Hydrological study of evapotranspiration types (ETo, ETa, MODIS ET) for Peloponnese (Greece) during 2016-2019, employing remote-sensed and meteorological stations’ data in GIS models, and pan evaporation measurements

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Δημητριάδου, Σταυρούλα

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The present dissertation develops 14 GIS models, of which 10 models to implement six empirical and physical-based methods for the estimation of Reference Evapotranspiration (ETo) and Actual Evapotranspiration (ETa) and 4 models implementing statistical measures. Specifically, FAO-56 Penman–Monteith (FAO PM), Hargreaves–Samani (HS) and Hansen, with the former used as a reference, were the methods selected for ETo estimation and Turc, Turc modified and Coutagne were the methods selected for ETa estimation. The study area is Peloponnese, Greece, a suitable Mediterranean testbed with substantial changes of relief over short distances. The study period is years 2016-2019 which constitute four out of the five warmest years since pre-industrial era. For the same period, pan evaporation (Epan) measurements in daily step of two Class-A pan stations in Peloponnese (Patras University at the northmost edge and Ladonas lake at the central part) with different environmental conditions were collected and intercomparisons were made. Moreover, 304 MODIS products have been processed and used in the research. Specifically, 200 images of a global remote-sensed product which estimates actual ET, MODIS net ET (184 images), and MODIS potential ET (ETp; 16 images) were derived (as 8-day composites) and processed for different time scales to detect similarities and differences with the aforementioned ET types. In addition, 104 MODIS LST day and night products have served as inputs to models, in particular, 96 monthly scale and 8 annual scale. Besides the intercomparisons of ET types for an interesting testbed, another novelty of the dissertation is that all the six applied methods and the three most reliable statistical indices for ET studies were implemented after developing GIS models in ArcMap model builder environment and as python scripts. The models of empirical methods include all the stages of pre-procession and the intermediate steps and the inputs are images in raster format. Thus, the output (ETo or ETa) is a useful map for the study area. For the statistical analyses three models implementing statistical indices (RMSD, MB, NMB). Specifically, approximately the same pattern was followed for ETo and ETa investigation. For the first parameter, to investigate the variability in the daily mean reference evapotranspiration (ETo) for the months December and August during 2016–2019, as representative winter and summer months according to availability and qualitative criteria of the data. Furthermore, to investigate the agreement between the three methods’ ETo estimates, and then to examine the former along with MODIS ET (daily) averaged products and daily Epan values. For the second case annual ETa estimates by the three empirical methods were examined along with annual MODIS ET and annual Epan values.The inputs of the models were ground-based and remote-sensed data. Meteorological data from sixty-two stations (over or adjacent to Peloponnese) under the National Observatory of Athens (NOA), MODIS Terra LST day and night products, and MODIS net ET products (8-day composites) have been employed. The parameter differentiating FAO PM from the temperature-based Hansen and Hargreaves-Samani methods was wind speed. The method depicts interactions among climate parameters (T, evaporative demand and water availability) in the frame of climate change. The years 2016–2019 are four of the warmest since the preindustrial era. Hargreaves–Samani’s estimations for the month December of 2016–2019 were almost identical to MODIS ET, despite their different physical meaning. As expected, for month August, there were considerable discrepancies between the three methods’ and MODIS’s estimates, attributed to the higher evaporative demand in the summertime. Thus H-S could serve as an alternative to MODIS ET for month December of years 2016-2019 for Peloponnese. The latter needs further investigation to find out if this is the case for other winter months. Turc method’s outcomes resemble MODIS ET ranges for all years, followed by Coutagne. Estimates by Turc modified, were almost identical to those of MODIS ET. Moreover, unpaired Student T-test has been performed between each empirical method and MODIS ET which verified that the outcomes were statistically significant (p ≤ 0.05) and exhibited low effect size according to Cohen’s D value (< 0,24). Therefore, Turc modified could be used as an alternative to MODIS ET for Peloponnese for years 2016-2019. Their discrepancies are attributed to the uncertainty which MODIS ET involves as a global product. The GIS models were proved accurate, reliable, time-saving, and they are adjustable to any study area. Moreover, Epan for Patras University station (semi-urban, low-elevation) exhibits an upward trend which resembles FAO PM trend through study-years and the same pan coefficients were determined for both December and August of each year which is an indication of consistency that needs further investigation. In contrast, Epan of Ladonas station (lakeside, higher elevation) is more complex. Therefore, pan coefficients based on FAO PM substantially vary and require investigation on monthly time scale. Additionally, the gradual decrease in days of Ladonas pan-water icing could imply undergoing decrease in snowpack-storage retention over the mountains of Peloponnese.

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Evapotranspiration, Actual evapotranspiration, Reference evapotranspiration, GIS models, Hydrology, Remote sensing, Climate change, Peloponnese Greece, ArcMap, Pan evaporation, Class A pan, Metorological data, ArcMap

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