Characteristics and impact factors of water and heat flux changes at different time scales during the growing season of desert grasslands

Water and heat flux is a key components for the energy exchange of surface air in atmospheric circulation. The driving mechanism can include the sensible heat flux (H) and latent heat flux (LE). There is a great variation in the water and heat flux in desert grasslands. The influencing factors are of great significance to understanding the regional exchange of water and heat flux against climate change. This study aims to explore the variation patterns of water and heat flux at different time scales. The eddy covariance and continuous observation were also utilized from automatic meteorological stations in Xilamuren Town, Damao Banner, Inner Mongolia, China. After that, the regulatory mechanisms of environmental factors were proposed for the desert grassland during growing seasons (April to October) in 2018 and 2019. A systematic analysis was made on the H, LE, and energy ratio (H/LE) in the desert grasslands at different time scales, together with their environmental control mechanisms. The results indicated that the H of plants was the main form of near-surface energy consumption in the desert grassland during the growth season. There was an unimodal variation in the H and LE on an hourly scale. The H and LE peaks mainly occurred between 12:00 and 14:00, where the time of LE peak occurrence was lagged by 1-2 h, compared with the H. The variation range of H and LE between 8:00—18:00 was 0.1-160.5 and 15.4-90.9 W/m², respectively. The main influencing factors of H were the wind speed (Ws), relative humidity (RH), vapor pressure deficit (VPD) and air temperature (Ta）with the correlation coefficients (r) of 0.72, -0.66, 0.61 and 0.38, respectively. The main influencing factors of LE were Ta, VPD, and ground soil temperature (Ts) and precipitation (P), with r of 0.76, 0.67, 0.61, and 0.37, respectively. The main influencing factors of H/LE were RH, Ws, and VPD, with r values of -0.74, 0.86, and 0.24, respectively. There was an outstanding seasonal variation in the H and LE on a daily scale. The maximum is also reached before and after the middle stage of plant growth. The daily variation ranges of H, LE, and H/LE during the plant growth season were 4.2-186.1, 1.3-160.9 W/m², and 0.2-29.4, respectively. The main influencing factors of H were RH, Ws, Ts, Ta, P, and VPD, with correlation coefficients of -0.41, 0.13, 0.17, 0.21, -0.15, and 0.39, respectively. The main influencing factors of LE were RH, Ws, Ts, and Ta, there was no significant correlation with P. The main influencing factor of H/LE was the RH with a correlation coefficient of -0.16, which was not significantly correlated with other factors. On a monthly scale, the H and LE were approximated a unimodal change. The average H and LE reached the maximum in June and August. The variation ranged of H and LE were 36.4-133.8 and 15.6-96.3 W/m², respectively, from April to October. The LE was the main mode of energy dissipation in summer. The main influencing factors of H were the Ts, Ta, and VPD, where there was the strongest positive correlation of VPD. The LE was significantly correlated with the RH, Ws, Ts, Ta, and P, where the P shared the strongest positive correlation (r =0.75). There was a significant correlation between H/LE with RH, Ws and P, in which the RH having the strongest positive correlation (r =-0.76). The finding can provide the theoretical basis and practical significance for vegetation restoration and environment protection in ecological functional grassland.