Evapotranspiration over the highest grassland ecosystem: alpine steppe of the Tibetan plateau

Ning Ma, Chinese Academy of Sciences

Abstract for Weekly Colloquium on Thursday, September 8, 2016 at 4 pm in PAS 220

Quantitative estimation of actual evapotranspiration (ETa) by in situ measurements and mathematical modeling is a fundamental task for physical understanding of ETa as well as the feedback mechanisms between land and the ambient atmosphere. While the release of latent heat from the ground surface of TP may significantly impact the Asian Monsoon system, the ETa information in the Tibetan Plateau (TP) has been greatly impeded by the extremely sparse ground observation network in the region. Here we used the highest (4947 m above sea level) in-situ flux measurement in a typical semiarid alpine steppe region of the TP to investigate how the environmental and biophysical factors influence the ETa process. Besides, considering the little data availability (e.g. soil and vegetation data, flux measurments) over this harsh region, we proposed the calibration of the complementary-relationship-based (CR-based) models to improve the ETa simulations.

We found that the symmetry of the CR could be achieved for dry regions of TP when (i) the Priestley-Taylor coefficient, (ii) the slope of the saturation vapor pressure curve and (iii) the wind function were locally calibrated by using the ETa observations in wet days, an estimate of the wet surface temperature and the Monin-Obukhov Similarity (MOS) theory, respectively. Besides, the asymmetric CR was confirmed in TP when the D20 above-ground and/or E601B sunken pan evaporation (Epan) were used as a proxy of the potential evapotranspiration (ETp). Thus daily ETa could also be estimated by coupling D20 above-ground and/or E601B sunken pans through CR. Additionally, to overcome the modification of the specific components in the CR, we also evaluated the other CR-based (Nonlinear-CR model and the Morton’s CRAE model) and Penman-Monteith-based (KP model and TD model) approaches. We found that all models except TD model are also able to simulate the daily ETa well, provided their parameter values have been locally calibrated. However, the fixed model physics made them bear some uncertainties because of their different assumptions on ETa process. Finally, we also extend the CR method into the lake evaporation simulation over TP, showing that the decrease of lake evaporation during 1998-2008 contributed to about 4% of recent rapid lake expansion.