Evapotranspiration and energy balance dynamics of a semi-arid mountainous steppe and shrubland site in Northern Mongolia

The semi-arid forest-steppe ecotone in the Khentii Mountains in Northern Mongolia is characterized by southerly exposed steppe slopes, shrublands in the floodplain of river valleys and northerly exposed taiga slopes. Here, the freshwater for the downstreaming agricultural, rural and mining sectors is generated. To gain insight into the main water-generating sources in mountainous river basins, we studied the actual evapotranspiration (ET) and its main influencing ecohydrometeorological parameters at a steppe and a shrubland site during the growing season 2012. Latent (LE) and sensible (H) heat fluxes were measured using the Bowen ratio energy balance method, and the surface soil heat flux (G(0)) was determined with the PlateCal and GradCal approach. During water-stressed soil conditions at the steppe site, volumetric water content (VWC) values of 0.05 m(3) m(-3) (-10 cm) resulted in significant limitations of the mean actual ET rates to 0.4 mm day(-1). In contrast, high VWC with maxima of >0.4 m(3) m(-3) and high mean actual ET rates >6.0 mm day(-1) were observed during water-unstressed soil conditions. As precipitation and change of stored soil moisture were approximately equal to actual ET, the contribution of freshwater from mountainous steppe within the forest-steppe ecotone can be neglected. At the shrubland site, which is underlain by a shallow aquifer system, VWC remained relatively constant with mean values of 0.4 m(3) m(-3) (-10 cm). A multiple regression analysis indicated that actual ET, with mean rates of 2.3 mm day(-1), was not significantly limited by precipitation and VWC, but by net radiation (R-n) and air temperature. As precipitation exceeded total actual ET, shrub covered river floodplains can contribute to freshwater generation.