Özet:

This study quantifies actual evapotranspiration (ETa) for a period from June to September 2016 measured by two weighable gravitation lysimeters in a semi-arid grassland in southwest Siberia. As part of a crop rotation system, the first lysimeter was fallow but covered with ruderal vegetation. The second lysimeter is permanently characterized by pristine steppe vegetation. In addition to ETa measurements, the reference evapotranspiration (ET0) is computed by a Penman-Monteith model. The estimates are related to the ETa records and the model is evaluated with regard to its performance in a semi-arid environment. The results indicated an ETa driven by energy but limited by water. Within 115 days the total amounts of ETa ranged from 205 mm to 374.1 mm, and daily values varied from 0.1 to 6.9 mm day-1. The large differences are caused by the different vegetation cover of the lysimeters. Due to the high and dense canopy of the pristine steppe vegetation, the transpiration term was considerably higher compared to the ruderal vegetation where soil evaporation took the major part. The daily ETa records differed on average by -91.1% to the ET0 estimates. The statistical analyses yielded a low correlation between ETa of the ruderal vegetation and ET0 but an acceptable model performance for the pristine steppe. However, it was observed that ETa occasionally exceeds ET0, particularly after precipitation. Due to the high water availability and the subsequent rise of ETa, ET0 was underestimated, whereas it was overestimated during dry periods. Finally, the quality of the Penman-Monteith model varied substantially with the water supply at the study site.

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