Physics

Evidence of global dimming/brightening is mainly based upon few measurements of solar radiation. A need for more research supported and extended with the use of other climatic variables, such as sunshine duration (SD), recorded for a longer time period and successfully used as a proxy for solar radiation over the past 80 years, has already been urged. Thus, in this study, residual SD series computed after removal of the cloudiness-related variability, from daily sunshine duration and cloudiness data measured at the National Observatory of Athens are used for highlighting global dimming or brightening periods in Athens, during the period 1951-2001. Furthermore, the consistency of trends in radiation records and their implications for the hydrological cycle and especially the trends in reference evapotranspiration are examined, during the period 1951-2001. The analysis focuses on the seasonal decadal variations, determines and explains the causes of the seasonal trends from the inter-annual and decadal variability of the SD during the last half past century, with a special emphasis in detecting possible subperiods in Athens. The signs of trends in the modeled annual and seasonal reference evapotranspiration are estimated, according to the indicated global dimming/brightening periods.

In this study daily pan evaporation measurements made at 13 sites in Greece are analyzed for evidence of long-term changes. Furthermore, the similarities between magnitude and trends in observed pan evaporation (Epan) and daily estimated Penman-Monteith reference evapotranspiration (ETp-m) are examined within the same period (1979-2004), for investigating the use of ETp-m estimates as a guide for future changes in evaporation. The results show that ETp-m and Epan are well correlated on a yearly, seasonal, warm and cold period and monthly basis. Trends in Epan and ETp-m are found positive for most of the stations on a yearly or warm and cold period or during the seasons (except of autumn). When examining Epan or ETp-m trends for every month separately, increases are mainly concluded for most of the months. Median trends in yearly and seasonal ETp-m and Epan are similar in sign. ETp-m and Epan median trends for each month are in agreement (mostly positive) for 75% of the months. When all stations are considered ‘as a whole’, annual, seasonal, warm and cold period Epan and ETp-m estimates show insignificant increases, while rainfall trends are significantly negative.

During the ongoing efforts for validating or invalidating the accelerating hydrological cycle, pan evaporation has come to the light, as an index of potential evaporation, often used together with precipitation measurements to obtain the surface moisture balance, and estimate its variability. Several studies have reported declines in pan evaporation rate throughout the Northern and Southern hemisphere for various periods since the 1950s. At individual sites, both increases and decreases in pan evaporation have also been reported. In this study, temporal analysis of daily pan evaporation measurements (Epan), Penman (ETp) and Penman-Monteith (ETp-m) evapotranspiration estimates, is presented for 16 stations in Greece, during the period 1979-1999. The high correlation between Epan and ETp-m on a yearly basis and the similarity in their annual time evolutions (in almost all stations of the study) indicate that pan evaporation can be used as an index of potential evapotranspiration in Greece, as has been reported in the literature for other regions. Furthermore, trends in Epan are examined, in order to investigate if they can represent the ETp-m trends as well. The decreasing Epan trend, reported widely in the literature, is confirmed in Greece for 10 stations, during the warm season only. In contrast, almost all stations indicate negative rainfall trends on annual, warm and cold season basis. When all stations are considered ‘as a whole’, annual, and cold season pan evaporation estimates show insignificant increases while rainfall trends are negative.

Abstract: Reference evapotranspiration (ET) is an important component of the hydrological cycle and its accurate quantification is crucial for designing and scheduling irrigation systems, preparing input data to hydrological water balance models and calculating potential evaporation for a land. Rainfall attributes are frequently available and thus an approach estimating ET using remote sensing-based surface temperature and local precipitation data could increase its spatial resolution in data-sparse areas. In the present study, in order to overcome low availability of meteorological data, MODIS surface temperature data are tested, as an alternative input for three ET models in Greece, during the growing season for the years 2002 and 2003. The air temperature range-based Hargreaves-Samani method and two methods depended upon precipitation besides air temperatures, such as the Droogers-Allen method and the improved Reference Evapotranspiration Model for Complex Terrains (REMCT) are considered. Daytime and night time land surface temperature (LST) data are used in replacement of maximum and minimum air temperature data in 22 ground stations. The obtained ET results are evaluated by comparisons with the corresponding ET estimates from the ground air temperature data and furthermore with Penman Monteith ET estimates based on ground data. The temperature-vegetation index method (TVX) is also used for improving the accuracy of the estimation of maximum air temperature and its results are tested in one selected station for investigating any improvement on the ET estimates obtained from the MODIS products. This study shows that in areas with lack of ground temperature data, rather similar results can be achieved using LST from the MODIS products in the selected minimum data-based ET methods, which were initially developed using ground air temperatures. The use of MODIS LST data in REMCT model can allow the spatial analysis of ET at higher spatial resolutions.

Drought is one of the major natural hazards, most often caused by a departure of the precipitation from the normal amount. Quantitative drought definitions identifying the beginning, end, spatial extent and severity of a drought, formulated in terms of drought indices integrate various meteorological and hydrological parameters. One of the most widely used drought indices, the Standardized Precipitation Index (SPI), is based only on rainfall data and is accepted as a good measure of short-and long-term meteorological drought, for a monthly temporal resolution. In this study, SPI is calculated on 1-, 2-, or 3-months scale (SPI 1 , SPI 2, SPI 3, respectively) in 11 stations in Crete, for the period 1974-2001. These SPIs are compared with two other indices, the evapotranspiration deficit index (EDI) and the water balance drought index (WBI). Both indices based on daily evapotranspiration and rainfall data and a daily single layer water balance model, are selected as estimating droughts...

Climate change is anticipated to impact adversely diverse levels of organized life. Mediterranean countries, counting Greece encounter the problem of increased desertification, with its islands being listed as areas of high risk. The Aegean islands are faced with major confrontations as areas of abandoned terraces with low vegetation cover. Nowadays, terrace fields are crucial as green infrastructures to mitigate climate change impacts, as they improve rainfall absorbency, reduce soil erosion, smooth extreme summer temperatures, moderate the risk of floods and forest fires while preserve the biodiversity and ecosystems services. LIFE–TERRASCAPE project aims to revitalize island terrace farming, through the implementation of Land Stewardship (LS) enterprise for the first time in Greece, in association to Information and Communication Technologies (ICT) applications. The study aims to measures of innovative agricultural establishment and production as an integrated adaptation strategy...

During the ongoing efforts for validating or invalidating the accelerating hydrological cycle, pan evaporation has come to the light, as an index of potential evaporation, often used together with precipitation measurements to obtain the surface moisture balance, and estimate its variability. Several studies have reported declines in pan evaporation rate throughout the Northern and Southern hemisphere for various periods since the 1950s. At individual sites, both increases and decreases in pan evaporation have also been reported. In this study, temporal analysis of daily pan evaporation measurements (Epan), Penman (ETp) and Penman-Monteith (ETp-m) evapotranspiration estimates, is presented for 16 stations in Greece, during the period 1979-1999. The high correlation between Epan and ETp-m on a yearly basis and the similarity in their annual time evolutions (in almost all stations of the study) indicate that pan evaporation can be used as an index of potential evapotranspiration in Gr...

The water balance model underlying calculations of the Palmer Drought Severity index (PDSI) has been recently used on a worldwide long–term basis leading to the conclusion of continental drying during recent decades. In its traditional formulation potential evaporation (Ep) is estimated from Thornthwaite's empirical formula, based solely on air temperature (Tair). Consequently, as Tair steadily increases with global warming, the calculated value of Ep in the model also steadily increases. In contrast, calculations of reference evapotranspiration from Penman-Monteith method have been reported as declining in some regions of the world, in general agreement with declining pan evaporation records. Thus, assessing the hydrologic impacts of climate change, the conflicting estimates of declining evaporative demand must be taken into account. In this paper, the effects of three different parameterisations of potential evaporation on long-term trends in actual evaporative flux (AET) and ...

To determine the influence of global changes on surface hydrological fluxes the feedback mechanisms between the land surface and atmosphere within the framework of Bouchet's complementary relationship between potential and actual evapotranspiration are investigated. Using meteorological observations and the advection-aridity model actual, potential and wet environment evapotranspiration are estimated for 28 or 16 stations in Greece for the period 1974 -2001 or 1957 -2001.Trends in actual and potential evapotranspiration are not increasing or decreasing uniformly over Greece. Directions of trends in actual and potential evapotranspiration (being in most cases opposite) are successfully used to offer observational evidence of the complementary relationship.

The Mediterranean area is one of the most visited tourist destinations of the world, but it has also been recognized as one of the most vulnerable to climate change areas worldwide with respect to increased thermal risk. The study focuses on a top worldwide tourist destination of the Mediterranean, Santorini Island in Greece, and aims to assess the past, present and future thermal environment in the island based on the advanced Universal Thermal Climate Index (UTCI). The study utilizes historical observations capturing past (late 19th to early 20th century) and more recent (1982–2019) time periods, while future projections are realized based on four regional climate models (RCMs) under the weak mitigation scenario (RCP4.5) and the non-mitigation scenario with high emissions (RCP8.5). The frequency of cold stress conditions at midday decreases during winter and early spring months by up to 19.8% (January) in the recent period compared to the historical one, while heat stress conditio...

The paper presents the analysis of a sub-set of high-resolution bias-adjusted simulations from the EURO-CORDEX initiative, in order to examine the changes in the mean climate and the extremes in three Mediterranean islands, namely, Sicily, Crete and Cyprus, in the near future (2031–2060) compared to the present climate (1971–2000), under two future scenarios, i.e. RCP4.5 and RCP8.5. The analysis entails commonly used climatic indices of interest related to the islands’ agricultural sector. The results indicate robust increases for both the mean maximum and minimum temperatures on a seasonal basis, as well as for the temperature related extremes under both climate scenarios. On the contrary, the changes in precipitation are less pronounced as the changes in the seasonal precipitation are not found statistically significant for the three islands under both scenarios. The projected warming combined with the projected unchanged precipitation pattern in the future, especially in spring a...