Simulated Daily Evapotranspiration Research Articles

Estimation of Evapotranspiration of a Jujube/Cotton Intercropping System in an Arid Area Based on the Dual Crop Coefficient Method

An accurate estimation of crop evapotranspiration (ETc) in intercropping is critical for a theoretical basis for formulating an intercropping irrigation system of fruit trees and improving the soil moisture condition of orchards in arid regions of Southern Xinjiang. Herein, observational data such as soil moisture and plant physiological indicators were measured in jujube/cotton intercropping in the Aksu region from 2015 to 2017. The evapotranspiration of single-crop jujube and cotton was estimated using the modified dual crop coefficient method. Then, based on the proportion of intercropping crops, the soil water stress coefficient was introduced to estimate the evapotranspiration of the jujube/cotton intercropping. The results show that the model has good applicability to simulate single-crop jujube and cotton, and jujube/cotton intercropping. However, compared with single cropping, the accuracy of simulated daily evapotranspiration was decreased. In addition, adjusting the cotton irrigation amount caused the simulation accuracy to further decrease. From the perspective of interspecies complementarity and competition, cotton dominated the intercropping system and had better tolerance to external environmental changes than jujube trees. Intercropping had negative effects on jujube trees in general, and very obvious positive effects on cotton. Based on the 3-year crop yield, 5-year-old jujube trees are recommended for intercropping. During this time, the yield of cotton under the effect of interspecific complementation increased by 26.47%, and the yield of intercropping jujube was similar to that of single crop. As the jujube tree age increases, the effect of increasing cotton production gradually diminishes. The jujube trees also had a significant reduction in yield due to interspecific competition. Our research supports the dual crop coefficient method as appropriate to estimate crop ETc in intercropping and may be further used to improve irrigation scheduling for jujube/cotton intercropping.

Effects of Urbanization on Watershed Evapotranspiration and Its Components in Southern China

Understanding the effects of land use change on evapotranspiration (ET) and its partitioning to transpiration and evaporation is important for accurately evaluating the likely environmental impacts on watershed water supply, climate moderation, and other ecosystem services (e.g., carbon sequestration and biodiversity). This study used a distributed hydrologic model, MIKE SHE, to partition evapotranspiration into soil evaporation, transpiration, ponded water evaporation, and interception, and examined how the ET partitions affected the water balance in the Qinhuai River Basin from 2000 to 2013. Simulated daily ET was compared to measurements at an eddy flux research site during 2016–2017 (R2 = 0.72). Degradation in rice-wheat rotation fields and expansion of impervious surfaces impacted not only total watershed evapotranspiration, which showed a significant downward trend (p < 0.05), but also its partitioning. A significant (p < 0.01) decrease in transpiration was detected. Ponded water evaporation was the only ET partition that exhibited a significant positive trend (p < 0.05). We concluded that the reduced transpiration as a result of land use and land cover change was the primary factor driving the variation of watershed scale evapotranspiration. In addition, there was an increase in annual water yield (23%) as a response to significant reduction in ET (7%) due to a 175% expansion of urban area in the study watershed. Our study provided insights to the mechanisms of land surface–water cycle interaction and better understanding of the effects of land use change on urban micro-climate such as “urban dry island” and “urban heat island” effects.

Seasonality of leaf area index and photosynthetic capacity for better estimation of carbon and water fluxes in evergreen conifer forests

Leaf area index (LAI), defined as one half the total leaf area per unit ground area, and Vcmax, representing the maximal carboxylation rate of leaves, are two most significant parameters used in most Terrestrial Biosphere Models (TBMs). The ability of TBMs to simulate gross primary productivity (GPP) and evapotranspiration (ET) for evergreen needle-leave forests (ENF) can be significantly hampered by uncertainties in LAI and Vcmax. Remotely sensed (RS) LAI for ENF is generally underestimated in winter, early spring and late autumn. Although constant Vcmax throughout the growing season is often used in TBMs for GPP and ET modeling, it could vary significantly under leaf aging and stressed conditions. There were recent studies that apply seasonal leaf chlorophyll constraints on GPP modeling for croplands and deciduous forests, but little attention is given to the influence of the seasonality of either LAI or Vcmax on GPP or ET estimations for the ENF biome. In this study, we pay special attention to this biome, with the purpose of investigating if the representations of seasonal LAI and Vmax variations are essential in TBMs. To serve this purpose, the University of Toronto LAI product Version 2 was corrected for its seasonal variation using leaf lifespan and in-situ measurements at eight ENF sites in Canada. Seasonal Vcmax variation was derived from the MERIS Terrestrial Chlorophyll Index (MTCI) through downscaling it to the leaf level using a scheme with a general vertical nitrogen distribution within the canopy. Leaf chlorophyll content (LCC) is thus derived from MTCI and converted to Vcmax using empirical equations. Four model cases with and without considerations of the seasonal LAI and Vcmax variations were tested and compared. Validation against eddy covariance measurements indicates that the case with both LAI and Vcmax variations produced the highest R2, lowest root mean square error (RMSE) and lowest mean absolute error (MAE) for both GPP and ET simulations, and thus outperforms all other cases without considering the variations or with consideration of one of the variations only. In this best case, the simulated daily GPP yields R2 of 0.91, RMSE of 0.91 g C m−2 and MAE of 0.65 g C m−2, while the simulated daily ET yields R2 of 0.8, RMSE of 0.52 mm and MAE of 0.34 mm. Most improvements were found in spring and autumn. Not only the correlations between the seasonal trajectories of model simulation and observation were improved, but also the annual total GPP and ET were more accurately estimated. The smallest mean absolute relative bias to eddy covariance measurements is 9% for GPP and 15% for ET, both were found in the best case. Moreover, improvements in GPP were more pronounced than in ET. Our results highlight the significance of considering both seasonal structural and physiological characteristics of leaves in TBMs. Considering the important role that evergreen coniferous forests play in global terrestrial ecosystems, global simulations of GPP and ET in space and time can benefit from the proper representation of seasonal variations in canopy structure and leaf physiology as represented by LAI and Vcmax, respectively.

An Algorithm Differentiating Sunlit and Shaded Leaves for Improving Canopy Conductance and Vapotranspiration Estimates

AbstractSurface conductance (Gs) is a key parameter in estimating land surface evapotranspiration (ET) and difficult to determine. Here we proposed an approach for determining Gs according to the stomatal conductance of sunlit and shaded leaves that is estimated from their respective gross primary production (GPP) with the Ball‐Berry model. Central to this approach, GPP is separately simulated for sunlit and shaded leaves with a revised two‐leaf light use efficiency model. We tested the approach at 17 FLUXNET sites with seven different vegetation types. The revised two‐leaf light use efficiency model outperforms its predecessor in estimating GPP at most sites. As to Gs estimation, although our proposed algorithm has higher Akaike information criterion values than has the model estimating Gs using vegetation indices, it was able to capture Gs variations at all sites, while models estimating Gs using leaf area index and vegetation indices performed poor at some sites. The proposed algorithm also improves ET estimation, indicated by lower Akaike information criterion, higher determination coefficient (R2), and lower root mean square error of simulated daily ET for both calibration and validation data sets. This study demonstrates the usefulness of differentiating sunlit and shaded leaves in improving canopy conductance and ET estimates.

Modeling Evapotranspiration and Crop Growth of Irrigated and Non-Irrigated Corn in the Texas High Plains Using RZWQM

Abstract. Accurate quantification and management of crop evapotranspiration (ET) are critical to optimizing crop water productivity for both dryland and irrigated agriculture, especially in the semiarid regions of the world. In this study, four weighing lysimeters in Bushland, Texas, were planted to maize in 1994 with two fully irrigated and two non-irrigated for measuring crop ET. The Root Zone Water Quality Model (RZWQM2) was used to evaluate soil water balance and crop production with potential evapotranspiration (PET) estimated from either the Shuttleworth-Wallace method (PTSW) or the ASCE standardized alfalfa reference ET multiplied by crop coefficients (PTASCE). As a result, two water stress factors were defined from actual transpiration (AT) and were tested in the model against the lysimeter data, i.e., AT/PTSW and AT/PTASCE. For both water stress factors, the simulated daily ET values were reasonably close to the measured values, with underestimated ET during mid-growing stage in both non-irrigated lysimeters. Root mean squared deviations (RMSDs) and relative RMSDs (RMSD/observed mean) values for leaf area index, biomass, soil water content, and daily ET were within simulation errors reported earlier in the literature. For example, the RMSDs of simulated daily ET were less than 1.52 mm for all irrigated and non-irrigated lysimeters. Overall, ET was simulated within 3% of the measured data for both fully irrigated lysimeters and undersimulated by less than 11% using both stress factors for the non-irrigated lysimeters. Our results suggest that both methods are promising for simulating crop production and ET under irrigated conditions, but the methods need to be improved for dryland and non-irrigated conditions. Keywords: ET, RZWQM modeling, Stress factor, Weighing lysimeter.

Quantitative estimation of land surface evapotranspiration in Taiwan based on MODIS data

Land surface evapotranspiration (ET) determines the local and regional water-heat balances. Accurate estimation of regional surface ET provides a scientific basis for the formulation and implementation of water conservation programs. This study set up a table of the momentum roughness length and zero-plane displacement related with land cover and an empirical relationship between land surface temperature and air temperature. A revised quantitative remote sensing ET model, the SEBS-Taiwan model, was developed. Based on Moderate Resolution Imaging Spectroradiometer (MODIS) data, SEBS-Taiwan was used to simulate and evaluate the typical actual daily ET values in different seasons of 2002 and 2003 in Taiwan. SEBS-Taiwan generally performed well and could accurately simulate the actual daily ET. The simulated daily ET values matched the observed values satisfactorily. The results indicate that the net regional solar radiation, evaporation ratio, and surface ET values for the whole area of Taiwan are larger in summer than in spring, and larger in autumn than in winter. The results also show that the regional average daily ET values of 2002 are a little higher than those of 2003. Through analysis of the ET values from different types of land cover, we found that forest has the largest ET value, while water areas, bare land, and urban areas have the lowest ET values. Generally, the Northern Taiwan area, including Ilan County, Nantou County, and Hualien County, has higher ET values, while other cities, such as Chiayi, Taichung, and Tainan, have lower ET values.

Modeling evapotranspiration and energy balance in a wheat–maize cropping system using the revised RZ-SHAW model

Correctly simulating evapotranspiration (ET) and surface energy balance is essential to simulating crop growth under water and heat stress conditions in agricultural systems. The revised hybrid model (RZ-SHAW), combining the Root Zone Water Quality Model (RZWQM) and Simultaneous Heat and Water (SHAW) model, was evaluated for simulating ET and surface energy balance components against observed data from an eddy covariance system in a wheat–maize double cropping system in the North China Plain (NCP), after it was calibrated for soil water content and crop growth. The average daily ET was slightly under-simulated by 0.05mm in the wheat seasons and over-simulated by 0.23mm in the maize seasons, compared with the observed latent heat flux (LE) from 2003 to 2005. The root mean squared error (RMSE) and model efficiency (ME) of simulated daily ET were 0.59mm and 0.86 for the three years. The goodness of simulation for Rn (net radiation), LE, H (sensible heat flux) and canopy temperature was better in the middle crop seasons than in the early crop seasons. The RMSE values for simulated Rn, H, LE, G (ground heat flux), and canopy temperature were 31.9, 37.2, 37.9, 21.8Wm−2, and 1.37°C, respectively, for middle wheat seasons and were 29.2, 27.1, 29.7, 19.7Wm−2, and 1.22°C, respectively, for middle maize seasons. These simulation results were comparable with previous modeling studies, indicating that the revised hybrid model is reasonable for simulating ET, surface energy balance as well as crop growth in the double cropping system.

Evapotranspiration from an Olive Orchard using Remote Sensing-Based Dual Crop Coefficient Approach

A remote sensing-based approach to estimate actual evapotranspiration (ET) was tested in an area covered by olive trees and characterized by Mediterranean climate. The methodology is a modified version of the standard FAO-56 dual crop coefficient procedure, in which the crop potential transpiration, T p, is obtained by directly applying the Penman-Monteith (PM) equation with actual canopy characteristics (i.e., leaf area index, albedo and canopy height) derived from optical remote sensing data. Due to the minimum requirement of in-situ ancillary inputs, the methodology is suitable also for applications on large areas where the use of tabled crop coefficient values become problematic, due to the need of corrections for specific crop parameters, i.e., percentage of ground cover, crop height, phenological cycles, etc. The methodology was applied using seven airborne remote sensing images acquired during spring-autumn 2008. The estimates based on PM approach always outperforms the ones obtained using simple crop coefficient constant values. Additionally, the comparison of simulated daily evapotranspiration and transpiration with the values observed by eddy correlation and sap flow techniques, respectively, shows a substantial agreement during both dry and wet days with an accuracy in the order of 0.5 and 0.3 mm d−1, respectively. The obtained results suggest the capability of the proposed approach to correctly partition evaporation and transpiration components during both the irrigation season and rainy period also under conditions of significant reduction of actual ET from the potential one.

Application of the Sebs Water Balance Model in Estimating Daily Evapotranspiration and Evaporative Fraction from Remote Sensing Data Over the Nile Delta

Estimation of evapotranspiration is always a major component in water resources management. The reliable estimation of daily evapotranspiration supports decision makers to review the current land use practices in terms of water management, while enabling them to propose proper land use changes. Traditional techniques of calculating daily evapotranspiration based on field measurements are valid only for local scales. Earth observation satellite sensors are used in conjunction with Surface Energy Balance (SEB) models to overcome difficulties in obtaining daily evapotranspiration measurements on a regional scale. In this study the SEB System (SEBS) is used to estimate daily evapotranspiration and evaporative fraction over the Nile Delta along with data acquired by the Advance Along Track Scanning Radiometer (AATSR) and the Medium Spectral Resolution Imaging Spectrometer (MERIS), and six in situ meteorological stations. The simulated daily evapotranspiration values are compared against actual ground-truth data taken from 92 points uniformly distributed all over the study area. The derived maps and the following correlation analysis show strong agreement, demonstrating SEBS’ applicability and accuracy in the estimation of daily evapotranspiration over agricultural areas.

Simulation of crop growth and energy and carbon dioxide fluxes at different time steps from hourly to daily

AbstractUnderstanding the exchange processes of energy and carbon dioxide (CO2) in the soil–vegetation–atmosphere system is important for assessing the role of the terrestrial ecosystem in the global water and carbon cycle and in climate change. We present a soil–vegetation–atmosphere integrated model (ChinaAgrosys) for simulating energy, water and CO2 fluxes, crop growth and development, with ample supply of nutrients and in the absence of pests, diseases and weed damage. Furthermore, we test the hypotheses of whether there is any significant difference between simulations over different time steps. CO2, water and heat fluxes were estimated by the improving parameterization method of the coupled photosynthesis–stomatal conductance–transpiration model. Soil water evaporation and plant transpiration were calculated using a multilayer water and heat‐transfer model. Field experiments were conducted in the Yucheng Integrated Agricultural Experimental Station on the North China Plain. Daily weather and crop growth variables were observed during 1998–2001, and hourly weather variables and water and heat fluxes were measured using the eddy covariance method during 2002–2003. The results showed that the model could effectively simulate diurnal and seasonal changes of net radiation, sensible and latent heat flux, soil heat flux and CO2 fluxes. The processes of evapotranspiration, soil temperature and leaf area index agree well with the measured values. Midday depression of canopy photosynthesis could be simulated by assessing the diurnal change in canopy water potential. Moreover, the comparisons of simulated daily evapotranspiration and net ecosystem exchange (NEE) under different time steps indicated that time steps used by a model affect the simulated results. There is no significant difference between simulated evapotranspiration using the model under different time steps. However, simulated NEE produces large differences in the response to different time steps. Therefore, the accurate calculation of average absorbed photosynthetic active radiation is important for the scaling of the model from hourly steps to daily steps in simulating energy and CO2 flux exchanges between winter wheat and the atmosphere. Copyright © 2007 John Wiley & Sons, Ltd.

BIOME‐BGC simulations of stand hydrologic processes for BOREAS

BIOME‐BGC is a general ecosystem model designed to simulate hydrologic and biogeochemical processes across multiple scales. The objectives of this investigation were to compare BIOME‐BGC estimates of hydrologic processes with observed data for different boreal forest stands and investigate factors that control simulated water fluxes. Model results explained 62 and 98% of the respective variances in observed daily evapotranspiration and soil water; simulations of the onset of spring thaw and the dates of snowpack disappearance and accumulation also generally tracked observations. Differences between observed and simulated evapotranspiration were attributed to model assumptions of constant, growing season, overstory leaf areas that did not account for phenological changes and understory effects on stand daily water fluxes. Vapor pressure deficit and solar radiation accounted for 58–74% of the variances in simulated daily evapotranspiration during the growing season, though low air temperature and photosynthetic light levels were found to be the major limiting factors regulating simulated canopy conductances to water vapor. Humidity and soil moisture were generally not low enough to induce physiological water stress in black spruce stands, though low soil water potentials resulted in approximate 34% reductions in simulated mean daily canopy conductances for aspen and jack pine stands. The sensitivity of evapotranspiration simulations to leaf area (LAI) was less than expected because of opposing responses of transpiration and evaporation to LAI. The results of this investigation identify several components within boreal forest stands that are sensitive to climate change.

Evapotranspiration predictions of CERES-Sorghum model in Southern Italy

The purpose of this study was to modify and calibrate the CERES-Sorghum water balance model for the dry, high radiation and windy conditions in an area in Southern Italy. The equation for estimating potential evapotranspiration ( E 0) was substituted by another one, calibrated in the study site and expressed as a function of equilibrium evaporation and maximum vapour pressure deficit (defined as the difference between the saturation vapour pressure at maximum and at minimum temperatures). To calibrate the E 0 equation included in CERES-Sorghum, two drainage lysimeters, located at the Istituto Sperimentale Agronomico experimental farm, Foggia (Italy), were used to measure weekly evapotranspiration of well-watered, irrigated fescue grass, from 1976 to 1986. A further drainage lysimeter, located in the same farm and cropped with well-watered grain sorghum (cv. NK 121) was used to calibrate the genetic coefficients input to the modified CERES-Sorghum model during the cropping seasons 1979 and 1980. Simulated phenological dates (anthesis and maturity), grain yield, LAI, biomass and crop evapotranspiration were then compared with the measured ones in a fourth drainage lysimeter cropped with sorghum. The modified model simulated grain yield accurately, but simulated daily evapotranspiration did not always match well the observed value, especially early in the crop cycle. Improvements are needed to the model in its simulation of soil evaporation and in the crop response function to temperature.