Measurement and estimation of evapotranspiration in semi-arid grassland during the summer season in southwest Siberia

Abstract

This study quantifies actual evapotranspiration (ET a ) 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 ET a measurements, the reference evapotranspiration (ET 0 ) is computed by a Penman-Monteith model. The estimates are related to the ET a records and the model is evaluated with regard to its performance in a semi-arid environment. The results indicated an ET a driven by energy but limited by water. Within 115 days the total amounts of ET a 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 ET a records differed on average by -91.1% to the ET 0 estimates. The statistical analyses yielded a low correlation between ET a of the ruderal vegetation and ET 0 but an acceptable model performance for the pristine steppe. However, it was observed that ET a occasionally exceeds ET 0 , particularly after precipitation. Due to the high water availability and the subsequent rise of ET a , ET 0 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.