Changes In Reference Evapotranspiration 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.

Reference evapotranspiration during the growing season in Poland (central Europe) in response to ongoing climate changes (1966–2020)

AbstractEvapotranspiration is an important part of the water cycle and influences, for example, water management and agricultural production. The article presents temporal and spatial changes in reference evapotranspiration (ETo) in the growing season (April–September) against the background of climatic conditions in Poland in the years 1966–2020. ETo was calculated using the Penman–Monteith method using data from 37 meteorological stations in the network of the Institute of Meteorology and Water Management‐National Research Institute. For the growing season of April–September, the average sum of ETo for Poland was 529 mm. In terms of spatial distribution, the values were lowest in northern Poland (Kołobrzeg, 480 mm; Lębork, 488 mm) and highest in the central lowlands (Warsaw, 570 mm; Poznań, 566 mm). For individual months, the average sum of ETo ranged from 53 to 63 mm in April and September to 108–112 mm in June and July. In Poland, in the years 1966–2020, ETo showed a statistically significant upward trend of 16.4 mm/10 year. This is the result of, inter alia, statistically significant increases in sunshine duration and air temperature and a decrease in relative humidity (respectively: 62 h/10 year, 0.40°C/10 year and −1%/10 year) during the growing season in Poland. Within the multiyear study period, the maximum ETo values occurred in 2018, reaching values of >700 mm. At that time, Poland was under the influence of very intense anticyclonic circulation. This is confirmed by the relationship between ETo and the anticyclonic circulation index for central Poland, which is statistically significant for the 55 study years.

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.

Reference evapotranspiration change in Heilongjiang Province, China from 1951 to 2018: The role of climate change and rice area expansion

It is of great significance to study the variation in reference evapotranspiration (ETo) and the factors that influence it to ensure the sustainable and efficient utilization of water resources. In this study, the spatio-temporal changes in ETo and rice areas as well as the relationship between them in Heilongjiang Province, China, from 1951 to 2018 were analysed by using trend, correlation and contribution analyses. The results showed that rice area expansion is an important factor affecting ETo changes. ETo in Heilongjiang Province decreased during the study period. The areas with high ETo in the Songnen and Sanjiang Plain showed a gradually shrinking trend. Climate change and rice area expansion both contributed to the decline in ETo. The decreases in sunshine duration and average wind speed in Heilongjiang Province were the main reasons for the decrease in ETo. Rice area in Heilongjiang province continued to increase at a rate of 0.54 × 105 ha yr−1. The expansion of rice area in Sanjiang Plain was the largest, followed by Songnen Plain. In response to the rice area expansion, the rise in average temperature slowed down, the average relative humidity was increased, while wind speed was reduced, which directly lead to the decrease in ETo. The effects of the transformation of different land types into rice fields on ETo were different. The cumulative effects of wetlands and dry fields conversion to rice fields on ETo were 7.67 mm yr−1 and − 20.15 mm yr−1 in Sanjiang Plain respectively.

Attribution analyses of reference evapotranspiration changes in China incorporating surface resistance change response to elevated CO2

Reference evapotranspiration (ET0) is a key component of the water cycle. In this study, a modified Penman–Monteith method incorporating the CO2 effect on surface resistance was used to estimate past change in ET0 in China for 1961–2019 and to project future changes for 2040–2099. A partial-differential equation was used to attribute the changes in ET0. Results indicated the following. (1) A shift point in the annual ET0 series occurred in 1993, in which annual ET0 in China had been decreasing and then increased significantly by −18.93 and 11.19 mm·decade-1 before and after 1993, respectively, with ET0 having changed most drastically within the temperate continental climate zone. (2) Wind speed and solar radiation were the major dominant factors responsible for the change in ET0 during 1961–1992, and their contribution rates were 48.30 and 38.92 %, respectively. During 1993–2019, increasing maximum air temperature was the dominant factor contributing to increasing ET0, with a contribution rate of 81.71%, yet wind speed was the dominant factor affecting ET0 changes at 45% of the stations in China. (3) In future projections, maximum air temperature is expected to be the dominant factor influencing the increase in ET0, the values of which were projected at 1.49 and 16.05 mm·decade-1 during 2040–2099 under RCP4.5 and 8.5 scenarios, respectively, with projections from five GCMs. The increasing surface resistance response to elevated CO2 was found to be an important contributor to the decrease in ET0. Particularly for the RCP8.5 scenario, the increase in surface resistance was found to lead to the decrease in ET0 by −0.68 mm·year−1 (-42.37%). This suggests that historical and future tendencies towards aridity in China may be considerably weaker and less extensive than previously assumed.

Detection and attribution of reference evapotranspiration change (1951–2020) in the Upper Yangtze River Basin of China

AbstractReference evapotranspiration (ET0) indicates atmospheric evaporating capability over a hypothetical reference surface. ET0 is an important hydrological and meteorological variable to reflect climate change. This is particularly true for the Upper Yangtze River Basin (UYRB), which is vulnerable and sensitive to changing environment. This study aims to provide a newer and longer description of ET0 change and the causes at basin and subbasin scales in the UYRB. Based on the observed data from 1951 to 2020, ET0 in the entire UYRB and subbasins is estimated using the Penman–Montieth method with local calibration. The spatial–temporal characteristics in ET0 change are identified from time-series analysis. Our results show that ET0 increases significantly by 3.3 mm/year in the entire UYRB. Stations with significant increases in annual ET0 are concentrated in the central part of the UYRB, where the mean annual ET0 is low. We further propose an improved method to assess the causes of ET0 change. Our results suggest that the relative humidity decrease has the most dominant effect, causing 4.69 mm/year of ET0 increase. Temperature increase tends to cause 1.26 mm/year of ET0 increase. Sunshine duration decrease and wind speed decrease contribute to 1.96 and 0.48 mm/year of ET0 decrease.

Temporal trend changes in reference evapotranspiration contrasting different land uses in southern Amazon basin

Due to the expansion of agricultural borders, land use and land cover changes in the southern part of the Brazilian Amazon have been altering water and energy balances over the last decades. Still, Brazilian agriculture, for the most part, depends on rainfall to supply the water needs of crops and evapotranspiration is the main mean of water transferring, in the form of vapor, to the process of cloud formation and consequently, rains. Reference evapotranspiration (ETo) is a climatic variable with great potential to indicate climate changes, which can be used to detect temporal trends and calculate aridity index (AI). Thus, the general objective is to analyze the behavior of the reference evapotranspiration and the components used to estimate it in contrast to the land use in four locations in the North of Mato Grosso state, Brazil, in the southern Amazon basin. The main factors related to possible changes in ETo over a mean period of 20 years were evaluated, identifying trends for the data set studied (ETo, sunshine duration, air temperature, wind speed, relative humidity, and precipitation) and the correlation (r) between reference evapotranspiration and the other variables. Besides, the behavior of the AI was analyzed to observe the degree of rainfall insufficiency to maintain the soil in humidity conditions suitable for plant development. To identify how significant the trends were, we used the Mann-Kendall trend analysis and linear regression analysis. We found at least two different significant trends for each location. The reference evapotranspiration showed a significant trend only in the station located in the municipality of Canarana, with a negative trend, varying 8.4 mm per year. The aridity index, for the driest months of the year, presented values close to 1, characterizing the region as medium to high aridity. The variable that most influenced the ETo variability was the insolation.

Elevation‐dependent changes in reference evapotranspiration due to climate change

AbstractThe Food and Agriculture Organizations' (FAO) Penman–Monteith reference evapotranspiration (ET0) is a crucial index in the research of water and energy balance. Temporal and spatial variations in ET0 from 1981–2017 were investigated in the Hengduan Mountains, China. The results showed a change point around the year 2000 in ET0 series. ET0 decreased and increased significantly by +3.200 mm/year (p < 0.01) from 1981–2000 and by +4.109 mm/year (p < 0.01) from 2001–2017, respectively. The contribution analysis shows that the positive significant contribution of air temperature (TA) was offset by negative effects of decreases in downward shortwave radiation (Rs) and wind speed (WS) and an increase in actual vapour pressure (ea), causing the decrease in ET0 from 1981 to 2000. WS was the largest contributing factor for the decrease in ET0 from 1981 to 2000 during spring, winter and annually, while Rs and ea were the largest negative contributors in summer and autumn, respectively. An increase in TA was responsible for the increase in ET0 in all seasons except winter and the annual scale in 2001–2017. The sensitivity analysis shows that ET0 was most sensitive to TA, and WS was the least sensitive variable. The trends of ET0 increased with elevation; we denote this as the elevation‐dependence of ET0 changes. The elevation‐dependence was also noted for the trends of WS and ea, with higher elevations showing larger changes in WS and lower changes in ea. Besides, the sensitivities of TA, Rs and ea decreased with elevation, while that of WS increased slightly with elevation. A comprehensive investigation into the trends of climatic drivers and their sensitivities revealed complex trends of the contributions of climatic variables on ET0 with elevation, with no uniform trend existed in seasons. The results will contribute to our understanding of the response of ET0 to climate change in a mountainous area, and provide a guideline for the water resources management under climate change.

Projected Changes in Reference Evapotranspiration in California and Nevada: Implications for Drought and Wildland Fire Danger

AbstractRecent high impact wildfires and droughts in California and Nevada have been linked to extremes in the Evaporative Demand Drought Index (EDDI) and Standardized Precipitation Evapotranspiration Index (SPEI), respectively. Both indices are dependent on reference evapotranspiration (ET0). Future changes in ET0 for California and Nevada are examined, calculated from global climate model simulations downscaled by Localized Constructed Analogs (LOCA). ET0 increases of 13–18% at seasonal timescales are projected by late century (2070–2099), with greatest relative increases in winter and spring. Seasonal ET0 increases are most strongly driven by warmer temperatures, with increasing specific humidity having a smaller, but noteworthy, counter tendency. Extreme (95th percentile) EDDI values on the 2‐week timescale have coincided with recent large wildfires in the area. Two‐week EDDI extremes are projected to increase by 6–10 times during summer and 4–6 times during autumn by the end of the century. On multiyear timescales, the occurrence of extreme droughts based on 3‐year SPEI below the historical fifth percentile, similar to that experienced during the 2012–2016 drought across the region, is projected to increase 3–15 times by late century. Positive trends in extreme multiyear droughts will further increase seasonal fire potential through degraded forests and increased fuel loads and flammability. Understanding how these drought metrics change on various climate timescales at the local level can provide fundamental information to support the development of long‐term adaptation strategies for wildland fire and water resource management.

Changes in reference evapotranspiration over the non‐monsoon region of China during 1961–2017: Relationships with atmospheric circulation and attributions

AbstractReference evapotranspiration (ET0) is an essential component of the hydrological cycle and is crucial to water resources management and assessment. Spatiotemporal variations in ET0 and their attribution to five climatic variables (maximum and minimum air temperatures, solar radiation, vapour pressure, and wind speed) were estimated for the non‐monsoon region of China from 1961–2017 meteorological data by using the Penman–Monteith equation and the partial‐differential method. The association between ET0 and atmospheric circulation patterns was explored by applying Pearson's correlation analysis. The annual ET0 series had two earlier shift points, the first showing a significant increase from 1961 to 1973 (period I) and the second showing a significant decrease from 1974 to 1994 (period II). ET0 has significantly increased again since 1994 (period III). Wind speed has been the dominant contributing factor to ET0 followed by maximum air temperature during all three periods, but the contribution gap between minimum and maximum air temperatures is narrowing. ET0 has been strongly correlated with eight selected teleconnection indices in some regions, but the locations and spatial extents have varied considerably for different indices. Eight relationships with ET0 were divided into four groups: North Atlantic Oscillation and Arctic Oscillation (mainly the northwest non‐monsoon region), Pacific North American Index and Pacific Decadal Oscillation (mainly the northern non‐monsoon region), Antarctic Oscillation and Atlantic Multidecadal Oscillation (mainly the Hexi Corridor and the southern Xinjiang), and East Asian Summer Monsoon Index and South China Sea Summer Monsoon Index (mainly the southeast non‐monsoon region). The Antarctic Oscillation, Atlantic Multidecadal Oscillation, and South China Sea Summer Monsoon Index indicated more potential for predicting ET0. When we used Pearson's correlation analysis to analyse the relationship between atmospheric circulation and the dominant factors affecting ET0, we found that the circulation patterns are likely to affect the variation in ET0 by influencing the dominant climatic variables.

Quantitative Analysis of the Impact of Meteorological Factors on Reference Evapotranspiration Changes in Beijing, 1958–2017

The effects of meteorological factors on reference evapotranspiration (ET0) are variable on different time scales, although research tends to focus only on certain time scales. Therefore, using the meteorological data from 1958 to 2017 of Beijing, China, ET0 values over the last 60 years were calculated using Penman–Monteith method. The variation in ET0 values was thus analyzed against four meteorological factors over different time scales. The sensitivity of ET0 to these factors was assessed using a sensitivity coefficient, while the contribution of each factor to ET0 change was quantified by combining this sensitivity coefficient with the factor’s relative change rate over multiple time scales. The results showed that the sensitivity coefficient of relative humidity over different time scales were all negative, while the sensitivity coefficients of net radiation, temperature and wind speed were mostly positive. The main sensitivity factors of ET0 on different time scales varied. On annual time scales, the main factors were relative humidity and temperature. Over annual time scales, relative humidity and net radiation alternated as the main sensitivity factor; while over interannual time scales, the most sensitive factor was relative humidity during 1958–1979 and net radiation thereafter. The contribution of these four meteorological factors to ET0 also fluctuated greatly on intra-annual time scales. On daily time scales, the contributions of temperature and wind speed at the start and end of the year were large, while net radiation and relative humidity were dominant mid-year. On monthly to seasonal time scales, the contributions of these four meteorological factors to ET0 were notable. The contribution of relative humidity was largest in spring and autumn; net radiation was dominant in summer, while temperature and wind speed were dominant in winter. This research on the temporal variability of ET0 response factors is of great significance for understanding regional climate change.

Changes in reference evapotranspiration and its driving factors in peninsular Malaysia

Trends in reference evapotranspiration (ETo) have been found highly diverse in different regions of the globe due to the contradictory changes in the meteorological variables that define ETo. Despite a significant impact of ETo in water resources and ecology, knowledge on the changes and the cause of the changes in ETo is very limited in tropical regions. The trends in ETo, the factors influencing the changes in ETo and the change point (year) that made the trend significant were evaluated in this study for tropical peninsular Malaysia. The modified version of Mann-Kendall (MK) test was used for the assessment of unidirectional changes in ETo and the driving meteorological variables. The innovative trend analysis (ITA) was conducted to understand the variations in change with time. Sobol's method was used to measure the sensitivity of ETo to different meteorological factors and the Sequential MK test was employed to identify the change point. The study revealed an increase in annual (0.009–0.026 mm/year) and seasonal (0.014–0.027 mm/year during southwest monsoon and 0.015–0.074 during northeast monsoon) ETo in peninsular Malaysia which contradicts to evapotranspiration paradox found in many regions. The minimum temperature (31.5–48.2%) was found as the most influencing factor followed by wind speed (15.1–32.8%.) in defining ETo in peninsular Malaysia. Analysis of ITA and sequential MK test results revealed that the rise in minimum temperature is the major cause of the increase in ETo in peninsular Malaysia. A faster rise in minimum temperature after 1981–1985 caused an increase in ETo after 1993–1996 in most of the locations. The minimum temperature in the region was noticed to rise much faster compared to the global average which indicates a large and continuous increase in ETo due to global warming and thus, reduction in atmospheric water balance in peninsular Malaysia.

Changes in reference evapotranspiration over Northwest China from 1957 to 2018: Variation characteristics, cause analysis and relationships with atmospheric circulation

The Ningxia Hui Autonomous Region (NX) in Northwest China has been challenged by water scarcity and drought for decades. In this study, to understand the spatio-temporal variation, cause analysis and relationship with atmospheric circulation of ET0 in Ningxia, ET0 and other climate factors at 20 national climate stations from 1957 to 2018 were analyzed. Results showed that ET0 in NX (Ningxia), NYR (Northern Yellow River Irrigation Area) and SMA (Southern Mountain Area) had increased significantly at annual scale, whilst the CAZ (Central Arid Zone) was the opposite trend, and ET0 had a trend of first rise and then decline from north to south in spatial distribution. ET0 was most sensitive to RH and Tmax at annual scale in Ningxia, while the greatest contribution rates were Tmax and SD. Ningxia was becoming drier in the past decades. The abrupt change in ET0 at approximately 1990, and it’s long and short period were 25a(15a) and 10a(5a) at annual scale, respectively. The four teleconnection indices could be used to predict changes in ET0 at annual and autumn scale, while the ENSO and PDO could predict changes in ET0 of summer and IOD and AO could predict changes in ET0 of spring and winter.

Variability in meteorological parameters and their impact on evapotranspiration in a humid zone of Pakistan

AbstractPakistan lies in the region over the globe where effects of climate change due to global warming are more likely to occur. These climatic changes are reflected in significant gradual fluctuations in measurements of meteorological parameters. Agricultural activity in any region depends upon the role of components of hydrological cycle over the region. An important component of hydrological cycle is evapotranspiration (ET), which is closely linked with changes in meteorological parameters. This study investigates impact of probable variation in four important meteorological parameters i.e. temperature (T), net solar radiation (Rn), vapour pressure (ea) and wind speed (U) on ET to estimate possible changes in reference ET in a humid zone of Pakistan by employing observed climatological records for 30 years (1981–2010). Analyses are done on the basis of possible ±20% changes in climatological mean values of the observed meteorological parameters. The analyses indicate that 20% rise in observed values of T, Rn and U increases value of total (annual) ET demand in the zone up to 11.93%, 16.37% and 2.83% respectively, and 20% rise in the observed values of vapour pressure decreases value of total (annual) ET till −2.5%. Moreover, the analyses also showed that ET is more sensitive to T and Rn in monsoon, vapour pressure in winter, and to wind speed in summer. Hence, this study might be useful in planning and formulating future strategies to meet expected future water requirements due to global warming in humid zone of Pakistan and its surrounding region.

Changes in Reference Evapotranspiration over Southwest China during 1960–2018: Attributions and Implications for Drought

Reference evapotranspiration (ET0) is important to the global energy balance and to hydrological cycling. However, the extent to which ET0 changes, the main driving factors, and especially the implications of its shift for drought in Southwest China are not clear. In this study, trends in Penman–Monteith ET0 and other climatic parameters at 79 stations in Southwest China from 1960 to 2018 were investigated by using the Mann–Kendall test. Furthermore, partial correlation analysis and multiple linear regression were used to determine the dominant climate driving factors in changes in ET0. The relative contribution of precipitation and ET0 to drought duration was also quantified based on spatial multiple linear regression. Results revealed that annual ET0 decreased significantly (p < 0.01) at a rate of 14.1 mm per decade from 1960 to 2000, and this decrease disappeared around 2000. For the entire study period, the sunshine duration (Tsun) was the most closely correlated with and played a dominant role in the variations in ET0 at both annual and seasonal (summer and autumn) timescales, whereas the relative humidity was the most dominant factor in the spring and winter. Trends in the Standardized Precipitation Evapotranspiration Index revealed that drought has become more serious in Southwest China, and ET0 has made a greater contribution to the duration of drought than precipitation. Our findings highlight that more attention should be paid to the impacts of ET0 changes on drought in Southwest China. Furthermore, these results can provide a reference for the allocation of water resources and the implementation of countermeasures to climate change.

Analysis of Changes in Reference Evapotranspiration, Pan Evaporation, and Actual Evapotranspiration and Their Influencing Factors in the North China Plain During 1998–2005

AbstractEvapotranspiration (ET) plays an important role in both energy and water balances. Determining changes in the different types of ET and their influencing factors under climate change is crucial to fully understand the mechanisms controlling ET and regional water cycles. Based on ET and meteorological data measured during 1998–2005 at the Yucheng Comprehensive Experimental Station of the Chinese Academy of Sciences, the variation of reference ET (ET0), pan evaporation (Epan), and lysimeter ET (the actual ET, Elys) were estimated. Together with a Mann‐Kendall trend test, correlation and sensitivity analyses were conducted to determine the effects of climatic factors, crop coefficients (Kc), and leaf area index (LAI) on different types of ET. During 1998–2005, precipitation presented an upward trend at all time scales, and average temperature (Ta), minimum temperature (Tmin), relative humidity, Epan, and ET0 exhibited downward trends. Elys displayed a downward trend at the daily scale and an upward trend at monthly and annual scales. The correlation analysis indicated that Epan and Elys were significantly positively correlated with all climatic factors except precipitation and relative humidity. The sensitivity analysis revealed that except for relative humidity, the influence of the other climatic factors on ET0 was positive. The highest sensitivity was found for relative humidity, followed by Ta, maximum temperature (Tmax), and Tmin. An analysis of the influence of Kc and LAI on the actual ET indicated that with an increase of Kc and LAI, the actual water demand of crops also displayed an upward trend.

Analysis of Reference Evapotranspiration Change in Korea by Climate Change Impact

In this study, the impact of climate change on the characteristics of evapotranspiration in Korea was assessed. The Regional Concentration Pathway (RCP) 4.5 and 8.5 scenarios were used to calculate the reference evapotranspiration for 62 stations of the Korea Meteorological Administration (KMA) and compared with the calculation results from observation data. The results of the analysis showed that the average annual evapotranspiration increases by 5.4% in the RCP 4.5 scenario and 9.5% in the RCP 8.5 scenario compared with the current one. On a monthly scale, the reference evapotranspiration for January, February, November and December is expected to increase by more than 10% compared to the current period. The ratio of annual rainfall to yearly evapotranspiration was used to examine the possibility of droughts for future periods, and the results were found to be relatively high in the RCP 8.5 scenario when compared to the RCP 4.5 scenario. The increase in the reference evapotranspiration due to climate change may lead to a decrease in the available water resources, and therefore, continuous monitoring and appropriate adaptation measures are necessary. Keywords: Reference Evapotranspiration, Climate Change, RCP Scenario, Hargreaves Equation

Changes in reference evapotranspiration over China during 1960–2012: Attributions and relationships with atmospheric circulation

AbstractThis study investigates reference evapotranspiration (ET0) trends in China from 1960 to 2012 based on the Penman–Monteith equation and gridded meteorological measurements. Under the combined impacts of factors influencing ET0 (i.e., net radiation [RN], mean temperature [TAVE], vapour pressure deficit [VPD], and wind speed [WND]), both seasonal and annual ET0 for the whole China and more than half of the grids decreased over the past 53 years. The attribution analyses suggest that for the whole China, the WND is responsible for annual and seasonal ET0 decreases (excluding summer, where RN is responsible). Across China, the annual cause of WND with the largest spatial extent (43.1% of grids) mainly derives from north of the Changjiang River Basin (CJRB), whereas VPD (RN) as a cause is dispersedly distributed (within and to the south of the CJRB). In summer, RN is dominant in more than half of the grids, but the dominance of VPD and WND accounts for approximately 90% of grids during the remaining seasons. Finally, the correlation coefficients between ET0 and the Atlantic Oscillation (AO), North AO, Indian Ocean Dipole (IOD), Pacific Decadal Oscillation (PDO), and El Niño Southern Oscillation (ENSO) indices with different lead times are calculated. For the whole China, annual and seasonal ET0 always significantly correlate with these indices (excluding the IOD) but with varied lead times. Additionally, near half of the grids show significant and maximum (i.e., the largest one between ET0 and a certain index with a lead time of 0–3 seasons) correlation coefficients of ET0 with PDO in spring and summer, ENSO in autumn, and AO in winter. This study is not only significant for understanding ET0 changes, but it also provides preliminary and fundamental reference information for ET0 prediction.

Trends in reference evapotranspiration and associated climate variables over the last 30 years (1984–2014) in the Pampa region of Argentina

Agricultural production is sensitive to weather and thus directly affected by climate change. In Argentina, the geographic region known as the “Pampa” is at the heart of agricultural production. In this context, the objective of the present study was to evaluate the presence of long-term trends and abrupt changes in reference evapotranspiration (ETo PM) and associated climate variables in 30 weather stations of the Pampa over the 1984–2014 period. The presence of temporal trends was evaluated using Mann-Kendall’s statistical test, whereas abrupt changes were detected through Pettitt test. Significant upward trends were observed in 80 and 43% of maximum temperature (Tmax) and ETo PM time series, whereas relative humidity (RH) and wind speed (WS), in contrast, presented decreasing trends in 57 and 47% of the locations. Abrupt changes were frequently observed in the years 2000–2003 for Tmax, RH, and ETo PM time series, a period that coincides with the occurrence of flooding events in the region. Weather stations of the Pampa region could be divided into two broad categories based on their trends in ETo PM and influencing climate variables: (A) stations exhibiting a rising trend in ETo PM and a concomitant decreasing trend in RH and (B) stations presenting invariant ETo PM and a decreasing trend in WS. The spatial distribution of the two categories of stations did not exhibit any specific geographic pattern. The information provided herein on modern trends in climate and evaporative demand is essential to the development of climate models and future scenarios necessary to evaluate food security prospects both regionally and globally.

Changes in reference evapotranspiration and its driving factors in the middle reaches of Yellow River Basin, China

Reference evapotranspiration (ET0) is important for agricultural, environmental and other studies, and understanding the attribution of its change is helpful to provide information for irrigation scheduling and water resources management. The present study investigates the attribution of the change of ET0 at 49 meteorological stations in the middle reaches of Yellow River basin (MRYRB) of China from 1960 to 2012. Results show that annual ET0 increases from the northwest to the southeast of MRYRB in space. We find that annual ET0 clearly presents a zigzag change pattern rather than a monotonically change during the whole period. The detected three breakpoints at 1972, 1988 and 1997 divide the whole period into four subperiods. The sensitivity analysis indicates that the ET0 is the most sensitive to surface solar radiation (Rs), followed by relative humidity (RH) and mean air temperature (T), and the least sensitive to wind speed (u) in our study area. Furthermore, we find that ET0 is becoming less sensitive to RH and more sensitive to T during 1960–2012. The attributions of the change in ET0 vary largely at different regions and subperiods. The declined wind speed is the dominant factor, followed by Rs to the ET0 reduction during 1960–2012. Further analysis shows that Rs and u are the two major contributing factors that control the change of ET0 at most stations and during most subperiods. Our study confirms that the change of ET0 is influenced by the complex interactions of climatic factors, and the dominant factor to the change of ET0 is different in various regions and time periods. The results presented here can provide a reference for agricultural production and water resources management in MRYRB as well as other semi-arid and semi-humid regions.