Annual ET0 Research Articles

Contribution of climatic variables and their interactions to reference evapotranspiration changes considering freeze-thaw cycles in the Tibetan Plateau during 1960–2022

While the direct impact of climate change on reference evapotranspiration (ET0) has been extensively studied, there is limited research on the indirect impact resulting from the interaction between climatic variables. This gap hinders a comprehensive understanding of climate change effects on ET0. Additionally, there is scarce exploration into the quantitative effect of freeze-thaw cycles on ET0 variation. In this study, we employed path analysis and dependent variable variance decomposition methods to discern the direct and interactive effects of climatic variables on ET0 in the Tibetan Plateau from 1960 to 2022. Annual ET0 exhibited variation across basins, with the coefficient of variability during the thawed period smaller than that during the non-thawed period. On an annual scale, the largest contribution to ET0 variation came from water vapor pressure deficit (VPD) at 47.7%. This contribution was amplified by its coupled interaction with temperature (T) at 47.1%, although the contribution was partially offset by the interactive effects of VPD with downward shortwave radiation and wind speed at −2.4% and − 27.6%, respectively. During different freezing-thawing periods, VPD primarily controlled ET0 variation, with its interaction with other climatic variables enhancing its impact. Furthermore, soil moisture, influenced by freeze-thaw cycles, exhibited a strong correlation with T and VPD, indicating the significant effect of freeze-thaw cycles on ET0 variation. The weak correlation between ET0 and NDVI suggested that vegetation growth had a limited regulatory effect on ET0. These findings provide valuable insights into the impact of interactions between climatic variables on hydrological processes, enhancing our understanding of the interactive roles of hydrometeorological variables.

Comparative Analysis of Climate Change Impacts on Climatic Variables and Reference Evapotranspiration in Tunisian Semi-Arid Region

Systematic biases in general circulation models (GCM) and regional climate models (RCM) impede their direct use in climate change impact research. Hence, the bias correction of GCM-RCMs outputs is a primary step in such studies. This study compares the potential of two bias correction methods (the method from the third phase of the Inter-Sectoral Impact Model Intercomparison Project (ISIMIP3) and Detrended Quantile Matching (DQM)) applied to the raw outputs of daily data of minimum and maximum air temperatures and precipitation, in the Cap-Bon region, from eight GCM-RCM combinations. The outputs of GCM/RCM combinations were acquired from the European branch of the coordinated regional climate downscaling experiment (EURO-CORDEX) dataset for historical periods and under two representative concentration pathway (RCP4.5 and RCP8.5) scenarios. Furthermore, the best combination of bias correction/GCM-RCM was used to assess the impact of climate change on reference evapotranspiration (ET0). Numerous statistical indicators were considered to evaluate the performance of the bias correction/historical GCM-RCMs compared to the observed data. Trends of the Hargreaves–Samani_ET0 model during the historical and projected periods were determined using the TFPMK method. A comparison of the bias correction methods revealed that, for all the studied model combinations, ISIMIP3 performs better in reducing biases in monthly precipitation. However, for Tmax and Tmin, the biases are greatly removed when the DQM bias correction method is applied. In general, better results were obtained when the HadCCLM model was used. Before applying bias correction, the set of used GCM-RCMs projected reductions in precipitation for most of the months compared to the reference period (1982–2006). However, Tmin and Tmax are expected to increase in all months and for the three studied periods. Hargreaves–Samani ET0 values obtained from the best combination (DQM/ HadCCLM) show that RCP8.5 (2075–2098) will exhibit the highest annual ET0 increase compared to the RCP4.5 scenario and the other periods, with a change rate equal to 11.85% compared to the historical period. Regarding spring and summer seasons, the change rates of ET0 are expected to reach 10.44 and 18.07%, respectively, under RCP8.5 (2075–2098). This study shows that the model can be used to determine long-term trends in ET0 patterns for diverse purposes, such as water resources planning, agricultural crop management and irrigation scheduling in the Cap-Bon region.

Change Trend and Attribution Analysis of Reference Evapotranspiration under Climate Change in the Northern China

Reference evapotranspiration (ET0), an essential variable used to estimate crop evapotranspiration, is expected to change significantly under climate change. Detecting and attributing the change trend in ET0 to underlying drivers is therefore important to the adoption of agricultural water management under climate change. In this study, we focus on a typical agricultural region of the Fenwei Plain in northern China and use the Mann–Kendall test and contribution rate to detect the change and trend in ET0 at annual and seasonal scales and determine the major contribution factors to ET0 change for the baseline period (1985–2015) and the future period (2030–2060) based on high-resolution gridded data and climatic data from the Coupled Model Intercomparison Project Phase 6 (CMIP6). The results indicate that the annual ET0 of the Fenwei Plain showed a significant decreasing trend in the baseline period but insignificant and significant increasing trends in the future period under the SSP245 and SSP585 scenarios, respectively. The annual ET0 of the plain under the SSP245 and SSP585 scenarios increase by 4.6% and 3.0%, respectively, compared to the baseline period. The change and trend in ET0 between the four seasons are different in the baseline and future periods. Winter and autumn show clear increases in ET0. VPD is the major contribution factor to the ET0 change in the plain. The change in ET0 is mainly driven by the climatic variables that change the most rather than by the climatic variables that are the most sensitive to the ET0 change. The change and trend in ET0 in the plain showed clear spatial differences, especially between the eastern and western area of the plain. To adapt to the impact of climate change on ET0, the irrigation schedule of the crops cultivated in the plain, the cropping system and management of the irrigation district in the plain need to be adjusted according to the change characteristics of spatial and temporal ET0 in the future. These results contribute to understanding the impacts of climate change on evapotranspiration in the study region and provide spatial and temporal references for adaptation in managing agricultural water use and crop cultivation under climate change.

Spatiotemporal Variations of Reference Evapotranspiration and Its Climatic Driving Factors in Guangdong, a Humid Subtropical Province of South China

It is of great importance to study the changes in reference evapotranspiration (ET0) and the factors that influence it to ensure sustainable and efficient water resource utilization. Daily ET0 data calculated using the Penman–Monteith method from 37 meteorological stations located within Guangdong Province in the humid zone of southern China from 1960 to 2020 were analyzed. The trend analysis and Mann–Kendall test were used to analyze the time series changes in ET0 and major climatic factors (air temperature (T), relative humidity (RH), sunshine duration (SD), and wind speed (u2)) for over 61 years. Sensitivity and contribution analyses were used to evaluate the driving factors of ET0. The main findings of the study are as follows: (1) the trend in average annual ET0 time series in Guangdong slightly increased at a trend rate of 1.61 mm/10a over the past 61 years, with most stations experiencing an increase in ET0. During the same period, air temperature significantly increased, while RH and SD decreased; u2 also decreased. (2) Sensitivity analysis showed that ET0 was more sensitive to RH and T than SD and u2, with ET0 being most sensitive to RH in spring and winter and T in summer and autumn. (3) The contribution analysis showed that T was the dominant factor for ET0 variation in Guangdong, followed by SD. SD was found to be the dominant factor in ET0 changes in areas where the “evaporation paradox” occurred, as well as in spring and summer. The study concludes that the climate in Guangdong became warmer and drier over the past 61 years, and if the current global warming trend continues, it will lead to higher evapotranspiration and drought occurrence in the future.

Development of machine learning-based reference evapotranspiration model for the semi-arid region of Punjab, India

Evapotranspiration (ET) is a critical element of the hydrological cycle, and its proper assessment is essential for irrigation scheduling, agricultural and hydro-meteorological studies, and water budget estimation. It is computed for most applications as a product of reference crop evapotranspiration (ET0) and crop coefficient, notably using the well-known two-step method. Accurate predictions of reference evapotranspiration (ET0) using limited meteorological inputs are critical in data-constrained circumstances. Due to the unavailability and heterogeneity of broad parameters of the FAO PM method, it becomes a major constraint for accurately estimating ET0. To overcome the complexity of calculation, the present study was focused on developing a Random Forest-based (RF) ET0 model to estimate the crop ET for the semi-arid region of northwest India. The RF-based model was developed by focusing on the easily available data at the farm level. For comparative study existing models like Hargreaves–Samani, Modified Penman and modified Hargreaves–Samani were used to estimate the ET0. The models' calibration and validation were done using meteorological data collected from the weather station of Punjab Agricultural University for 21 years (1990–2010) and nine years (2011–2019), respectively, and the FAO PM model was taken as a standard. The mean absolute error (MAE) and root-mean-square error (RMSE) were found to be least as 0.95 mm/d and 1.32 mm/d, respectively for the developed RF model, with an r2 value of 0.92. The seasonal ET0 estimated by modified Hargreaves–Samani (MHS) and RF were found as 498.3, 482.1 mm in rabi season and 755, 744.8 mm in kharif season respectively, whereas the annual ET0 was 1380.2 and 1355.7 mm respectively. The predicted ET0 values by RF-based model were used for irrigation scheduling of two growing seasons (2020–2021) of maize and wheat crops. The outcome of the field trial also demonstrates that there was no appreciable yield drop in the crop when compared to irrigation scheduling by the FAO PM model, demonstrating the applicability of the developed model for irrigation in the semiarid region of the Punjab in India.

Spatiotemporal variations and driving factors of reference evapotranspiration in the Yiluo river basin

The variations in the reference evapotranspiration (ET0) are closely related to meteorological factors. The purpose of this study is to explore the relationships between the meteorological factors and the ET0. Based on meteorological data from 26 meteorological stations in the Yiluo River Basin (YLRB) and its surrounding areas from 1958 to 2020, in this study, the temporal and spatial variations and driving factors of the ET0 in the YLRB are investigated. The results are as follows. Spatially, the annual ET0 decreases from the northeast to the southwest in the YLRB. Temporally, the annual ET0 exhibits a fluctuating decreasing trend rather than a monotonic decreasing trend during the entire period. The trend of the ET0 contains two mutation points, in 1972 and 1994. Thus, the research period can be divided into three periods. It is concluded that the variations in the ET0 are the most sensitive to the relative humidity, but the driving factor that contributes the most to the variations in the ET0 is the wind speed. The driving factors are closely related to the rates of relative change of the meteorological factors.

Elevation‐dependent changes in the trend of reference evapotranspiration in the Tibetan Plateau during 1960–2017

AbstractElevation plays a crucial role in modulating the spatiotemporal distributions of climatic variables in mountainous regions, which affects water and energy balances, among which reference evapotranspiration (ET0) is a key hydrological indicator. However, the response of ET0 to climate change with elevation continues to be poorly understood, especially in the Tibetan Plateau (TP) which has elevation variations of more than 4,000 m. The spatiotemporal variations of ET0 with elevation were investigated using long‐term (1960–2017) meteorological observations from 82 stations on the TP. The results suggest that the average annual ET0 showed an insignificant increasing trend. A significant negative correlation between ET0 and elevation was found (p < .01). The positive trends of ET0 decreased with elevation, whereas the negative trends of ET0 increased significantly with elevation (p < .05). The magnitude of trends of ET0 becomes smaller at higher‐elevation stations. Sensitivity analysis indicated that ET0 was most sensitive to shortwave radiation (Rs). Moreover, the sensitivities of temperature (T) and wind speed (U) significantly decreased with elevation, whereas those of Rs and vapour pressure deficit (VPD) increased slightly with elevation. The contribution and path analyse indicated that increasing VPD was the dominant contributor to the increase in ET0. The effect of elevation on ET0 variation mainly depended on the tradeoff between the contributions of U and VPD. U was the largest contributing factor for the change in ET0 below 2,500 m, whereas VPD was the primary contributor to the increase in ET0 above 2,500 m. This study provides insights into the response of ET0 to climate change with elevation on the TP, which is of great significance to hydrometeorological processes in high‐altitude regions.

Analysis of the variation tendency and influencing factors of reference evapotranspiration in Shandong Province, China

AbstractReference evapotranspiration (ET0) is an important part of the water and energy cycles during crop growth. The study of its changing trend is of significance for guiding efficient water utilization and reasonable regulation of agricultural production. Based on daily meteorological data, including temperature, relative humidity, wind speed and sunshine hours, from 38 surface meteorological stations in Shandong Province from 1980 to 2019, the FAO56 Penman–Monteith formula was used to calculate the ET0 and analyse trend and influencing factors, combined with the methods of climate tendency rate, Morlet wavelet, sensitivity analysis and contribution rate analysis. ET0 had 26–30 main oscillation cycles during the 1980–2019 period, and 28‐, 18‐ and 8‐year time scales corresponded to the first, second and third main cycles of ET0, respectively, which determined the characteristics of ET0 change in the past 40 years. The annual average ET0 was significantly positively correlated with average temperature (T), wind speed (WS) and sunshine daily hours (SSD) and significantly negatively correlated with average relative humidity (RH), among which WS, SSD and RH were the main factors affecting ET0. On this basis, because T was not the dominant factor affecting ET0 change, a simplified ET0 calculation algorithm based on seasonal temperature was established that had high goodness of fit R2 accuracy. This study quantitatively analysed the periodic changes and causes of ET0 from a spatio‐temporal view and provided a scientific basis for the rational regulation of agricultural production in Shandong Province.

Spatial and Temporal Variation in Reference Evapotranspiration and Its Climatic Drivers in Northeast China

Reference evapotranspiration (ET0) is an important component of the global water cycle, and its long-term change directly influences the regional water supply and demand balance. Under the background of global change, investigating spatiotemporal trends in ET0 and its response to climate change is of great importance for the conservation and rational utilization of water resources. Based on daily climate data from 91 meteorological stations during 1960–2017 in Northeast China, this study calculated ET0 using the Penman-Monteith method and analyzed its spatiotemporal change trends and primary driving factors. The results show the following: (1) During 1960–2017, the annual ET0 in Northeast China showed a nonsignificant upward trend at a rate of 1.45 mm/10a. A mutation point of ET0 was detected in 1993. From 1960 to 1993, ET0 experienced a significant decrease (p < 0.1), while annual air temperature showed a significant upward trend (p < 0.01), which indicated the appearance of an evaporation paradox. This was because the remarkable drop in wind speed and sunshine duration played a great role in the reduction of ET0. From 1994 to 2017, the evaporation paradox disappeared. (2) ET0 trend in Northeast China was significantly and positively related to altitude. In the lower altitude regions (<500 m), ET0 generally decreased, while in the higher altitude areas (>500 m), ET0 displayed an upward trend. (3) Based on the results of multiple regression analysis, relative humidity was the primary driving factor for ET0 trends in Northeast China during 1960–2017. At diverse altitudes, the primary climatic factors influencing ET0 were different. In high-altitude areas (>500 m), the change in ET0 was mainly influenced by relative humidity, while wind speed was the primary driving factor at low altitudes (<500 m).

Prediction of Future Spatial and Temporal Evolution Trends of Reference Evapotranspiration in the Yellow River Basin, China

Reference evapotranspiration (ET0) is an integral part of the regional hydrological cycle and energy balance and is extremely sensitive to climate change. Based on temperature data from 24 global climate models (GCMs) in the Coupled Model Intercomparison Project Phase 6 (CMIP6), this study developed a multi-model ensemble based on delta statistical downscaling with multiple interpolation methods and evaluation indicators to predict the spatial and temporal evolution trends of ET0 in the Yellow River Basin (YRB) under four emission scenarios (SSP126, SSP245, SSP370, and SSP585) for the near- (2022–2040), mid- (2041–2060), and long- (2081–2100) term future. Results demonstrate that regional data generated based on delta statistical downscaling had good simulation performance for the monthly mean, maximum, and minimum temperatures in the YRB, and the developed multi-model ensemble had better simulation capability than any single model. Compared to the historical period (1901–2014), the annual ET0 showed a highly significant increase for different future emission scenarios, and the increase is faster with increasing radiative forcing. The first main cycle of ET0 change was 52, 53, 60, and 48 years for the SSP126, SSP245, SSP370, and SSP585, respectively. ET0 in the YRB had positive values for EOF1 under all four emission scenarios, responding to a spatially consistent trend across the region. Compared to the historical period, the spatial distribution of ET0 under different future emission scenarios was characterized by being larger in the west and smaller in the east. As the radiative forcing scenario increased and time extended, ET0 significantly increased, with a maximum variation of 112.91% occurring in the western part of the YRB in the long-term future under the SSP585 scenario. This study can provide insight into the water cycle patterns of watersheds and scientific decision support for relevant departments to address the challenges of climate change.

Spatiotemporal Variations in Reference Evapotranspiration and Its Contributing Climatic Variables at Various Spatial Scales across China for 1984–2019

Reference evapotranspiration (ET0) is of great significance in studies of hydrological cycle, agricultural water resources, and hydrometeorology. The present study collected daily meteorological data at 536 meteorological stations in China from 1984 to 2019, calculated daily ET0 using the FAO Penman–Monteith equation, analyzed the spatial distribution and temporal variation characteristics of ET0 and meteorological variables at four different spatial scales (continental, regional, provincial, and local), and discussed the sensitivity of ET0 to the meteorological variables and the contribution rates of the meteorological variables to the ET0 variations. The results showed that ET0 increased at 406 out of the 536 stations (75.7%), with the trends being significant at 65 stations at the 5% significance level, and 147 at the 1% significance level. The slope calculated using Sen’s method and linear trend method showed that the annual ET0 at the continental scale increased by approximately 12 mm/decade. Most of the stations showed decreasing trends in relative humidity (Hm), sunshine duration (SD), and wind speed at 2 m height (U2) while increasing trends in the maximum air temperature (Tmax) and minimum air temperature (Tmin). ET0 was most sensitive to Hm (sensitivity coefficient, St = −0.66), followed by Tmax (St = 0.29), SD (St = 0.18), U2 (St = 0.16), and Tmin (St = 0.07). Most of the stations showed increasing trends in St for Hm (56.16%), Tmax (72.95%), Tmin (87.31%), and U2 (90.49%), and decreasing trends for SD (69.78%). The variations in Hm, Tmax, and Tmin increased the ET0 at most of the stations (82.28%, 98.13%, and 69.03%, respectively). The variations in SD and U2 decreased ET0 at most of the stations (66.04% and 56.34%, respectively). Some ET0 characteristics in a few regions can be well described using a single spatial scale. However, most regions exhibited significantly different ET0 characteristics across spatial scales. The results of this project can provide reference for hydrological analysis and agricultural water management under climate change conditions and provide data and information for other hydrology-related applications.

Responses of Soybean Water Supply and Requirement to Future Climate Conditions in Heilongjiang Province

Understanding future changes in water supply and requirement under climate change is of great significance for long-term water resource management and agricultural planning. In this study, daily minimum temperature (Tmin), maximum temperature (Tmax), solar radiation (Rad), and precipitation for 26 meteorological stations under RCP4.5 and RCP8.5 of MIRCO5 for the future period 2021–2080 were downscaled by the LARS-WG model, daily average relative humidity (RH) was estimated using the method recommended by FAO-56, and reference crop evapotranspiration (ET0), crop water requirement (ETc), irrigation water requirement (Ir), effective precipitation (Pe), and coupling degree of ETc and Pe (CD) for soybean during the growth period were calculated by the CROPWAT model in Heilongjiang Province, China. The spatial and temporal distribution of these variables and meteorological factors were analyzed, and the response of soybean water supply and requirement to climate change was explored. The result showed that the average Tmin, Tmax, and Rad under RCP4.5 and RCP8.5 increased by 0.2656 and 0.5368 °C, 0.3509 and 0.5897 °C, and 0.0830 and 0.0465 MJ/m², respectively, while the average RH decreased by 0.0920% and 0.0870% per decade from 2021 to 2080. The annual average ET0, ETc, Pe, and Ir under RCP4.5 for 2021–2080 were 542.89, 414.35, 354.10, and 102.44 mm, respectively, and they increased by 1.92%, 1.64%, 2.33%, and −2.12% under the RCP8.5, respectively. The ranges of CD under RCP4.5 and RCP8.5 were 0.66–0.95 and 0.66–0.96, respectively, with an average value of 0.84 for 2021–2080. Spatially, the CD showed a general trend of increasing first and then decreasing from west to east. In addition, ET0, ETc, and Pe increased by 9.55, 7.16, and 8.77 mm per decade, respectively, under RCP8.5, while Ir decreased by 0.65 mm per decade. Under RCP4.5 and RCP8.5, ETc, Pe, and Ir showed an overall increasing trend from 2021 to 2080. This study provides a basis for water resources management policy in Heilongjiang Province, China.

Quantitative driving analysis of climate on potential evapotranspiration in Loess Plateau incorporating synergistic effects

Potential evapotranspiration (ET0) is an important part of regional hydrological cycle and energy cycle. In the context of global climate change, it is of great significance to reveal the spatiotemporal variation of ET0 and its response to climate change for further understanding the evolution mechanism of water cycle in changing environment. In this study, the Penman-monteith formula was utilized to calculate ET0 in 6 sub-basins of the Loess Plateau (LP) during 1974–2019 using the daily observed data from 84 meteorological stations. The Mann-Kendall trend test and Kriging interpolation were applied to analyze the temporal and spatial characteristics of ET0 on annual and seasonal scales. In addition, the multivariate adaptive regression splines model, sensitivity analysis and contribution rate were used to identify the driving factors of ET0 variations by considering the synergistic effect of multiple factors. Results showed that (1) The ET0 of LP showed an insignificant increase at a rate of 5.448 mm/10a on the annual scale. At the seasonal scale, ET0 increased in spring and winter, but decreased in summer and autumn. Spatially, the annual average, spring, summer and autumn ET0 showed a trend of higher north–south and lower east–west; (2) The annual ET0 of LP is most sensitive to relative humidity (RH). At the seasonal scale, ET0 was most sensitive to sunshine hours(S) in spring and autumn, average temperature (T) in summer and RH in winter; (3) RH is the main influencing factor of ET0 change in the LP. At the seasonal scale, wind speed (WS) and T dominated the ET0 variation in spring and winter, respectively. S dominated the ET0 variation in summer and autumn; (4) The variation trend and driving factors of ET0 in each sub-catchment were different from those in the LP, and there were significant differences among the sub-catchments. This study provides a reference for the sustainable development of vegetation restoration and the management of agricultural water resources in the LP.

Interaction between wind speed and net radiation controls reference evapotranspiration variance in the inland river basin of Northwest China

AbstractExploring the variance in reference evapotranspiration (ET0) and its dominant influencing factors is important for climate change, hydrological cycles and water management. Temperature (T), wind speed (U2), net radiation (Rn) and actual vapour pressure (ea) are the major climatic input variables in Penman–Monteith equation. Previous studies have successfully applied the total differential method to determine the relative contributions of changes in these variables to variation in ET0 on different timescales. However, the interaction of climatic variables has not been incorporated into this method. Taking the inland river basin of Northwest China as the study area, we extended the total differential method to decompose the ET0 variance into temporal variance and covariance of T, U2, Rn and ea on intra‐annual and annual scales during 1960–2017. The results indicated that the variance in ET0 on the intra‐annual scale was larger than that on the annual scale. Among the four single climatic variables, U2 variance made larger contributions to intra‐annual and annual ET0 variance with relative contributions of 6.8% and 1.1%, respectively. However, the interaction of climatic variables played a dominant role in the variation in ET0. Specifically, on intra‐annual scale, coupled U2 and Rn primarily controlled the ET0 variance (76.1%), followed by coupled Rn and ea (9.6%); but their positive effects were weakened by the negative effects of coupled T and ea (−2.3%). On annual scale, coupled U2 and Rn still governed the variance in ET0 (97.3%), but the effects were also weakened by other groups of interaction effects. Furthermore, ET0 was highly related to NDVI on an intra‐annual scale (R2 = 0.68, p = 0.00), indicating the strong effect of seasonal vegetation dynamics on ET0 variance. This study provides new insights to quantitatively assess the interaction effects of environmental factors on key hydrometeorological variables.

Spatiotemporal pattern of reference crop evapotranspiration and its response to meteorological factors in Northwest China over years 2000-2019.

Agricultural irrigation water in Northwest China accounts for more than 80% of total local water consumption, which is 1.23 times that of China. However, Northwest China is the most water-scarce place in China. Water scarcity in restricts crop growth and production. Reference crop evapotranspiration (ET0) is important for agricultural water management. Understanding the reason for ET0 change is helpful to provide a basis for rational planning of agricultural irrigation systems to conserve water. This study investigated the temporal and spatial variation characteristics of ET0 at 181 meteorological stations in Northwest China from 2000 to 2019. And the sensitive factors and dominant factors affecting ET0 change were quantitatively identified based on sensitivity analysis and contribution rate evaluation. Results showed that (1) a significant increase in maximum and minimum temperature (Tmax and Tmin), a significant decrease in sunshine duration (SD) and relative humidity (RH), and a slight increase in wind speed at 10m height (U10) were observed. (2) Annual ET0 had an insignificant increasing trend. Spring and autumn ET0 contributed greatly to the growth of annual ET0, especially in March, May, September, October, and November. ET0 in HH (Yellow River Basin area) had decreased at annual scale, while other subregions were the opposite trend. Significant differences in monthly and seasonal changes in the spatial distribution of ET0. (3) U10 was the dominating contribution factor related to annual ET0 variability, followed by Tmin, RH, Tmax, and SD. In seasonal time scale, Tmin, SD, U10, and RH were the most dominant factors in spring, summer, autumn, and winter respectively. (4) Spatial distribution for contribution rates of various meteorological factors showed significant diversity among various subregions. The positive contribution of U10 was the major cause of the increase in ET0 in semi-arid grassland area (BGH), the southwest of "Qice line" (QCXXN), and the southeast of "Qice line" (QCXDN); the significant increase in Tmin contributed most in Qaidam Basin (CDM), Hexi inland river basin (HX), the northeast of "Qice line" (QCXDB), and the northwest of "Qice line" (QCXXB), while the contribution of decreasing SD offsets the positive effects of other factors, leading to the decrease in ET0 in HH. Our work illustrates that water management measures should be different at different spatial and temporal scales. The effect of U10 can be offset by covering, to reduce evaporation and maintain water in BGH, QCXXN, and QCXDN. And high-temperature resistant varieties are planted to adapt to temperature growth in CDM, HX, QCXDB, and QCXXB. Agricultural water management strategies should be formulated and selected according to local conditions.

Optimal Alternative for Quantifying Reference Evapotranspiration in Northern Xinjiang

Accurate estimation of reference evapotranspiration is a key step in irrigation and water resources planning. The Penman Monteith (FAO56-PM) formula recommended by FAO56-PM is the standard for calculating the reference evapotranspiration. However, the FAO56-PM model is limited in the observation of meteorological variables, so it is necessary to choose an alternative ET0 model which requires less meteorological data. Based on the daily climate data of eight meteorological stations in northern Xinjiang from 2000 to 2020, seven empirical models (Hargreaves, Berti, Dorji, Dalton, Meyer, WMO, Albrecht) and four optimization algorithms (RF model, LS-SVR model, Bi-LSTM model and GA-BP model) combined with seven different parameters were evaluated comprehensively. The results show that the accurate of the empirical model based on temperature is obviously better than the empirical model based on air mass transport. The annual and multi-year alternative ET0 models of different input parameter combinations are: LS-SVR1, RF2, LS-SVR3, LS-SVR4, GA-BP5, LS-SVR6, GA-BP7. It can be used as a substitute for the reference evapotranspiration model without relevant meteorological data. Only the LS-SVR6 model and GA-BP7 model are recommended as the best alternative models for northern Xinjiang reference evapotranspiration at daily, monthly and seasonal scales.

Spatio‐temporal variability of reference crop evapotranspiration at different timescales in Sichuan Province, south‐west China*

AbstractAiming to analyse the variation of seasonal reference crop evapotranspiration (ET0) and show the impact of meteorological factors on ET0 across Sichuan Province, this study mainly discussed the variation of ET0 during the periods of 1967–2016 in three subregions of Sichuan Province with the Mann–Kendall test (MK), Morlet wavelet analysis and path analysis. Meanwhile, future ET0 trend variation characteristics at different timescales were evaluated by Hurst exponents based on the rescaled range analysis (R/S) method. The results indicated that over the past 50 years, annual ET0 had a decreasing trend with a rate of −0.64 mm per10 years, confirming the existence of an ‘evaporation paradox’. Through spatial analysis, the distribution of average annual ET0 exhibited a declining trend from south to north over the study region. On the seasonal scale, the highest values of ET0 in summer were distributed in the Sichuan central basin while the highest values in other seasons were in the south‐western mountain area. The Hurst exponents showed that the trends of annual and seasonal ET0 in the future are consistent with past ones in the whole region (H > 0.5). The annual ET0 showed the obvious periodic variation of 28 years in the study regions based on Morlet wavelet analysis. The decreases in ET0 were mostly attributed to the decline in sunshine hours. These findings can help water managers in better water planning and irrigation scheduling for local agricultural production.

Applicability and improvement of different evapotranspiration methods of reference crops in Jiangxi Province

To achieve accurate evaluation of evapotranspiration of reference crops (ET0) in Jiangxi, China, in the absence of systematic climatological data, with reference to the FAO-56 Penman–Monteith (P-M) equation, the Priestley-Taylor (P–T) method, the Makkink method, the Hargreaves-Samani (H–S) method, the Irmak-Allen (I-A) method, the Penman1948 (48PM) method, the Penman-Van Bavel (PVB) method, the Baier-Robertson (B-R) method, the improved Baier-Robertson (M-B-R) method, the Schendel (Sch) method, the Turc method, the Jensen-Haise (J-H) method, and the Brutsaert-Stricker (B-S) method were used to evaluate the daily climatological data collected by 26 weather stations in Jiangxi, China, and 17 weather stations in adjacent provinces. The results were compared with each other and parameter rate determination was conducted. The results indicated that the Turc method exhibited optimized applicability before parameter rate determination and the average root mean square error (RMSE) and the average normalized root mean square error (NRMSE) by this method were 0.39 mm/d and 0.157 mm, respectively. However, parameter rate determination led to negligible improvement in accuracy for this method. The Turc method could be directly applied in Jiangxi (except Nanchang). For special distribution of error after parameter rate determination, all methods exhibited significant errors in Northern Jiangxi. Herein, the 48PM method and the B-S method showed good applicability after parameter rate determination and RMSE and NRMSE of data by these methods ranged in 0.06 ~ 0.34 mm/d and 0.08 ~ 0.27, 8 ~ 27%, respectively, and their d-indices were close to 1. The annual over-estimations in weather stations in Jiangxi were below 30 mm. In the absence of data about relative humidity and wind speed, the P–T method was an appropriate simplified method for Jiangxi. In this case, α was slightly lower than the default value (1.05 ~ 1.18), RMSE was within 0.21 ~ 0.66 mm/d, and NRMSE was within 0.08 ~ 0.308 ~ 30%. Accuracy of RMSE, d-index, and NRMSE of data by the P–T method, the I-A method, and the PVB method was consistent with all stations, while that by the Mak method was slightly lower, which could be attributed to severe over-estimation in July and August. RMSE of the H–S method, the B-R method, the M-B-R method, the J-H method, and the Sch method were above 0.75 mm/d and these methods were not suitable for accurate evaluation of ET0 in Jiangxi, China. The annual ET0 was calculated by various methods (except the 48PM method and the B-S method) exhibited significant variation around 2003. This may be attributed to significant changes in certain meteorological factors over recent years.

Effect of Elevation on Variation in Reference Evapotranspiration under Climate Change in Northwest China

Through its effects on water and energy cycles, elevation plays an important role in modulating the spatial distribution of climatic changes in mountainous regions. A key hydrological indicator, reference evapotranspiration (ET0) reflects the maximum amount of water transferred to the atmosphere from the land surface. The current scarcity of information regarding elevation’s impact on variation in ET0 under climate change limits our understanding of the extent to which elevation modulates interactions between ET0 and climate change and of the attendant processes involved. Drawing upon long-term (1960–2017) meteorological observations from 84 stations in Northwest China (NWC), we examined (i) spatial and temporal variations in ET0; (ii) the sensitivity and contribution of air temperature (T), sunshine duration (SD), relative humidity (RH), and wind speed (WS) to ET0; (iii) the existence of a relationship between elevation and ET0 trends; and (iv) the major factor in controlling this relationship by using attribution analysis. Overall, annual ET0 in NWC showed a declining trend between 1960 and 2017, though at a change point in 1993, the trend shifted from a decline to a rise. A significant correlation between temporal change in ET0 and elevation confirmed the existence of a relationship between elevation and ET0 variation. The effect of elevation on changes in ET0 depended mainly on the elevation-based tradeoff between the contributions of T and WS: WS was the primary factor contributing to the decrease in ET0 below 2000 m, and T was the dominant factor contributing to the increase of ET0 above 2000 m. The rate of reduction in WS declined as elevation increased, thereby diminishing its contribution to variation in ET0. The present study’s results can serve to guide agricultural irrigation in different elevation zones under NWC’s evolving climatic conditions.

Assessment of spatiotemporal variability and trend analysis of reference crop evapotranspiration for the southern region of Peninsular India.

Accurate estimation of reference evapotranspiration (ET0) is an essential requirement for water resource management and scheduling agricultural activities. Several empirical methods have been employed in estimating ET0 across diverse climate regimes over the past decades. In this study, the Python implementation for estimation of daily and monthly ET0 values of representative stations of ten agro-climatic zones of Karnataka from 1979 to 2014 using the standard FAO Penman-Monteith method was carried out. The assessment of temporal and spatial variability of monthly ET0 values across the various agro-climatic zones done by the various statistical measures revealed that the variation in spatial ET0 values was higher than temporal variation, indicating major difference in ET0 values was with respect to the stations rather than years under study. The nonparametric Mann-Kendall test conducted at 1% significance level on the annual ET0 values revealed a statistically significant increasing trend for all the ten stations during the study period. The trend test conducted on the climate variables like mean air temperature, wind speed, relative humidity, and solar radiation signifies their influence on the annual ET0 values. The magnitude changes in the trends detected by the Theil Sen's slope indicated that increasing values of mean temperature, solar radiation, and decreasing values of relative humidity predominantly contributed to the annual upward trend in ET0 values for the 10 stations. A trivial impact of wind speed on annual ET0 values was observed for the stations. Kalburgi and Udupi stations exhibited a positive ET0 trend with the highest and lowest annual values among ten stations.