Trends In Potential Evapotranspiration Research Articles

Implications of uncontrolled water withdrawal and climate change on the water supply and demand gap in the Lake Tana sub-basin

This study evaluates the implications of multiple climatic and non-climatic factors on the water resource of the Lake Tana sub-basin, Ethiopia. The study focuses on three drivers: land use change, irrigation expansion, and climate change (CC), and their impact on the current and future water availability across the sub-basin. The study uses a random forest (RF) machine learning classifier in the Google Earth Engine (GEE) platform to detect land use land cover change (LULC) and for mapping the actual irrigated-area. The Climate Hazards Group InfraRed Precipitation and temperature data were used with station data to evaluate the implication of CC on the water availability of Lake Tana sub-basin. The supply-demand relationship was done for Gumara catchment within Tana sub-basin as an experimental site. Based on the LULC analysis, plantation, cropland, bare lands, built-up, and wetland showed an increasing trend, while forest, bush land, and grassland showed a decreasing trend. The increasing rate of crop land in the expense of natural forest, shrub land, grassland may cause runoff/flood, high soil erosion, and lower rate of groundwater recharge. The actual irrigated area mapping analysis shows a widespread irrigation near the Lake Shore and upstream parts of the sub-basin. The climate change analysis shows an increasing trend of potential evapotranspiration during the irrigation period attributed to increase in maximum temperature. The rising of potential evapotranspiration during the irrigation period creates a water shortage in the study and future periods as the crops need more water for growth. The estimated stream flow for Gumara catchment shows an increasing trend for the future period. The supply-demand relationship of the catchment shows an uneven distribution of water in the Gumara catchment both in the current and future periods resulting in unbalance abstraction of irrigation water. The study reveals that climate change, land cover change, and uncontrolled expansion of irrigated land area are likely to increase the unmet demand by increasing irrigation water demand, and this can cause conflict of interest between the users. Therefore, sustainable water resource management practices such as land management practices, and adaptive management of irrigation should be applicable to prevent or reduce the occurrence of conflict over the water demand.

Temporal trends of potential evapotranspiration in the state of Acre

A evapotranspiração é considerada um importante parâmetro agrometeorológico, utilizado em estudos relacionados à demanda hídrica. Esse trabalho teve como objetivo, caracterizar e avaliar as tendências das séries históricas climatológicas de evapotranspiração potencial do estado do Acre. Utilizou-se os dados de 43 anos de 1971 a 2020, das estações de Cruzeiro do Sul, Tarauacá e Rio Branco municipios do Acre. As séries de ETo foram ajustadas em modelos de regressão linear, para identificar sua tendência na série temporal, para verificar as estacionariedade se utilizou o teste não paramétrico de Spearman’s Rho (SR). A evapotranspiração média diária dos três municipios se comportaram de maneira similar ao longo do ano, na ordem de 4 a 5,6 mm diário. As maiores demandas de ETo mensal ocorreram em outubro, 156,4 (Cruzeiro do Sul) 174,5 (Rio Branco) e 158,9mm (Tarauacá). O teste estatístico de Spearman, mostrou tendência monotônica decrescente nos dados de evapotranspiração diária e máximas

An Assessment of Trends of Potential Evapotranspiration at Multiple Timescales and Locations in Sicily from 2002 to 2022

Climate change and the related temperature rise can cause an increase in evapotranspiration. Thus, the assessment of potential evapotranspiration (PET) trends is important to identify possible ongoing signals of climate change, in order to develop adaptation measures for water resource management and improve irrigation efficiency. In this study, we capitalize on the data available from a network of 46 complete meteorological stations in Sicily that cover a period of about 21 years (2002–2022) to estimate PET by the Food and Agriculture Organization (FAO) using the Penman–Monteith method at the daily time scale in Sicily (southern Italy). We then analyse the trends of PET and assess their significance by Sen’s Slope and the Mann–Kendall test at multiple temporal scales (monthly, seasonal, and annual). Most of the locations do not show significant trends. For instance, at the annual timescale, only five locations have a significantly increasing trend. However, there are many locations where the monthly trend is statistically significant. The number of locations where monthly trend is significant is maximum for August, where 18 out of these 46 stations have an increasing trend. In contrast, in March, there are no locations with a significant trend. The location with the highest increasing trend of PET indicates trend slopes of 1.73, 3.42, and 10.68 mm/year at monthly (August), seasonal (summer), and annual timescales, respectively. In contrast, decreasing PET trends are present only at the monthly and seasonal scales, with a maximum of, respectively, −1.82 (July) and −3.28 (summer) mm/year. Overall, the findings of this study are useful for climate change adaptation strategies to be pursued in the region.

Inter-annual variations of vegetation dynamics to climate change in Ordos, Inner Mongolia, China

To reveal the characteristics of climate change and the controlling factors for vegetation dynamics in the Ordos, Inner Mongolia, China, 34 years (1982-2015) of regional climate variables and vegetation dynamics were investigated. The results show that: Annual mean air temperature (TMP) significantly increased with a linear slope of 0.473°C/10yr. Annual precipitation (PRE) had a non-significant positive trend nearly 5 times lower than the trend of potential evapotranspiration (PET). The average Normalized Difference Vegetation Index (NDVI) computed for the region was found to show a significant positive trend (6.131×10-4/yr). However, all climate variables displayed non-significant correlations with NDVI at annual scale. The reduction of desert and the increase of grassland over the past decades were accountable for the increased NDVI. Principal components analysis revealed that the regional climate change can be characterized as changes in temperature, humidity and the availability of radiant energy. Based on principal components regression coefficients, NDVI was mostly sensitive to humidity component, followed by growing season warmth (WMI). Spatially, 93.1% of the pixels displayed positive trend and 61.8% of the pixels displayed significant change over the past decades. Both principal regression analysis and partial correlation analysis revealed that NDVI in eastern part of Ordos was sensitive to TMP, whereas, NDVI in southern and western areas of Ordos displayed the high sensitivity to combined effects of PRE and cloud coverage (CLD). Partial correlation analyses also revealed that TMX was a surrogate for aridity, TMN was a representative of humidity, and temperature variations below the threshold of 5°C (CDI) were less important than WMI. We conclude that regional climate change can be characterized by warming and increased aridity. The significant positive trend of regional NDVI and the non-significant correlations between NDVI and climate variables at annual scale suggests the hidden role of the human activities.

Spatiotemporal heterogeneity and attributions of streamflow and baseflow changes across the headstreams of the Tarim River Basin, Northwest China

Understanding spatiotemporal heterogeneity of streamflow and baseflow and revealing their changes contributed by climatic factors and human activities in the alpine region of inland river basin are critical for regional water management. However, the hydrology heterogeneity in the alpine region has remained unclear, which limits the scientific understanding of the interaction mechanism between the hydrological cycle and terrain, and further constrains the effective utilization of regional water resources in the water-shortage areas. In this study, the hydrological process and regimes for headstreams of Tarim River Basin (HTRB) during 1985–2011 were simulated by the Soil and Water Assessment Tool. We systematically characterized the spatial and temporal patterns of streamflow and baseflow through geostatistical and trend analyses, and subsequently investigated their heterogeneity responses to climate change and human activities at different sub-basins and elevation zones. Results show that the spatial distributions of streamflow and baseflow are highly related to terrain and river direction. Increased trends in precipitation enhanced with altitude, whereas decreased trends in potential evapotranspiration (PET) weakened with altitude, meanwhile, increased trends in streamflow and baseflow of HTRB are most pronounced in mid-altitude areas during 1985–2011. The climate elasticities of streamflow and baseflow are highly reliant on the altitudinal gradient. Increases in streamflow and baseflow in high-lying areas are more sensitive to precipitation variation, while they are more sensitive to PET change in low-lying areas. The magnitude and change rate with altitude bands of the precipitation has greater effects on streamflow and baseflow variations than those of PET. Furthermore, the percentage of sub-basins where climate changes dominate streamflow variation in each elevation band increases with height but decreases abruptly at elevations above 5000 m. The percentage of sub-basins where climate changes dominate baseflow variations gradually decreases in elevation bands above 3000 m. Our results indicate that climate change rather than human activities dominants the variation in streamflow and baseflow in most sub-basins and elevation bands.

Spatial distribution of the trends in potential evapotranspiration and its influencing climatic factors in Iraq

Understanding the spatial variations in potential evapotranspiration (PET) and its influencing climatic variables is essential for sustainable agriculture and water resources management. However, little published research has investigated the alternation of PET due to climate change in the case of Iraq. The objective of the present study was to analyze the spatial trends in annual and seasonal PET in Iraq. Accordingly, the latest global ERA5-Land dataset of the European Centre for Medium-Range Weather Forecasts for 1981–2021 was employed. The PET was estimated using the FAO-Penman–Monteith method. The modified Mann–Kendall statistical test was applied to evaluate the significance of the trends in PET, which can separate unidirectional trends caused by climate change from the natural variability of climate. The attained results indicate that: (1) Over the past four decades, the annual and seasonal PET witnessed a significant increasing trend in almost all of Iraq, except for the alluvial plain in the eastern and southeastern parts. (2) The increasing trend in PET confirmed the patterns of the trend significance, with the highest increase of 0.28–0.65 mm/decade in southwest Iraq. (3) Summer had the highest upward trend of 0.35–0.65 mm/decade, followed by spring, autumn, and winter. (4) The air temperature was the predominant driving factor of rising PET, showing a positive correlation ranging from 0.77 to 0.88 and a contribution of 26 to 94%, mainly in the south, central, and northwest regions. The reverse contribution of wind speed and surface pressure to PET, particularly in the southeast and southwest, remains offset by the influence of air temperature and net solar radiation. Overall, the PET has risen drastically due to global climate change, indicating the potential for increased atmospheric water demand in the region.

Effects of various driving factors on potential evapotranspiration trends over the main grain-production area of China while accounting for vegetation dynamics

Potential evapotranspiration (PET) is an important consideration in the study of agricultural water management and climate change. Although a number of studies have already explored the effects of climate factors on changes in PET, the quantitative effects of various driving factors needs to be further studied, as most studies have not accounted for vegetation dynamics by characterizing changes in leaf area and stomatal resistance. Here, we used the Shuttleworth–Wallace (S-W) model based on climate, vegetation, and energy factors to quantitatively analyze the effects of various driving factors at multiple spatial scales over the main grain-producing area of China (MGPAC) from 1982 to 2016. The S-W model had a satisfactory performance because S-W based Standardized Precipitation Evapotranspiration Index (SPEI) had a higher determination coefficient (0.33 VS. 0.26) than Penman-Monteith based SPEI for soil moisture standardized anomaly over MGPAC. MGPAC had a mean annual PET of 926.20 mm and showed a marked increase of 3.844 mm yr−1 during the study period. Evergreen broadleaf forest and grassland had the highest and lowest annual average PET values (1535.65 mm and 633.67 mm), respectively. Although climate factors could directly explain 79.42% of the PET increase over the MGPAC, vegetation dynamics were also identified to be a non-negligible factor explaining the increase in PET (directly explaining 18.91% of the PET trend). Specifically, vegetation dynamics contributed to explaining 67.30% and 48.91% of the PET trend in the Huang-Huai-Hai and Loess Plateau regions. These findings are closely related to changes in the temperature and leaf area index (LAI), which notably increased by 0.04 °C yr−1 (P < 0.01) and 0.0038 m2 m−2 yr−1 (P < 0.01), respectively. Regarding single drivers, variation in temperature and LAI significantly increased the PET by 3.582 mm yr−1 (P < 0.01) and 1.127 mm yr−1 (P < 0.01) across the MGPAC, respectively. This study can provide a new insight to improve agricultural production and management in China.

Trend analysis of potential evapotranspiration in peninsular Malaysia

Potential evapotranspiration (PET) plays a prominent part in hydrologic water cycle which is responsible for the water from the Earth’s surface travel back to the atmosphere. Climate change brought contradictory changes in PET which led to the inconsistency of PET trend globally. Hence, the present study carried out the trend of potential evapotranspiration using meteorological data from 7 different meteorological stations located in Peninsular Malaysia. Mann-Kendall and Sen’s slope estimator tests were employed in this article for PET trend analysis. Bayan Lepas experienced significant increasing trend for both FAO 56-Penman Monteith and Mahringer models during Southwest monsoon. For Mahringer model, significant increasing trends were observed at Ipoh and Pulau Langkawi stations during Northeast and inter monsoon 2, respectively. There were only a few meteorological stations experienced decreasing trend throughout the period. Although the combinations of increasing and decreasing PET trends were observed, it can be highlighted that majority of the PET time series exhibited increasing trend in Peninsular Malaysia.

Study on the Variation Trend of Potential Evapotranspiration in the Three-River Headwaters Region in China Over the Past 20 years

To study the variation trend of potential evapotranspiration (PET) in the Three-River Headwaters (TRH) region of the Qinghai-Tibet Plateau in China, we use 2-m temperature and surface pressure observation data from 14 meteorological weather stations in the TRH region, and the surface PET is calculated by the Penman-Monteith formula. The global land surface data assimilation system (GLDAS) from 2000 to 2018 were used to compare and verify the accuracy and applicability of the calculated PET in the THR region. The results show that in the past 20 years, the PET of 14 weather stations in the TRH region has shown an increasing trend, with annual averaged growth rate of 1.4±1.2 mm·a-1, and the spatial distribution of the annual variation rate of PET has obvious difference. PET is higher in the eastern area of TRH region, and lower in the western area. The drought in this area increased from southeast to northwest, which was consistent with the spatial distribution of precipitation. The aridity index K has fluctuated and increased begore 2015, but there was a sudden change in 2018, and the aridity index K in the TRH began to decline after 2018, and the climate changed from dry to wet.

Regional differentiation in climate change induced drought trends in the Netherlands

The summer of 2018 was characterized by high temperatures and low precipitation values in the Netherlands. The drought negatively impacted different sectors, resulting in an estimated damage of 450 to 2080 million Euros. Strong regional differences were observed in the precipitation shortfall across the country, with highest deficits in the southern and eastern regions. This raised two questions: (i) have increasing global temperatures contributed to changes in meteorological and agricultural droughts as severe or worse as in 2018? And (ii) are trends in these types of droughts different for coastal and inland regions? In this paper we show that there is no trend in summer drought (Apr-Sep) near the coast. However, a trend in agricultural drought is observed for the inland region where water supply is mainly dependent on local precipitation. This trend is driven by strong trends in temperature and global radiation rather than a trend in precipitation, resulting in an overall trend in potential evapotranspiration. Climate model analyses confirm that this trend in agricultural drought can at least in part be attributed to global climate change.

Investigating the impacts of climate change and human activities on hydrological drought using non-stationary approaches

Climate variation and human activities are two prime drivers influencing the hydrological cycle and the stationarity of hydrologic systems. Hydrological drought is caused by significant negative variation from normal hydrologic conditions, influenced by climate variation and human activities. Traditional methods employed for quantifying the impacts of climate change and human activities are based on the assumption of stationarity, which is no longer valid under the current changing environment. In this study, a heuristic segmentation method was used to identify the change point in streamflow time series by considering its non-linearity, and trend analysis was performed on hydro-meteorological variables before and after the change point. A non-stationary Standardized Runoff Index (SRINS) was constructed using Generalized Additive Models in Location, Scale, and Shape (GAMLSS). Finally, the influences of climate change and human activities on hydrological drought were quantified at various time scales. The results reveal change points in the streamflow time series after the 1990s. Significant decreases in precipitation and streamflow were also observed after the change points in all study watersheds, whereas the trend of potential evapotranspiration increased at a higher rate after the change points. The impact of climate change on the seasonal (three-month) time scale was greater in the winter and spring seasons, whereas the impact of human activities was significantly higher in the summer and autumn seasons. The influence of climate change on hydrological drought was dominant at longer (6- and 12-month) time scales, whereas human activities accounted for 25.6% and 20% of the changes in watersheds #1018 and #1019 on the 12-month time scale, respectively.

Divergence of potential evapotranspiration trends over Pakistan during 1967–2016

Knowledge of the variability and the changes in potential evapotranspiration (PET) is imperative for agriculture and water resource planning and management. There is a growing concern on alteration of PET because of global climate change. The spatial patterns of the changes in PET for annual and two key cropping seasons of Pakistan namely, Kharif and Rabi, are investigated in the present study for the period 1967–2016. The gauge-based gridded PET data of Climatic Research Unit (CRU) is used for this purpose. The rate of changes in PET over various CRU grid cells of Pakistan is assessed with Sen’s Slope estimator, and the significance of the changes is evaluated using a modified version of Mann-Kendall trend test that has the ability to separate the natural variability of PET from unidirectional trend due to global climate change. Besides, trends in annual and seasonal PET for different 30 years with an interval of 10 years are estimated to assess the variations of the trends with time. The results show higher PET in the southern coastal region and lower PET in the northern mountainous regions. The spatial characteristics of annual PET trend showed a significant decrease (0.74 to 1.65 mm/year) in the south and an increase (1.04 to 1.59 mm/year) in the east during 1967–2016. Among the two crop growing seasons, the PET is found to increase (0.51 to 0.66 mm/year) in a small area in the southeast and decrease over a large area in the center (0.87 to 1.29 mm/year) during Kharif, and only increase (0.38 to 0.85 mm/year) in the southeast during Rabi. The time-varying trends in annual and two crop growing seasons reveal that PET has increased significantly at more grid cells in recent years which indicates higher impact of climate change on PET in recent years in Pakistan.

Spatial distribution of potential evapotranspiration trends in the Inner Mongolia Autonomous Region (1971–2016)

Evapotranspiration is a critical hydrological and meteorological variable impacted by climate change. In this study, the annual potential evapotranspiration (ETo) was estimated using the Penman-Montieth method at 104 meteorological stations in Inner Mongolia, China, from 1971 to 2016. The characteristics of seasonal and annual ETo were analyzed using the least squares method and trends of ETo, and its driving meteorological factors were investigated using the Mann-Kendall test. The most influential meteorological factors driving ETo were identified using stepwise regression and partial correlation analysis. Results showed that the annual average (range) ETo was approximately 1050 mm (996–1165 mm), and air temperature was approximately 4 °C (3.25–6.01 °C). ETo decreased over time, suggesting that the evaporation paradox exists. The ETo in summer was the largest followed by that in spring, autumn, and winter. Relative humidity, precipitation, sunshine duration, minimum temperature, maximum temperature, and mean temperature were the main factors influencing the distribution of ETo. Trends of ETo showed a high degree of spatial variability; however, stations showing significant negative trends were generally concentrated in central Inner Mongolia. At an annual time scale, decreased wind speed and sunshine duration were the most important drivers of reduced ETo.

Quantifying the effects of climate variability, direct and indirect land use change, and human activities on runoff

Much attention has been recently focused on the direct hydrological effects (DHEs, such as runoff yield and concentration) of climate variability and land use/cover change (LUCC); however, the influence of LUCC on regional climate change and corresponding runoff change (indirect hydrological effect, IHE) have rarely been assessed quantitatively. This study employed the Mann-Kendall test and ensemble empirical mode decomposition (EEMD) method to analyze linear and nonlinear trends in potential evapotranspiration (PET), precipitation and runoff and their correlation in typical basin (Yihe River) in eastern China during 1951–2013. This study presents a framework to quantify runoff change in a detection and attribution study based on the Soil and Water Assessment Tool (SWAT) and Weather Research and Forecasting (WRF) model, and it quantifies the effects of climate variability, DHE and IHE of LUCC (1990–2010) and other human activities on runoff. The results indicate that (1) the annual PET in the Yihe River basin had an increasing trend, with a rate of 6.3 mm/decade, while the precipitation and runoff during the period 1951–2013 exhibited decreasing trends, at rates of 26.6 mm/decade (a = 0.05) and 0.16 × 108 m3/decade, respectively. (2) The runoff, PET and precipitation had similar periodic at inter-annual scales (approximately 3 years and 6 years) and inter-decadal scales (approximately 13–16 years). The contribution rate of runoff to total change from the inter-annual scale was the largest, reaching 58%, and those from the inter-decadal scale and multi-decadal scale were 6% and 11%, respectively. (3) The results also revealed that climate variability was likely to be the principle cause of an obvious decrease in runoff, and the contribution rate was as high as 90% before the 1980s while declining to 66.3% in 1990–1999. Furthermore, DHE, IHE and other human activities were responsible for 18.5%, 9.2% and 6.0%, respectively, of the runoff change in the 1990s. It is worth noting that the percentage of downward runoff accounted for 35.8% and 21.3% from DHE and IHE, respectively, during 2000–2013, which implied that human activities have gradually become the dominant factors affecting runoff change, with a contribution of 77.1%.

Identifying the effect of climate change on desertification in northern China via trend analysis of potential evapotranspiration and precipitation

Abstract Desertification and its causes are dynamic processes, which therefore make it necessary to discuss the factors that drive desertification in a specific spatiotemporal sense. Here the temporal trends in potential evapotranspiration (PE) and precipitation were selected as climate change indicators, five possible scenarios were established and relationships between desertification and the climate change indicators were examined for different time periods across northern China. The results indicate that climate change was the primary or one of drivers of desertification reversion in the northeastern and northwestern regions of China between 1975 and. 1990, whereas the desertification reversion near Mu Us Desert was influenced primarily by human intervention. Climate change triggered desertification variation across the entire region, including the Hulenbur, Otigdag, Horqin, Horbq, and Mu Us deserts and the Sounite Grassland between 1990 and 2000. However, the desertification reversion near the Otindag, Mu Us and Horqin deserts was due primarily to human intervention. Desertification reversion occurred across 54,609 km2 between 2000 and 2005 in northern China and climate change was thought to be related to majority reversion, whereas the regions that experienced desertification development and aggravation, including the areas near the Hulunbuir and Nengjiang deserts, were influenced primarily by human intervention. Broad desertification reversion occurred across the North China region between 2005 and 2010, with 45,787 km2 experiencing desertification reversion compared with only 8026 km2 experiencing desertification development and aggravation, however, climate change had a limited effect on desertification in this period. The study provides a new understanding of the causes of desertification, and an easier and more effective framework for determining the role of climate change on desertification.

Potential evapotranspiration trends in West Africa from 1906 to 2015

In this study, Climate Research Unit monthly observations were used to assess the trends of potential evapotranspiration (PET) over West Africa from 1906 to 2015. Trends and changes in PET during the 110-year study period and two reference climatological periods (1931–1960 and 1961–1990) were examined. Nonparametric trend test of the Mann–Kendall and Kolmogorov–Smirnov was employed to find the changes in trends of PET and their significance or otherwise. The contributions of some meteorological parameters (air temperatures, precipitation and cloud cover) to the observed trends in PET were also examined. Results of long-term data analysis showed mixed trends in PET in the three designated zones (Guinea, Savanna and Sahel) but notable significant increasing trends (0.165 mm per year) at p = 0.1 in Sahel. However, very sharp differences in PET were observed in both reference periods across the zones with significant decreasing trends (at p = 0.01) in PET during the first period but increasing significant trends (at p = 0.1) during the second. In spite of this pattern of variations, general reductions in PET (− 0.6% to − 1.2%) were observed, which were found to be triggered by decrease in temperature (− 0.13 to − 1.07%) and vapour pressure (− 0.02 to − 0.33%) as well as increase in cloudiness (0.01–0.05%). However, the magnitudes of changes in PET were found to be insignificant in all the zones. Maximum temperature, more than other dominant climatic parameters, contributed significantly (Theil–Sen’s regression: 23.5% ≤ β ≤ 50.3%; p = 0.01) to observed variations in PET.

Impacts of Climate Variability and Human Activities on Streamflow in the Wanquan River Basin along the East Coast of Hainan Island, Southern China

Zhao, Y.; Zou, X.; Liu, Q., and Chen, Y., 2019. Impacts of climate variability and human activities on streamflow in the Wanquan River Basin along the east coast of Hainan Island, southern China. Journal of Coastal Research, 35(2), 410–419. Coconut Creek (Florida), ISSN 0749-0208.The influences of climate change and human activities on streamflow change have received significant attention recently. In this study, daily data from seven precipitation gauges and one hydrological station are analyzed to assess the impacts of climate variability and human activities on streamflow over the past 54 years in the Wanquan River Basin (WRB). The Mann-Kendall test and Morlet wavelet method were employed to analyze the streamflow series, and the relative effects of climate change and human activities were empirically determined based on a coupled water and energy budget analysis. The results show that annual streamflow exhibited a statistically insignificant decreasing trend throughout the entire basin at an annual rate of –0.015 × 108 m3/y. Meteorological factors, such as temperature, exhibited significant increases throughout the entire basin. A decreasing trend in precipitation and an increasing trend in potential evapotranspiration were identified in the upstream region, whereas the opposite trends were observed in the downstream region. Moreover, an abrupt change in streamflow at Jiaji station occurred in 1974, exhibiting periodicities of 2 to 4 and 8 to 11 years at a 95% confidence level during the periods of 1965–80 and 1970–75, respectively. Regarding the catchment-averaged water balance, a quantitative analysis revealed that climate change resulted in an increase in streamflow from the 1970s to the 2000s throughout the entire basin, and streamflow increased by 146%, 259%, 473%, and 128% in the 1970s, the 1980s, the 1990s, and 2000–13 relative to streamflow in the 1960s. However, human activities may have decreased streamflow (–46%, –159%, –373%, and –28% in the 1970s, the 1980s, the 1990s, and 2000–13, respectively). Therefore, the relative effect of climate change was greater than that of human activities. Further research is needed to clarify the mechanisms associated with the anthropogenic effects on runoff changes.

The Trend and Influencing Factors of Potential Evapotranspiration in the Yellow River

Based on the Penman-Monteith formula recommended by the World Food and Agriculture Organization (FAO) and the Mann-Kendall trend test method, the variation trend of potential evapotranspiration in the Yellow River Basin from 1952 to 2014 is analyzed. The results showed that the potential evapotranspiration of 43.3% of the 90 meteorological stations in the Yellow River Basin showed a significant upward trend. 30% of the stations showed a significant downward trend, and 26.7% of the stations had no obvious trend of change. In all the secondary areas of water resources, the stations above Longyangxia are basically marked upward. The Longyangxia to the northern part of Lanzhou, the Longmen to the east of Sanmenxia and the Sanmenxia to Huayuankou are all significant descending sites. The change trend of the sites below the Huayuankou is not obvious. In other partition three kinds of sites are distributed. Through the correlation analysis, it is found that the increase of temperature has a great influence on the stations where the potential evapotranspiration is significantly increased, and the decrease of wind speed is the main reason for the significant decrease of potential evapotranspiration in some stations.

Tendências observadas da evapotranspiração potencial no estado de Alagoas (1961-2016) (Observed trends of potential evapotranspiration in the state of Alagoas (1961-2016))

Neste estudo utilizou-se a equação de Penman-Monteith para analisar os padrões temporais de mudanças da Evapotranspiração Potencial (ETP), para as três mesorregiões do Estado de Alagoas: sertão, agreste e litoral, com base na composição regional das series temporais desta variável a partir de estações meteorológicas localizadas em cada uma das regiões, para o período 1961-2016. Os maiores valores médios diários da ETP ocorrem no sertão, seguido do agreste e litoral, com máximos de outubro a março, e mínimos de abril a setembro. As tendências foram analisadas para trimestres representativos das estações do ano, e em nível anual, via regressão linear pelo método dos mínimos quadrados, e pelo teste seqüencial de Mann-Kendall, com o objetivo de encontrar também os pontos de ruptura que indicam o provável inicio das tendências. No verão observou-se tendências positivas e estatisticamente significativas, por regressão linear e teste de MK apenas para o agreste, assim como no outono, a partir da década de 80. O inverno foi a única estação onde as três regiões apresentaram a mesma tendência, negativa, e com significância estatística. Na primavera o sertão apresenta tendência de decréscimo dos valores diários da ETP, estatisticamente significante, enquanto no agreste a tendência é de aumento também com significância pelos dois métodos de análise, com o litoral apresenta tendência negativa mas sem significância. Em suma, anualmente, percebe-se tendências estatisticamente significantes de decréscimo da ETP média diária no sertão e de aumento no agreste, enquanto há predomínio de neutralidade no litoral. Investigar estas diferenças torna-se um objetivo futuro de pesquisa, a fim de analisar a maior contribuição para as tendências, seja pela componente radiativa ou pela componente aerodinâmica da equação de PM. A B S T R A C TIn this study, the Penman-Monteith equation was used to analyze the temporal patterns of Potential Evapotranspiration (PET) changes for the three mesoregions of the State of Alagoas: sertão, agreste and litoral, based on the regional composition of the time series of this variable from meteorological stations located in each of the regions, for the period 1961-2016. The highest daily mean values of ETP occur in the sertão, followed by the agreste and litoral, with maximum and minimum values from October to March, and April to September, respectively. The trends were analyzed for the three representative months of the seasons, and at the annual level, using linear regression using the least squares method, and by the Mann-Kendall sequential test, with the objective of also finding the break points that indicate the probable beginning trends. In the summer, positive and statistically significant trends were observed, by linear regression and MK test, only for the agreste, as well as in autumn, starting from the 80's. Winter was the only season where the three regions showed the same trend, negative, and with statistical significance. In the spring the sertão shows a tendency to decrease the daily values of the ETP, statistically significant, while in the agreste the trend is also increased with significance by the two methods of analysis, with the litoral showing a negative but not significant trend. In summary, annually, there are statistically significant decreases in the mean daily ETP in the sertão and increase in the agreste, while there is a predominance of neutrality in the littoral. Investigating these differences becomes a future research goal in order to analyze the major contribution to the trends, be it by the radiative component or the aerodynamic component of the PM equation.

Spatiotemporal variations of potential evapotranspiration and aridity index in relation to influencing factors over Southwest China during 1960–2013

This study investigated the spatial–temporal patterns and trends of potential evapotranspiration (ET0) and aridity index (AI) over Southwest China during 1960–2013 based on daily temperature, precipitation, wind speed, sunshine duration, total solar radiation, and relative humidity data from 108 meteorological stations. The Penman–Monteith model, Mann–Kendall (M–K) test, moving t test, and Morlet wavelet method were used. The results indicated that ET0 and AI across the region displayed decreasing trends, but the former was significant. After 2000, regionally average trends in ET0 and AI increased rapidly, indicating that droughts increased over Southwest China in recent years. Spatially, the changes of ET0 and AI were dissimilar and not clustered, either. Temporally, both ET0 and AI displayed obvious abrupt change points over different timescales and that of AI was during the winter monsoon period. Significant periodic variations with periods of 27, 13, and 5 years were found in ET0, but only of 13 and 5 years existed in AI. Correlation analysis revealed that the sunshine duration and wind speed were the dominant factors affecting ET0 and that AI showed strong negative correlation with precipitation. The findings of this study enhance the understanding of the relationship between climate change and drought in Southwest China, while the mechanism controlling the variation in drought requires further study.