Decrease In Evapotranspiration Research Articles

Irrigation-induced decreases in reference evapotranspiration over the North China Plain

Irrigation-induced decreases in reference evapotranspiration over the North China Plain

Drought-Induced Alterations in Carbon and Water Dynamics of Chinese Fir Plantations at the Trunk Wood Stage.

Over the past three decades, China has implemented extensive reforestation programs, primarily utilizing Chinese fir (Cunninghamia lanceolata (Lamb.) Hook) in southern China, to mitigate greenhouse gas emissions and counter extreme climate events. However, the effects of drought on the carbon sequestration capacity of these forests, particularly during the trunk wood stage, remain unclear. This study, conducted in Huitong, Hunan, China, from 2008 to 2013, employed the eddy covariance method to measure carbon dioxide (CO2) and water fluxes in Chinese fir forests, covering a severe drought year in 2011. The purpose was to elucidate the dynamics of carbon and water fluxes during a drought year and across multi-normal year averages. The results showed that changes in soil water content (-8.00%), precipitation (-18.45%), and relative humidity (-5.10%), decreases in air temperature (-0.09 °C) and soil temperature (-0.79 °C), and increases in vapor pressure deficit (19.18%) and net radiation (8.39%) were found in the drought year compared to the normal years. These changes in environmental factors led to considerable decreases in net ecosystem exchange (-40.00%), ecosystem respiration (-13.09%), and gross ecosystem productivity (-18.52%), evapotranspiration (-12.50%), and water use efficiency (-5.83%) in the studied forests in the drought year. In this study, the occurrence of seasonal drought due to uneven precipitation distribution led to a decrease in gross ecosystem productivity (GEP) and evapotranspiration (ET). However, the impact of drought on GEP was greater than its effect on ET, resulting in a reduced water use efficiency (WUE). This study emphasized the crucial role of water availability in determining forest productivity and suggested the need for adjusting vegetation management strategies under severe drought conditions. Our results contributed to improving management practices for Chinese fir plantations in response to changing climate conditions.

Assessing the Impact of Urbanization and Climate Change on Hydrological Processes in a Suburban Catchment

Global urbanization, population growth, and climate change have considerably impacted water resources, making sustainable water resource management (WRM) essential. Understanding the changes in hydrological components is important for effective WRM, particularly in cities such as Higashi-Hiroshima, which is known for its saké brewing industry. This study used the Soil and Water Assessment Tool (SWAT) with Hydrological Response Units (HRUs) to achieve high spatial precision in assessing the impacts of land use change and climate variability on hydrological components in a suburban catchment in western Japan. Over the 30-year study period (1980s–2000s), land use change was the main driver of hydrological variability, whereas climate change played a minor role. Increased surface runoff, along with decrease in groundwater recharge, evapotranspiration, and baseflow, resulted in an overall reduction in water yield, with a 34.9% decrease in groundwater recharge attributed to the transformation of paddy fields into residential areas. Sustainable WRM practices, including water conservation, recharge zone protection, and green infrastructure, are recommended to balance urban development with water sustainability. These findings offer valuable insights into the strategies for managing water resources in rapidly urbanizing regions worldwide, emphasizing the need for an integrated WRM system that considers both land use and climate change impacts.

Surface energy balance-based surface urban heat island decomposition at high resolution

Urban heat island (UHI) is among the most pronounced human impacts on Earth. To formulate locally adapted mitigation strategies, a comprehensive understanding of the influencing mechanisms of UHI at high resolution is imperative. Based on surface energy balance, we attributed surface UHI (SUHI) into five biophysical terms (surface radiation, anthropogenic heat, convection, evapotranspiration and heat storage term) using Sentinel-2 and Landsat-8 images in Beijing. The simulated SUHI intensity, derived by combining all five contribution terms, exhibited a good consistency but a higher spatial resolution, than SUHI intensity extracted from Landsat-8 land surface temperature product. SUHI intensity tended to decrease from the old city to outsides, attributed to the decrease of evapotranspiration, solar radiation and anthropogenic heat term. The convection and heat storage term play a positive role in reducing SUHI. Among urban morphological blocks, low-rise and high-density blocks had the strongest SUHI, with the evapotranspiration term contributing the most. The results highlighted the capacity of the urban surface to evaporate water in affecting Beijing SUHI. The proposed method provides one useful tool to analyze the drivers of SUHI from the aspect of heat formation, which can be potentially applied worldwide for large-scale comparisons of how urbanization affects UHI.

Forest Dieback Alters Nutrient Pathways in a Temperate Headwater Catchment

ABSTRACTForested headwater catchments ensure good water quality for downstream ecosystems and human consumption. Climate change and the exacerbating likelihood of extreme events elevate the risk of severe forest dieback. However, the effects of forest dieback on the quantity and quality of stream water are not fully understood. Here, we analyse high‐frequency observations of discharge, electrical conductivity (EC), dissolved organic carbon (DOC) and nitrate (NO3N) obtained before, during and after a drought‐induced forest dieback in a headwater stream in the German Harz Mountains. We focus on the characteristics of concentration‐discharge (C‐Q) relationships at the scale of runoff events to assess the effects of forest dieback on the sources, mobilisation and pathways of EC, DOC and NO3N. When comparing pre‐ and post‐dieback conditions, we found a significant increase in runoff efficiency and a doubling of DOC loads exported from the catchment, while DOC concentrations increased only moderately and their C‐Q patterns did not change. EC exhibit no changes in concentrations but a steepening of C‐Q dilution patterns. We explain these findings with a dieback‐induced decrease in evapotranspiration, which leads to more intensive drainage of the upper organic soil layers in the riparian zone. In contrast, we observed a strong increase in NO3N concentrations and fluxes by a factor of ~5, while C‐Q patterns at the event scale changed from enrichment to dilution. We argue that the dieback led to an excess of NO3N on the hillslopes that connect to the stream via surficial flowpaths. In this way, NO3N bypasses the riparian zone, reducing the catchment's efficiency in attenuating this nutrient. Our study emphasises the pivotal role of riparian zones in mediating water quality in headwater streams. Different configurations of the riparian zone and its connection to the hillslopes and the stream network may be a missing piece in explaining differences in water quality responses of catchments to forest dieback.

Vegetation Restoration Increases the Drought Risk on the Loess Plateau.

The extensive implementation of the 'Grain for Green' project over the Loess Plateau has improved environmental quality. However, it has resulted in a greater consumption of soil water, and its overall hydrological effects remain highly controversial. Our study utilized a coupled land-atmosphere model to evaluate the effects of vegetation changes resulting from revegetation or reclamation on the hydrology of the Loess Plateau. Revegetation was found to stimulate an increase in precipitation, evapotranspiration, and atmospheric water content. However, the increase in precipitation was insufficient to compensate for soil water loss driven by intensified evapotranspiration, resulting in a decrease in both runoff and soil water content. In contrast to revegetation, reclamation would reduce precipitation, although the reduction was less than the decrease in evapotranspiration. This could lead to an increase in both runoff and soil water content. The results provide an important scientific basis for the hydrological effects of vegetation changes on the Loess Plateau, which is particularly important for guiding current and future revegetation activities toward sustainable ecosystem development and water resources management.

Impacts of urban expansion on air temperature and humidity during 2022 mega-heatwave over the Yangtze River Delta, China

The Yangtze River Delta (YRD) experienced record-breaking heat in the summer of 2022. However, the urban heat pattern and the role of urban expansion over the last two decades in this hot summer have not been explored. Using the advanced mesoscale Weather Research and Forecasting (WRF) model, we reproduced the fine spatial features and investigated the urban heat island (UHI) and dry island (UDI) effects during the two identified heatwaves in 2022. We further replace the current (2020) land use with the historical (2001) land use in WRF to evaluate the impacts of urban expansion from 2001 to 2020 on air temperature and moisture. Our finding revealed that the conversion of land use resulted in near-surface warming and drying, with pronounced diurnal variations, especially during the July heatwave. The analysis of surface energy balance demonstrated that the substantial decrease in evapotranspiration (ET) was the primary driver of daytime warming, elevating temperatures by 7 °C (July heatwave) and 2 °C (August heatwave). This ET reduction also led to the strong daytime coupling of warming and drying effects over new urban areas. At night, the release of stored heat resulted in the temperature increase of 2 °C (1 °C) during July (August) heatwave, highlighting the nighttime as a critical period for heightened thermal risk. Additionally, urban expansion at the periphery contributed modestly to the warming of urban cores, exacerbating conditions in an already hot environment. This study enhances understanding of the impacts of urban expansion on air temperature and humidity during extreme heatwaves, thereby supporting targeted adaptation and mitigation for extreme events within large cities.

Spatiotemporal interactions between soil moisture and water availability across the Yellow River Basin, China

Study regionYellow River Basin (YRB) in China. Study focusThis study attempts to shed new light on regional land-atmosphere coupling and relevant impacts on basin-scale water resource management. Specifically, the objectives are to investigate the driving factors and physical mechanisms of SM changes via land-atmosphere coupling. New hydrological insights for the regionEcological conservation and the high-quality development in the YRB stand as a pivotal national strategy. The equilibrium of water availability (PME，precipitation minus evapotranspiration) poses a significant challenge to the sustainability of the basin's ecosystem. Unfortunately, a comprehensive examination of the regional response, spatiotemporal heterogeneity, and the mechanism governing soil moisture (SM) in response to PME remains limited. An enhanced multiple linear regression method is implemented to quantify monthly sensitivity coefficients, revealing a notable correlation: reduced SM in arid regions correlates with heightened PME. This decline in SM triggers a decrease in evapotranspiration, attenuates the cooling effect of evapotranspiration, and amplifies temperature disparities. Consequently, this process results in an intensified boundary layer and tropospheric ascending motion, thereby increasing water vapor transport. This feedback loop is most pronounced during drought conditions, particularly in summer arid areas (sensitivity coefficient =-0.27). The findings underscore the intricate interplay between land and atmosphere, elucidating the discernible impact of climate change on water resources at a sub-basin scale.

Enhanced decreases in rice evapotranspiration in response to elevated atmospheric carbon dioxide under warmer environments.

A short period of exposure to elevated CO2 is known to decrease evapotranspiration via stomatal closure. Based on theoretical evaluation of a canopy transpiration model, we hypothesized that this decrease in the evapotranspiration of rice under elevated CO2 was greater under higher temperature conditions due to an increased sensitivity of transpiration to changes in CO2 induced by the greater vapour pressure deficit. In a temperature gradient chamber-based experiment, a 200 ppm increase in CO2 concentration led to 0.4 mm (-7%) and 1.5 mm (-15%) decreases in 12 h evapotranspiration under ambient temperature and high temperature (+3.7°C) conditions, respectively. Model simulations revealed that the greater vapour pressure deficit under higher temperature conditions explained the variations in the reduction of evapotranspiration observed under elevated CO2 levels between the temperature treatments. Our study suggests the utility of a simple modelling framework for mechanistic understanding of evapotranspiration and crop energy balance system under changing environmental conditions.

Evidence of field‐scale shifts in transpiration dynamics following bark beetle infestation: Stomatal conductance responses

AbstractAmplified eruptive outbreaks of bark beetles as a consequence of climate change can cause tree mortality that significantly affects terrestrial water and carbon fluxes. However, the lack of field‐scale observations of underlying physiological mechanisms currently hampers the expression of such ecosystem disturbances in predictive modelling. Based on a unique flux tower dataset from a subalpine forest located in the Rocky Mountains, mechanisms of stomatal response to an extensive bark beetle outbreak were investigated using various models and parametrizations. The datasets cover a decade, including the periods of pre‐infestation, infestation, and post‐infestation. Field measurements showed considerable decreases in evapotranspiration (ET), transpiration (T), and leaf area index (LAI) during the two‐year infestation period compared to the pre‐infestation period. Model interpretations of observed water and carbon fluxes indicated that the overall reductions in T were not solely due to decreased LAI, but also to changes in physiological behaviours. The summer season's canopy‐scale stomatal conductance was significantly reduced during the infestation period, from 0.0018 to 0.0011 m s−1. One primary reason for the observed variations is likely that the bark beetle infestation hampers the water transport in the xylem. The damage of xylem has important implications for water use efficiency (WUE), which also significantly influences the parameterization of stomatal conductance. When using stomatal conductance models to forecast ecosystem dynamics, it is crucial to recalibrate the model's parameters to ensure the accurate depiction of stomatal dynamics during various infestation periods. The neglect of the temporal variability of canopy‐scale stomatal conductance under ecosystem disturbances (e.g., bark beetle infestations) in current earth system models, therefore, requires specific attention in assessments of large‐scale water and carbon balances.

Assessment of the effects of climate change on water balance components in the upper Erer subbasin, Ethiopia

Eastern Ethiopia watersheds are located in transition zone from Arid to semi-humid climate and in expanding to westwards the west annual rainfall is highly declining. This paper explains future hydrological response impacts under changing climate using ensemble average of the CORDEX RCMs for historical (1979–2014) and future (2024–2070) periods. The result revels the monthly average temperature varies (0.04–6.25°C) for RCP-4.5, while it varies (0.03–6.59°oC) for RCP-8.5. The monthly average rainfall to be decline by 90.71 mm and rise by 211. 22 mm for RCP-4.5, while it is going to decline by 84.97 mm and rise by 235.62 mm for RCP-8.5. The adjusted SWAT model was used to detect the changes of projected hydrological response from reference period. Balance components of the baseline period was compared to future period. The result shows the change in decrease of annual mean surface flow (4.98 %–5.63 %), groundwater flow (5.63 %–6.68 %), evapotranspiration (2.45 %–2.57 %) and water yield (5.54 %–5.21 %) to be expected from RCP-4.5 to RCP-8.5. The findings of this paper provide valuable assistance to water resource planners by enhancing their comprehension of change in climate effects at local level.

Revisiting the hydrological legacy of revegetation on China's Loess Plateau using Eagleson's ecohydrological perspective

Revegetation has resulted in a trend of increasing vegetation greenness on the Chinese Loess Plateau. However, it remains unclear whether the regional vegetation coverage exceeds hydroclimatic limitations in the context of revegetation, and the hydrological effects of greening are controversial. Eagleson's optimality hypothesis can explain some of the hydrological effects on the Loess Plateau. Here, building on previous research, the geospatial vegetation states were estimated for pre- and post-revegetation periods on the Loess Plateau from 1982 to 2015 using Eagleson's ecological optimality theory. Additionally, a drought composite analysis approach was utilized to investigate the hydrological effects related to drought (including sensitivity and partitioning) under various vegetation states. It was found that revegetation increased the proportion of catchments in the equilibrium state and decreased the proportion in the disturbed state, owing to a wetter climate compared with the pre-revegetation period. Root-zone soil drought, driven by precipitation (P) deficit, asymmetrically triggered hydrological effects for both the pre- and post-revegetation periods, with reduced runoff (Q) for both periods and a decrease in evapotranspiration (ET) during the pre-revegetation period but an increase in ET during the post-revegetation period. Moreover, catchments in an equilibrium state exhibited lower sensitivity between ET and P, and more stable partitioning of ET with regards to P, compared with those in a disturbed state. These results underscore the theoretical framework that an equilibrium state is crucial for maintaining ecosystem ET. Our results highlight the necessity of considering the hydrologic regulation of vegetation states when assessing the hydrological effects of vegetation change.

Constraints on hopanes and brGDGTs as pH proxies in peat

pH is one of the major parameters governing peat functioning, and pH variations in modern peatlands affect carbon and methane production and consumption. Paleo-pH reconstructions have been limited thus far, but they could help to better understand peat functioning in variety of settings and also serve as an indirect proxy for climatic and environmental variations such as precipitation. Bacterial hopanes and branched Glycerol Dialkyl Glycerol Tetraethers (brGDGTs) were investigated in a 10-ka peat record from North-East Cameroon (NGaoundaba, Western Central Africa). Recently developed pH proxies using the hopane ββ/(αβ + ββ) ratio and the brGDGT cyclization ratio (CBTpeat) were applied and compared with previously published bulk organic data from the same core. Different hypotheses are usually proposed to explain the high abundance of the “thermally mature” C31 αβ hopane in peats: acid-catalyzed isomerization of ββ to αβ isomers with or without biological mediation or the direct input of C31 αβ hopanes by bacteria. In the NGaoundaba peat deposit, the opposite variation in the ββ/(αβ + ββ) ratio of C30 and C31 hopanes and the carbon isotopic composition of C31 ββ and αβ hopanes suggest that an eventual transformation of C31 ββ to αβ isomers is not possible without biological mediation. A linear correlation between hopane- and brGDGT-reconstructed pH is observed, suggesting a similar response to environmental changes for these two independent proxies. A large increase in reconstructed pH values coincides with changes in vegetation and precipitation at the end of the African Humid Period (AHP). Lower pH values are observed during the AHP, coinciding with a period of increased precipitation and consistent with bulk organic data and slightly higher C31 ββ hopane δ13C values compared to middle and late Holocene. Bulk organic data indicate an interruption of the AHP by a drier intermission that coincides with a decrease in reconstructed pH values, probably reflecting a decrease in evapotranspiration. Variations in pH values could be interpreted in terms of preservation of peat organic matter and might reflect past changes in methane cycling in the investigated peatland. The present study reinforces the idea that reconstruction of pH in peat deposits represents a promising proxy of environmental change, enabling a better understanding of changes in peat functioning over large timescales and various locations.

Why does a decrease in cloud amount increase terrestrial evapotranspiration in a monsoon transition zone?

Terrestrial evapotranspiration plays a critical role in drought monitoring and water resource management. Changes in evapotranspiration are significantly influenced by cloud-related precipitation and radiation effects. However, the impact of cloud amount (CA) on evapotranspiration through its influence on precipitation remains uncertain, especially in the transition zone affected by the East Asian summer monsoon (EASM), which limits the understanding of the water cycle. Therefore, this study deeply explores the impact of CA on evapotranspiration and its potential physical mechanisms in Northwest China. The results show that the correlation between 31-year average evapotranspiration and CA is negative only in the semi-arid region and is positive in other climatic regions of Northwest China. This unique negative correlation is related to the change of precipitation pattern in the semi-arid region caused by the weak EASM. Smaller CA in weak monsoons results in more short-wave radiation reaching the surface, larger sensible heat, and weaker convective inhibition. Consequently, the proportion of convective clouds (CCs) increases and precipitation from these CCs enhances evapotranspiration. Less CA increases evapotranspiration and potentially exacerbates aridity in the semi-arid region of Northwest China. These results emphasize the role of cloud type in evapotranspiration. It is well known that global warming can change cloud type with more CCs. Therefore, this study sheds new light on evapotranspiration change under global warming.

Future water storage changes over the Mediterranean, Middle East, and North Africa in response to global warming and stratospheric aerosol intervention

Abstract. Water storage plays a profound role in the lives of people across the Middle East and North Africa (MENA) as it is the most water-stressed region worldwide. The lands around the Caspian and Mediterranean seas are simulated to be very sensitive to future climate warming. Available water capacity depends on hydroclimate variables such as temperature and precipitation that will depend on socioeconomic pathways and changes in climate. This work explores changes in both the mean and extreme terrestrial water storage (TWS) under an unmitigated greenhouse gas (GHG) scenario (SSP5-8.5) and stratospheric aerosol intervention (SAI) designed to offset GHG-induced warming above 1.5 ∘C and compares both with historical period simulations. Both mean TWS and extreme TWS are projected to significantly decrease under SSP5-8.5 over the domain, except for the Arabian Peninsula, particularly in the wetter lands around the Caspian and Mediterranean seas. Relative to global warming, SAI partially ameliorates the decreased mean TWS in the wet regions, while it has no significant effect on the increased TWS in drier lands. In the entire domain studied, the mean TWS is larger under SAI than pure GHG forcing, mainly due to the significant cooling and, in turn, a substantial decrease in evapotranspiration under SAI relative to SSP5-8.5. Changes in extreme water storage excursions under global warming are reduced by SAI. Extreme TWS under both future climate scenarios is larger than throughout the historical period across Iran, Iraq, and the Arabian Peninsula, but the response of the more continental eastern North Africa hyper-arid climate is different from the neighboring dry lands. In the latter case, we note a reduction in the mean TWS trend under both GHG and SAI scenarios, with extreme TWS values also showing a decline compared to historical conditions.

Effects of land use/land cover change on hydrological responses of a watershed in the Central Rift Valley of Ethiopia

Abstract The purpose of this study is to evaluate the impacts of LULC changes on the hydrological components of a watershed using multivariate statistics, and hydrological modeling approaches. The study analyzed the LULC distributions, and changes corresponding to the years 2000, 2010, and 2020. The SWAT model was then applied to assess the hydrological impacts of these changes in the studied watershed. Finally, changes in LULC types were correlated with the water balance components using a Partial Least Squares Regression (PLSR) method. The result showed a continuous expansion of barren, built-up, and cropland areas, while forests, shrubs, and grassland decreased by about 67.12, 41, and 36.88%, respectively. The modeling result showed that surface runoff, water yield, and evapotranspiration decreased by 16.1, 2.9, and 9.3%, respectively. In contrast, base flow, soil water storage, and lateral flow of the watershed increased by up to 19.1%, 55.9%, and 150.4%, respectively, due to LULC changes. The PLSR model identified the cropland, forest, and shrub LULC types as the major factors affecting the water resource components. The study results provide useful information for policymakers and planners in the implementation of sustainable water resource planning and management in the context of environmental change.

Influence of human-induced land use change on hydrological processes in semi-humid and semi-arid region: A case in the Fenhe River Basin

Study Region: The Fenhe River Basin (FRB) is an important tributary of the Yellow River Basin and locate in a semi-arid and semi-humid region in northwest China. Over the past four decades, significant anthropogenic activities have resulted in significant land use and land cover change (LUCC) that have markedly impacted hydrological processes in FRB.Study Focus: To analyze land use changes under multiple historical land use scenarios from LU1980 to LU2015, and to establish the Water and Energy transfer Processes in Large basins (WEP-L) model to quantify the influences of LUCC on hydrological processes, particularly on baseflow.New hydrological insights for the region: LUCC from LU1980 to LU2015 resulted in a 14.5 % increase in water yield (WY) and a 2.7 % and 1.4 % decrease in baseflow (BF) and evapotranspiration, respectively. Urbanization led to a significant increase in WY and a reduction in BF, exhibiting strong spatial gradients between urban centers and their peripheries. Increases in forestland decreased WY, while ET showed no significant change due to large absolute values and moisture limitations. Furthermore, the response of BF to forest cover exhibits spatial variation. A negative correlation between BF and forest cover was evident in upstream, whereas no significant correlation was observed in downstream. The findings of this study offer valuable insights for land use planning and sustainable watershed management in semi-arid semi-humid region.

The modelling of tomato crop response to the climate change with different irrigation schemes

The inequality between available water supplies and growing water demand from diverse sectors, as well as the predicted climate changes are putting significant pressures on Egypt’s food security. There is a nation-wide demand for new scientifically proven on-farm practices to boost water productivity of major food crops. The objective of this study was to explore the use of various deficit irrigation schemes to improve water productivity ( WP) of tomato cultivated in Egypt under distinct climate change scenarios, RCP4.5 and RCP8.5, in three time-steps of the reference period (2006– 2016), 2030s, and 2050s. The AquaCrop model was used to simulate the influence of climate change on the tomato crop, as well as two deficit irrigation application schemes for the full growing season and the regulated application for the initial and maturity crop stages. With the same irrigation method, the predicted WP increased in a general pattern across all climate change scenarios. The combination of irrigation schedule with the 80% deficit irrigation can enhance WP near the optimum level (approximately 2.2 kg∙m–3), especially during early and mature stages of the crop, saving up to 16% of water. The results showed that the expected temperature rise by 2050s would reduce the crop growth cycle by 3– 11 days for all irrigation treatments, resulting in a 1–6% decrease in crop evapotranspiration ( ETc) and affecting the dry tomato yield with different patterns of increase and decrease due to climate change.

Spatiotemporal changes and driving factors of reference evapotranspiration and crop evapotranspiration for cotton production in China from 1960 to 2019

Introduction: Understanding and tracking changes in crop water requirements is crucial for effective irrigation, water planning, and future decisions. Determining the reference evapotranspiration (ETO) and crop evapotranspiration (ETC) of China cotton is essential for water resource management.Methods: This study analyzed the spatiotemporal changes in ETO and ETC at 248 standard stations in cotton production regions of China from 1960 to 2019, and the ETO and ETC of each station were quantified by using the CropWat 8.0 and non-parametric Mann-Kendall test. The impacts of climate change on ETO and ETC were evaluated by analyzing the contribution rate and sensitivity coefficient of climate change.Discussion: The results revealed distinct distributions of ETO and ETC across various growth stages and spatial scales in the cotton production regions of China. In the Huanghe Valley, the rate of decline for ETO decreased from 787.23 mm to 769.84 mm, while in the Yangtze Valley cotton region, it decreased from 749.19 mm to 735.01 mm. Similarly, in the Northwest inland cotton regions, the rate of decline for ETO reduced from 991.19 mm to 982.70 mm. As for ETC, the rate of decline decreased from 677.62 mm to 654.33 mm in the Huanghe Valley, from 653.02 mm to 625.50 mm in the Yangtze Valley, and from 916.25 mm to 886.74 mm in the Northwest inland cotton regions. ETO was highly sensitive to maximum air temperature (Tmax), followed by relative humidity (RH), sunshine duration (SD), wind speed at 2 m height (WS), and minimum air temperature (Tmin). WS was the most influential climate variable associated with ETO change, followed by Tmax, SD, RH, and Tmin. Significant declines in WS and SD were indicated in the decrease in ETO in the Huanghe Valley and Yangtze Valley cotton regions. WS showed a significant decrease in ETO in the northwestern inland cotton region. However, decreased RH and increased temperature commonly reversed the trend of ETO from 2000 to 2019, and the northwestern inland cotton region had the most significant upward trend. Amidst high temperatures and drought stress, the irrigation needs of cotton were rising, posing a significant threat to both cotton production and water resources.

Contribution rates of climate variables to the change in reference evapotranspiration in northern Thailand from 1991 to 2019

AbstractHigher temperatures are expected to increase evaporation and reference evapotranspiration (ETo). However, a decrease in ETo has been observed in numerous regions worldwide. ETo is controlled by climate variables besides temperature; wind speed, air humidity, and radiation, which vary according to regional climatic conditions, may also be drivers. Few studies have quantified the sensitivity and contributions of climate variables to ETo change in tropical savanna climates. Long‐term (1991–2019) climate time series in northern Thailand was used to assess the trends in ETo and determine which climate variables exert the strongest control on ETo, during the rainy and non‐rainy season, as an example for the tropical savanna climate. Furthermore, this study used the proportion of contribution and sum of positive (negative) contribution and classified the results into five groups to analyse the change in ETo. This helped to determine the climate variable with the highest contribution. From this study, ETo was the most sensitive to the maximum temperature, followed by sunshine duration in both rainy and non‐rainy seasons. Furthermore, the variables that contributed the most to the long‐term change in ETo were sunshine duration and wind speed. The increase (decrease) in sunshine duration and wind speed contributed to the increase (decrease) in ETo in this area. Understanding ETo changes and determining whether ETo has a strong tendency to change are the basis for water resource planning and management in the region.