4.4 Evapotranspiration
Assessment of evapotranspiration (ET) is very problematic for this region due to the lack of data. Annual values of ET for goltsy landscape at the Morozova Creek watershed (altitude range 1100-1700 m a.s.l.) was estimated based on the water balance data of the Kolyma water balance station (KWBS) in the range from 70 mm (Lebedeva et al., 2017) to 92 mm (Makarieva et al., 2018a). In the Upper Wolf-Creek catchment (part of the southern headwaters of the Yukon River, Canada) with tundra and shrub-tundra environment and elevation reaching 2250 m, the annual value of ET reached 135 mm (Janowicz et al., 2004). The assessment for the Upper Kuparuk (elevation range of 698-1464 m) and Imnavait River basins (elevation range from 844 to 960 m) (Alaska) from tundra landscape was about 140 and 178 mm respectively (Schramm et al., 2007). In the Tana River Basin (Finnish Tenojoki), with a mean air temperature -6˚C at the highest mountain tops (1010 m), annual values of snow sublimation and evapotranspiration was estimated to be 90 and 58 mm respectively (Dankers & Chrisrensen, 2005). The assessment of total annual evaporation at the Axel Heiberg Island at the Canadian Arctic Archipelago was about 140 mm (Ohmura, 1982).
The observations of ET from the ground surface at the Suntar-Khayata station were carried out in 1958 (Grave, 1959). Two land evaporimeters GGI-500 were used (Makarieva et al., 2018a). The evaporimeters were installed in early June 1958, when the snowpack was continuous, had not started melting yet, and ground temperature was below zero. Evaporation tanks were filled with soil from the Suntar-Khayata Station site and left under snow until it completely melted at the site on July 20-27, 1958. The observations continued throughout August 1958. Evaporation tanks were weighed every 5 days, and precipitation was registered daily in direct proximity to them (Grave, 1959). In August 1958 observed values of precipitation accounted for 77 mm, infiltration rate was 36 mm, and ET was 44 mm (about 1.4 mm per day).
Compared to the ET assessments for the KWBS watersheds where the snow-free season lasts twice as long and mean summer values of air moisture deficit are 1.5 times higher than at the Suntar-Khayata Station, we question this single result of observations and suggest that the value of 44 mm is significantly overestimated.
In the Hydrograph model, the amount of ET is calculated taking into account the potential evaporation, initial amount of moisture in soil layers, maximum water holding capacity of the soil, the fraction of contribution of a given soil layer to total evaporation, the value of which depends on soil and vegetation type (Semenova et al., 2013). To estimate ET, we adopted the evaporation coefficient parameter from the modelling studies conducted for the Kolyma water-balance station (Semenova et al., 2013; Makarieva et al., 2020). Its value for the goltsy landscape is 9*10-10 m (hPa*s)-1. Simulated ET values were 19 mm in August 1958 and 43 mm on average during the snow free season for the whole period of simulations. Adding 11 mm of snow evaporation gives us about 54 mm of annual total evaporation from the goltsy landscape.