Research on the Characteristics of Surface Flux and Surface Parameters of Meadow Underlying Surface in the Source Region of Yellow River

Abstract

Based on observation data from January 1st to December 31st in 2014 at Maduo Site, which is located near the Yellow River source area in the Qinghai-Tibet Plateau, this research examined how the surface flow of alpine meadow underlying surface varies. The applicability of the land-surface process models CLM5.0 and CLM4.5 in alpine grassland was also tested. The findings revealed that total radiation in the Yellow River source area was lower in the winter but quickly increased after mid-March, peaked in June or July, and then gradually decreased. The surface reflected radiation was affected by both the total radiation and surface albedo. Its seasonal variation showed different characteristics than the total radiation. Due to the snow on the surface, the short-wave radiation reflected by the surface was larger in winter, while the surface reflected radiation in summer was relatively small, especially after the rainy season. Long-wave surface radiation followed the same seasonal pattern as air counter radiation, with a low point in winter and a high point in summer. Due to cloud cover and other variables, however, long-wave surface radiation was larger than atmospheric counter radiation. The sensible heat flux and net radiation varied similarly until July (rainy season), but after July, the latent heat flux and net radiation followed the same pattern until the soil was frozen in early November. For the whole year, the study of regional surface flux was mainly focused on latent heat flux. Corresponding to sensible heat and latent heat, the daily average of soil heat flux was above 0 from mid-March to late August, which meant that the soil heat flux was transmitted from the surface soil to the deep layer, while the rest of the time showed a negative, with soil heat flux transmitted from the deep layer to the surface soil. In late March, the daily average heat flux of soil was abnormally large, which was related to the melting of frozen soil. Generally, for the whole year, soil heat flux was transmitted from the deep layer to the surface soil. The simulation results of CLM5.0 and CLM4.5 for radiation flux and surface heat flux are very close to the observed values, especially CLM5.0. This indicates that the simulation effect of the model in Alpine Grassland in the source region of the Yellow River has been significantly improved after improving many parameterization schemes and dynamic processes in CLM5.0. However, the soil freezing and thawing process and snow parameterization scheme in CLM need to be further improved. Generally, the daily average CO2 flux during the frozen period of soil was 0. In May, September, and October, the respiration of alpine meadows dominated, surface releasing CO2, while in early and middle June, photosynthesis played the leading role, surface absorbing CO2. During the period from late June to mid-August, when both the moisture and temperature of soil were higher, and the surface vegetation experienced rapid growth, the daily average CO2 flux increased sharply. On the contrary, when the frozen soil melted, the CO2 flux released by the surface increased significantly.