestration services over time. 2) Artificial forestlands had greater soil conservation and carbon sequestration services than natural grassland but had lower water conservation services. 3) Leaves had a greater impact on carbon sequestration and water conservation services than did root length and root biomass density. 4) Root biomass density had a greater effect on soil conservation services than did leaf carbon content and soil organic matter. 5) Leaf carbon content, specific root length, and root biomass density had significant effects on the trade-off value between any two ecosystem services with increasing time after restoration of artificial forestland. 6) Specific leaf area had a greater effect on the trade-off values among the three services than did the other functional traits in the natural grassland. In arid ecosystems, natural grasslands are the best restoration strategy given their higher water conservation services. However, in soil erosion-affected areas, restoration through artificial forestlands is more appropriate. To mitigate the trade-offs between ecosystem services, it is recommended that artificial forestlands be thinned before the leaf carbon content, specific root length, and root biomass density reach a maximum (i.e., mature forestland).
Mixed-species plantations are promoted to restore degraded ecosystems and improve soil quality worldwide. However, differences of soil water conditions between pure and mixed plantations are still controversial and how species mixtures affect soil water storage (SWS) was not well quantified. In this study, vegetation characteristics, soil properties and SWS were continuously monitored and quantified in three pure plantations (Armeniaca sibirica (AS), Robinia pseudoacacia (RP) and Hippophae rhamnoides (HR)) and their corresponding mixed plantations (Pinus tabuliformis-Armeniaca sibirica (PT-AS), Robinia pseudoacacia-Pinus tabuliformis-Armeniaca sibirica (RP-PT-AS), Platycladus orientalis-Hippophae rhamnoides plantation (PO-HR), Populus simonii-Hippophae rhamnoides (PS-HR)). The results found that SWS of 0-500 cm in RP (333.60 ± 75.91 mm) and AS (479.52 ± 37.50 mm) pure plantations were higher than those in their corresponding mixed plantations (p > 0.05). SWS in the HR pure plantation (375.81 ± 81.64 mm) was lower than that in its mixed plantation (p > 0.05). It is suggested that the effect of species mixing on SWS was species specific. Additionally, soil properties exerted more contributions (38.05-67.24 %) to SWS than vegetation characteristics (26.80-35.36 %) and slope topography (5.96-29.91 %) at different soil depths and the whole 0-500 cm soil profile. Furthermore, by excluding the effects of soil properties and topographic factors, plant density and height were particularly important to SWS (with standard coefficients 0.787 and 0.690 respectively). The results implied that not all the mixed plantations exhibits the better soil water conditions than the compared pure plantations, which was tightly related to species selected for mixing. Our study provides scientific support for revegetation technique improvement (structural adjustment and species optimization) in this region.
Soil moisture (VWC) and atmospheric dryness (vapor pressure deficit, VPD) have an increasing impact on tree transpiration and productivity with climate change. However, the interaction of VWC and VPD on canopy transpiration (Ec) of plantations is still unclear, especially in water-scarce areas. Here, we selected Pinus tabulaeformis Carr. and Platycladus orientalis (L.) Franco plantations in the semiarid loess hills of China to examine the impacts of soil and atmospheric water conditions on Ec during the growing seasons (May–September) of 2015–2020. The results showed that the magnitudes of Ec were similar in the two plantations throughout the study period. The lowest Ec occurred in September because of little precipitation and low atmospheric evaporative demand, while Ec was highest in July due to the increase in soil water availability and atmospheric evaporative demand. There were obvious inflection points in the response relationship between VWC and VPD with Ec for both plantations. Under non-drought conditions, Ec reached a maximum in both plantations and was mainly impacted by solar radiation (Rs) and air temperature. As VPD increased, Rs still was the dominant factor, but canopy conductance decreased significantly, resulting in negative correlations between Ec and VPD. Under soil drought and compound drought conditions, Ec was mainly controlled by Rs and VWC, and the inhibitory effect of VPD on Ec was enhanced. In addition, the ratio of Ec to precipitation was close to 50% for both plantations at the interannual scale. Considering that soil water depletion in the Loess Plateau will accelerate with climate change and increasing drought events, the increasing water use stress will further hinder tree gas exchange, inhibiting tree growth. It is thus necessary to pay special attention to the impact of soil and atmospheric drought on water use for forest management in the Loess Plateau and similar regions of the world.
In the era of water scarcity and severe droughts, the accurate estimation of evapotranspiration (ET) is crucial for the efficient management of water resources, understanding hydrological and ecological processes, and comprehending the relationships between the atmosphere, hydrosphere, and biosphere. ET is a complex phenomenon influenced by a set of biophysical and environmental factors. Its estimation becomes more complicated in heterogeneous environments, demanding detailed data and accurate model calibration. Combining remote sensing imagery and machine learning (ML) models has provided a considerable capacity for estimating ET, which relaxes a number of assumptions and requires less data than traditional approaches. Satellite imagery provides influential variables for ET estimation using ML models. Nevertheless, a growing number of ML models and emerging satellite imagery has opened up a wide and complex potential before researchers. While previous studies have reviewed physical-based methods for ET estimation, this paper offers a recent decade review of the progress, challenges, and opportunities provided by the RS and ML models for the ET estimation and future outlook.
Orchards consist of complex agricultural systems, with a variety of characteristics (planting density, tree height, training system, canopy cover, irrigation method, interrow management) influencing crop evapotranspiration (ETc). Thus, irrigation water management requires finding crop coefficients (Kc) that represent the characteristics of local orchards, evidencing the need for site specific data. The main objective of this study was to derive the Kc of almond, olive, citrus, and pomegranate orchards in Alentejo, southern Portugal, wherein they became dominant over the last decade. Monitoring was carried out in nine orchards, which management decisions were performed by the farmers. The ETc was estimated from the soil water balance computed for each orchard using the FAO56 dual-Kc approach with the SIMDualKc model. The model successfully simulated the soil water contents measured in the various fields along two growing seasons, with root mean square error values lower than 0.005 m3 m−3 and modeling efficiencies from 0.363 to 0.782. The estimated basal crop coefficients (Kcb) for the initial, mid- and end-seasons were respectively 0.22, 0.58, and 0.50 for almond; 0.32-0.33, 0.35-0.36 and 0.33-0.34 for olive; 0.40, 0.40-41, and 0.40-0.41 for citrus; and 0.24, 0.60, and 0.52 for pomegranate. Small variations in olive and citrus Kcb values were found to be related to differences in the fraction of the ground covered by trees’ canopies and tree height. The single Kc values, which included the component relative to soil evaporation, were also estimated. Furthermore, evaluation of the soil water balance in the nine case studies showed salinity effects in one almond orchard, mild irrigation water deficits in olive systems, and large non-consumptive water use in citrus and pomegranate orchards. These results evidence the need for better management of orchards irrigation water in the region, and the current study provides for reliable information on the Kc of tree crops to support improving the management of local orchard systems and the preservation of soil and water resources. Aimed at these resources and the sustainability of their use, simulated alternative irrigation schedules were performed, which identified possible water savings of 20 mm in case of olives, up to 855 mm for citrus.
Evapotranspiration (ET) is a critical component of the water, carbon and energy cycles of the land surface. Remote sensing-based models provide the possibility of mapping ET from field to regional and even global scales. A key boundary condition of the Two-Source Energy Balance (TSEB) model for computing ET is land surface temperature (LST). The proposed modification to TSEB is to use a combination of near surface soil moisture (SM) derived by microwave and LST as dual boundary conditions along with a more physically-based transpiration formulation for computing ET. Both TSEB and TSEB-SM were applied to an irrigated agricultural area with surrounding semi-arid sparse natural vegetation. In general, TSEB-SM model performance when compared to measured fluxes was similar to the TSEB model. However, the TSEB-SM model computed more reliable estimates of LE under dry soil surface/low vegetation cover conditions, primarily in the arid and semiarid natural ecosystems surrounding the irrigated agricultural area. These comparisons indicate that TSEB-SM may provide more reliable flux estimation and ET partitioning over sparsely vegetated areas compared to the traditional formulations used in TSEB. With reliable microwave remote sensing of soil moisture, TSEB-SM has potential for ET monitoring in both natural and agricultural landscapes.
Study region
A typical grassland (Medicago sativa plot) and a typical forest land (Robinia pseudoacacia plot) located in the semi-arid Loess Plateau, China.
Study focus
Field observations and scenario simulation approaches were used to investigate the potential impact of future warming and drying climate on the whole water budget and to assess whether, how, and to what degree the highly recommended man–driven tree–to–grass conversion could alleviate the adverse impact of such climate change on soil water.
New hydrological insights for the region
Differences of interception and transpiration between two plots resulted in their different soil water storage. Under the warming and drying future climate, interception and transpiration in two plots would experience significant reductions, especially transpiration, and M. sativa plot could maintain more water than R. pseudoacacia plot. When R. pseudoacacia was replaced by M. sativa, although interception would dramatically increase, the transpiration would significantly decrease, which made the soil water storage experience an insignificant change. Climate change and tree–to–grass conversion mainly exerted their influences in months with larger precipitation, and vegetation conversion would reduce monthly T/ET. Our study improves the understanding of the hydrological processes under climate change and vegetation conversion, and also provides valuable guidelines for the management of water resources and the restoration of ecological environment in the semi–arid Loess Plateau.
Several versions of the Complementary Relationship (CR) between actual regional evaporation and apparent potential evaporation have recently been proposed. Few studies have compared multiple CR versions side-by-side using datasets spanning various climates and land surfaces. Filling this lack is one purpose of this project. It also investigates how various CR versions respond to changes in spatial and temporal averaging. This study uses multiple years of data from seven eddy-covariance flux stations in Australia, representing a wide range of biomes, along with global ERA5 reanalysis data products. Daily and monthly averages were used for both datasets, and the Australian observations also used weekly and yearly averages. The ERA5 data represent a scale of about 30 km, much larger than the scale represented by the flux station data. A set of five questions regarding the impact of spatial and temporal scaling on CR parameter values and performance are asked and assessed using the two datasets. Four recent CR versions are considered in answering the questions. Due to important differences between FLUXNET and ERA5 data, questions regarding temporal scaling were answered with greater confidence than those regarding spatial scaling. With these data, rescaled versions of the CR performed best overall.
Black locust (Robinia pseudoacacia) is widely planted throughout semiarid and subhumid regions of the Loess Plateau of China. Determining the changes in transpiration of this species in different climatic areas is important for revealing the acclimation mechanism of black locust and developing suitable forest management practices, particularly in the context of global climate change. Here, sap flow and canopy conductance of black locust plantation trees in semiarid (Yan’an) and subhumid (Yongshou) sites were quantified using Granier-type thermal dissipation probes and concurrent environmental observations from 2012 to 2017. Several physiological parameters were measured throughout the growing season. The results showed that sap flow was correlated with phenological factors across seasons within a year. However, interannual changes in sap flow were affected mainly by the reference evapotranspiration (ET0) at the Yongshou site, and jointly by precipitation (P), soil water content, and P/ET0 at the Yan’an site. Sap flow response to meteorological factors showed less discrepancy between pre- and post-rainfall periods at the Yan’an site. Moreover, canopy conductance fluctuated less with a wider range of vapor pressure deficit (VPD) and the slope of canopy resistance as a function of VPD was lower, indicating relatively lower sensitivity of stomatal conductance to environmental factors in Yan’an site. Physiological parameters, except for predawn leaf water potential, were significantly different between the two sites. The results suggested that black locust tended to reduce transpiration, modify leaf morphology, and improve water use efficiency to enhance its adaptability to the dryer site. The species changes stomatal regulation characteristics and general growth rate to acclimatize to distinct water habitats.
Partitioning of evapotranspiration (ET) into evaporation (E) and transpiration (T) is challenging but important to better understand the mechanisms of water loss from the ground surface to the atmosphere, especially in semiarid and arid regions. In this study, the underlying water use efficiency (uWUE) method is compared with hydrometric and remote sensing-based ET partitioning methods. The uWUE method is considered to be a promising method for partitioning ET, and can also be used to obtain temporal continuous data with fine spatial representation. Thus, the uWUE method was used to partition ET in six typical ecosystems in the Heihe River basin (HRB), which is the second largest endorheic river basin in western China. During 2008–2016, the daily average contributions of T to ET (T/ET) were 0.53, 0.52, 0.59, 0.37, 0.56, and 0.59 in the alpine meadow, Qinghai spruce, maize, desert, Tamarix, and Populus euphratica–Tamarix ecosystems, respectively. The T/ET ratio exhibited obvious seasonal variations in alpine meadow, maize, Tamarix, and Populus euphratica–Tamarix ecosystems. The ET partitioning results were related to air temperature in all ecosystems, especially in areas with sufficient precipitation or irrigation water supply. The vapor pressure deficit was also a main controlling factor in the upper- and middle-reach ecosystems, especially in the Qinghai spruce ecosystem. Groundwater and/or soil moisture made high relative contributions (RCs, %) to the T/ET ratio of the riparian forest in the lower reaches. Additionally, the leaf area index also made a high RC to the T/ET ratio of deciduous vegetation. Determining the quantitative contribution of T to ET in the HRB is beneficial for water resource management to guide the rational allocation and efficient use of water resources.
Partitioning of evapotranspiration (ET) into soil water evaporation and transpiration allows separate assessment of soil and plant water, energy, and carbon exchange. Remote sensing-based models are ideally suited to monitor ET over large areas, but ET partitioning estimates vary widely. The objective of this study was to evaluate the two-source energy balance (TSEB) model for seasonal ET partitioning using total, soil, and vine canopy energy balance fluxes measured over a vineyard in the Negev desert in Israel. Energy fluxes were evaluated with the original TSEB and three adapted versions using (1) measured soil heat flux, (2) optimized plant transpiration parameterization, and (3) measured soil and vine temperatures instead of composite surface temperature as model inputs. Optimization of plant transpiration parameters revealed a model tendency to underestimate transpiration due to underestimation of available energy and potential transpiration. Adaptations included, among others, accounting for higher leaf radiation absorption expected in dense clumped canopies, which increases available energy, and increasing the Priestley-Taylor coefficient from 1.26 to 2, which increases potential transpiration. While the original TSEB gave reasonable total energy fluxes, the vine energy fluxes were greatly underestimated. Both soil heat flux and plant transpiration adaptations improved modeled vine energy fluxes throughout the season under both well-watered and water-stressed conditions. However, the performance of the TSEB version using measured soil and vine temperatures was inferior to applying the standard TSEB with composite temperature. While daily energy fluxes could be estimated with reasonable accuracy, sub-daily fluxes proved to be more challenging and merit further research. Finally, changes in ET partitioning with canopy development and in response to water stress could be detected quite well albeit it with an underestimation of the transpiration fraction of ET, which, on average, amounted to 39% using standard TSEB and 6% with optimized plant transpiration parameters.
Estimation of daily Evapotranspiration (ET) over cloudy regions highly desires models which rely on meteorological data only. Notwithstanding, the conventional crop coefficient (Kc) 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 six years (2007–2012) 15‐min micrometeorological records of these sites revealed that there is an indubitable strong interaction between relative humidity (RH), reference ET (ETo), and actual ET at different timescales. This allowed to bypass the need for the non–meteorological inputs and express Kc as a 2nd order polynomial function of RH and ETo, 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.
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Accurately modelling and predicting forest transpiration under changing environment and canopy structure are essential for understanding the interactions among the atmosphere, soil, and vegetation and precisely integrating forest water management. To enhance water management in larch plantations, the canopy transpiration (T), reference evapotranspiration (ETo), canopy leaf area index (LAI), and relative extractable water (REW) of the 0–60 cm soil layer were synchronously monitored during the growing season (May–September) in 2016 and 2018. The response functions of the basal crop coefficient (Kcb) in an ETo-based transpiration model to individual driving factors (LAI and REW) were determined using upper boundary lines and then coupled to form the T model using the crop coefficient method. The results showed that (1) the mean (and range) of the daily canopy T was 0.83 (0.003–1.60) mm·d⁻¹ in dry year of 2016, and 0.70 (0.02–1.55) mm·d⁻¹ in wet year of 2018. (2) The Kcb increased with rising LAI, following a positive linear relation. With rising REW, Kcb first increased rapidly and then stabilized gradually following a saturated exponential function. (3) The daily T model was fitted as T=(0.056 + 0.083(1-exp(-3.915REW))) (0.961LAI) ETo and had good performance in both the calibration period (R² = 0.78; NSE = 0.77) and validation period (R² = 0.85; NSE = 0.75). (4) The relative importance of ETo, LAI, and REW on T were analysed based on the developed T model. The results showed that the average contribution of ETo, LAI, and REW to T differed in various years. The dominant factor contributing to the changes in T was ETo followed by REW and LAI in dry year of 2016 and LAI followed by ETo and REW in wet year of 2018 compared with the reference T under the long-term means of the daily ETo, LAI, and REW. The newly developed simple T model in this study represents a key step for the comprehensive integrated management of forests and water and accurately predicts the response of the daily canopy T to varying LAI values under a given REW and ETo, which is helpful for designing precise adjustments of stand structure.
The complementary principle provides a simple method for estimating land surface evaporation using standard meteorological observations and has received increasing attention in recent years. In this study, we propose an exponential generalized complementary function that satisfies the physical boundary conditions and constraints proposed by Brutsaert (2015). With one adjustable parameter, the new exponential function is validated using observations from 21 FLUXNET stations, and the results show that the exponential function can efficiently simulate actual daily evaporation. A comparison between the exponential function and the polynomial function proposed in previous studies suggests that the exponential function can improve the accuracy, reasonableness and flexibility of land surface evaporation estimation. Overall, the exponential function provides a new method for estimating land surface evaporation and may enhance our understanding of the complementary principle.
Investigating the effects of climate change on transpiration (T) and evaporation (E) in natural and constructed grasslands is of theoretical and practical significance for vegetation restoration and water resource management in the Loess Plateau, China. In this study, a two-year field experiment was conducted in one natural (Imperata cylindrica plot) and two constructed grasslands (Pennisetum giganteum and Medicago sativa plots) in the semi-arid Loess Plateau. Hydrus-1D models were then established and validated to simulate T and E processes under scenarios combining climate characteristics in both future periods and different hydrological years. The results showed that under all climate scenarios, T in the natural grassland, and P. giganteum and M. sativa plots ranged from 63.7–179.3, 126.8–245.5, and 96.9–243.1 mm, respectively, while E ranged from 90.6–131.9, 68.7–92.8, and 70.4–92.4 mm, respectively. Both T and E showed a decreasing trend in the future periods, exhibiting the highest values in wet years and the lowest values in dry years. The effects of climate change on T were greater than that on E in all three grasslands. Of the studied grasslands, T of M. sativa plot exhibited the strongest response, followed by that in the natural grassland and P. giganteum plot. However, E of the natural grassland responded to the strongest degree, followed by that of the P. giganteum and M. sativa plots. Precipitation during the growing-season was the most dominant climatic factor affecting T and E, and was positively linearly related with T and E (P < 0.01) in all three grasslands. The conversion thresholds of precipitation for T-E dominance were 435.8, 149.4, and 253.3 mm for the natural grassland, and P. giganteum and M. sativa plots, respectively. In constructed grasslands, more water was lost through T than through E; hence, the T/ET ratios were high (>0.55). In contrast, T was high in the natural grassland only in conditions of sufficient precipitation; hence, the T/ET ratio was relatively lower (around 0.5). Precipitation distribution strongly affected the soil water supply, and the natural grassland was more efficient in maintaining soil water than the constructed grasslands. Our study not only improves our understanding of the hydrological processes and water budget under different grass restoration measures, but also provides valuable guidelines for the management of water resources and the restoration of ecological environment in the Loess Plateau.
Generalized complementary functions, which describe the relationship between the ratio of actual evaporation over the Penman potential evaporation (E/EPen) and the proportion of the radiation term in EPen (Erad/EPen) have not been widely used at annual time scales. In this study, the generalized nonlinear advection-aridity function (GNAA) and sigmoid generalized complementary function (SGCF) were evaluated for annual evaporation estimation with calibrated parameters in the Loess Plateau of China. For all 15 catchments, the lowest values of annual Erad/EPen were found to be much larger than zero, and the annual E/EPen increased approximately linearly with annual Erad/EPen. This complementary curve at an annual timescale differs from those at daily timescales, which require different parameterizations of the GNAA and SGCF. For the GNAA, parameter c, which is often set to zero for daily time scales, must be well calibrated. For the SGCF, the upper and lower limits of Erad/EPen must be constrained. After calibration, both the SGCF and GNAA performed well at estimating annual evaporation. In addition, the calibrated Priestley-Taylor coefficient from the SGCF was found to be closer to the widely accepted value (1.26) than that determined from the GNAA.
Transpiration (T) from plant tissues is an important variable in ecological and agricultural studies. T is usually estimated using the Priestley-Taylor (PT) equation by an analog to the wet environment evaporation (ETw), because vegetation can use water from deep soils. However, the relation between the PT coefficients of T and ETw (αc and αe) was still unclear. In this paper, we tested whether αe can be used as a reasonable indicator of αc at intra-annual and annual scales. We obtained αe by inverting a complementary relationship model with measured evapotranspiration (ET) at the flux sites and obtained αc after ET was partitioned into evaporation (E) and T. We found that αc tends to converge to αe at annual scales, and a linear equation can adequately fit the αc = f(αe) functions at annual scales. With the αc = f(αe) functions determined, satisfactory accuracy (R² = 0.89) was achieved in estimating T from ET following the “ET → αe → αc → T” calculating process at annual scales. In comparison, the αc ~ αe relationship at 8-day scales are modulated by leaf area index and vapor pressure deficit. However, similar αc = f(αe) functions are identified in the region with the same climate type at 8-day scales, indicating that a single αc = f(αe) function is applicable in the same region. The accuracy in T predictions at 8 day scales deteriorated compared to those at annual scales. However, R² were still larger than 0.58 at most of the sites. Because great progress in ET estimation has been achieved with the help of thermal remote sensing technique in the last two decades, our results here can provide a new method to estimate T from remotely sensed ET.
A higher drought risk in Java Island is generally known than the other regions in Indonesia. Tracking soil moisture can be an alternative way to monitor drought rather than precipitation-based drought indices. The objective of this study was to assess root-zone water storage (defined by root-zone soil moisture contents) based on a linked approach between the generalized complementary relationship (GCR) and a single bucket model in Java Island. Since it does not require precipitation for estimating actual evapotranspiration (ETa), the GCR allowed implementation of a simple single bucket model. The ETa and root-zone soil moisture estimated in this study were compared against the Global Land Evaporation Amsterdam Model (GLEAM) and the root-zone water storage additionally compared with the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5 reanalysis data products. Overall, the GCR ETa estimates were higher than those from GLEAM, and similar patterns of the root-zone water storage were found in the comparisons of both GLEAM and ERA5. The comparative evaluation suggests a further study on the adjustment of Priestley-Taylor coefficient value in Java for better application of the GCR. The soil moisture estimated by the single bucket model and the root-zone soil moisture products of GLEAM were highly correlated (0.8 or greater Pearson correlation coefficients). Low root-zone water storage and high ETa rates were found in eastern Java relative to the other areas, indicating high water shortage risks in dry season. This study found that El Niño clearly contributed to the variability of the root-zone water storage in Java especially in wet seasons (December to February). It is also suggested that the proposed approach can be useful to operationally provide soil water availability in Java from readily available meteorological observations.
A generalized implementation of the complementary principle was applied to estimate global land surface evaporation and its spatial distribution. The single parameter in the method was calibrated as a function of aridity index, mainly on the basis of runoff and precipitation data for 524 catchments in different parts of the world. The spatial distribution of annual evaporation from the Earth’s land surfaces for 2001-2013 was then calculated at a spatial resolution of 0.5 degree, by means of an available global net radiation data set (commonly referred to as CERES SYN1deg-Daily) and a global forcing data set (referred to as (CRU-NCEP v7) for near surface temperature, humidity, wind speed and air pressure. The results are shown to agree with reliable previous estimates by more elaborate methods. The global average evaporation for 2001-2013 was found to be 472.65 mm a ⁻¹ or 36.96 W m ⁻² . The present method should allow not only future updates but also retroactive historical analyses with routine data of net radiation, near-surface air temperature, humidity, wind speed and precipitation; its main advantage is that the environmental aridity is deduced from atmospheric conditions, and requires no knowledge of surface characteristics, such as soil moisture, vegetation, and terrain which are highly variable and often difficult to quantify at larger spatial scales. Because they are strictly measurement based, the results can serve also as a reality check for different aspects of climate and related models.
The world's largest afforestation programs implemented by China made a great contribution to the global “greening up.” These programs have received worldwide attention due to its contribution towards achieving the United Nations Sustainable Development Goals. However, emerging studies have suggested that these campaigns, when not properly implemented, resulted in unintended ecological and water security concerns at the regional scale. While mounting evidence shows that afforestation causes substantial reduction in water yield at the watershed scale, process‐based studies on how forest plantations alter the partitioning of rainwater and affect water balance components in natural vegetation are still lacking at the plot scale. This lack of science‐based data prevents a comprehensive understanding of forest‐related ecosystem services such as soil conservation and water supply under climate change. The present study represents the first ‘Paired Plot’ study of the water balance of afforestation on the Loess Plateau. We investigate the effects of forest structure and environmental factors on the full water cycle in a typical multi‐layer plantation forest composed of black locust, one of the most popular tree species for plantations worldwide. We measure the ecohydrological components of a black locust versus natural grassland on adjacent sites. The startling finding of this study is that, contrary to the general belief, the understory – instead of the overstory – was the main water consumer in this plantation. Moreover, there is a strict physiological regulation of forest transpiration. In contrast to grassland, annual seepage under the forest was minor in years with an average rainfall. We conclude that global long‐term greening efforts in drylands require careful ecohydrologic evaluation so that green and blue water tradeoffs are properly addressed. This is especially important for reforestation‐based watershed land management, that aims at carbon sequestration in mitigating climate change while maintaining regional water security, to be effective on a large scale.
Estimating stand transpiration of natural forests using traditional methods through up-scaling of sap flux density from sample trees based on stand sapwood area only is difficult because of the complexity of species, ages, and hierarchical structure of natural forests. To improve stand transpiration estimation, we developed an up-scaling method by considering the tree dominance effect based on the assumption that individual tree transpiration is affected by crown dominance and species, in addition to factors previously considered such as meteorological conditions, sapwood area, and soil moisture. In this study, the meteorological factors, soil moisture, and sap flux density of 15 sample trees of different species and dominance in a natural evergreen and deciduous broadleaved mixed forest were simultaneously monitored from March 2012 to February 2014 in the Karst mountain region in southwestern China. After establishing a single tree transpiration model which considers the effects of dominance and species, an up-scaling method was explored to estimate stand transpiration. The results show that the transpiration intensity increased exponentially with increasing tree dominance. The contribution to annual stand transpiration from a few dominant trees (5.4% of trees, 28.2% of basal area) was up to 65.0%. The corresponding contribution was 16.2% from sub-dominant trees (7.6% of trees, 16.2% of basal area) and 22.8% from middle- and lower-layer trees (87.0% of trees, 55.6% of basal area). The variation of individual tree transpiration was mainly (97.9%) explained by tree dominance, but very weakly by tree species. The estimated annual stand transpiration was 300.2 mm when using the newly developed method which considers tree dominance, 52.5 mm (14.9%) lower than the estimation (352.7 mm) of traditional method which considers only the sapwood area effect, and 8.5 mm (2.7%) lower than the estimation (308.6 mm) which considers the effects of both species composition and sapwood area. The main tree characteristics affecting stand transpiration are tree size (sapwood area) and dominance. Consideration of tree dominance will significantly improve stand transpiration estimation and provide a more solid basis for guiding integrated forest–water management at stand scale.
Planted grassland is one of the most quickly and effective way for vegetation restoration in arid and semi-arid regions. However, inappropriate grasslands construction can cause soil water deficit and soil drying in arid areas. In the present study, we evaluated the soil water storage deficit and soil water balance characteristics of different grassland types under natural conditions. The relative growth rates of planted grasslands, especially the planted legume grasslands, were approximately double higher than that of the natural grassland. Soil water storage deficit degree was the highest in the 0-50 cm soil layers than below the 50 cm layers. Soil water deficit degree was higher in legume grasslands than in gramineous grasslands below the depth of 50 cm, and it was significantly lower in the natural grassland than in planted grasslands below the depth of 200 cm (P < 0.05). The precipitation (P) was lower than the evapotranspiration (ET) in planted grasslands, thus resulting in soil water deficiency and soil desiccation. The average ET/P value was significantly higher in planted grasslands than in the natural grassland, and followed the order of planted gramineous grasslands > planted legume grassland and shrubland > natural grassland. None of the grasslands got sufficient water supply during the growing season for transpiration and growth. Our findings provided evidence that natural grasslands have lower water consumption , and indicated which vegetation types can maintain more soil water and facilitate vegetation sustainability when undertaking restoration in water-limited environments.