Long-term energy and CO2 flux observations over an agricultural field in southeastern Tibetan Plateau

Information on the exchange of energy, momentum and mass (H2O, CO2, CH4, etc.) over complex topography is critical for determining the development of the boundary layer, carbon and water cycles, weather and climate. This information can also improve the numerical modelling of physical atmosphere-land processes. Based on a 12-year (2007– 1

0.66 to 31.57 MJ m -2 d -1 , with a 12-year average value of 17.35 MJ m -2 . The annual integrated R g ranges from 5066.1 to 6702.7 MJ m -2 yr -1 , with an average value of 6154.1 MJ m -2 and a standard deviation of 450.2 MJ m -2 ( Table 1).
The T a at different heights of the near-surface layer in the wet season are higher than those in the dry season. For monthly average T a , the fastest increase period occurs between March and April, whereas a decrease period occurs between October 95 and November. The daily average T a values at heights of 2, 4, 10, and 20 m range from 0.4 to 24.7, 0.6 to 24.3, 0.3 to 24. The RH of the near-surface layer usually decreases with measurement height, and in the wet season, it is higher than in the dry season. The monthly average RH reaches the highest in August and the lowest in February. The daily average RH values 100 at heights of 2, 4, 10, and 20 m range from 14.5 % to 100 %, 13.4 % to 99.5 %, 12.4 % to 95.4 %, and 11.8 % to 96.3 %, with 12-year average values of 64.6 %, 62.3 %, 57.2 %, and 56.8 %.
The WS of the near-surface layer commonly increases with measurement height, and in the wet season, it is weaker than that in the dry season. The monthly average WS reaches the strongest in February and the weakest in September. The maximum daily average WS values at heights of 2, 4, 10, and 20 m are observed in January 2008 (7.1, 8.4, 9.9, and 11.1 m s -105 Local complicated terrain often affects air flow within the atmospheric boundary layer, which not only changes wind speed but also changes wind direction. In the study area, the predominant wind direction is easterly and east-southeasterly throughout the year (Fig. 3). The prevailing and sub-prevailing winds at a height of 10-m display a clear diurnal cycle, with east-southeasterly and easterly winds during the daytime and westerly and static winds at night (Fig. 4). The time when the 110 prevailing winds switch exhibits good correspondence with the sunrise and sunset (solar radiation heating effect). This result may be chiefly caused by the shapes of the Diancangshan Mountains and Erhai Lake, which extends from northwest to southeast.
The T s values at different soil depths in the wet season are higher than those in the dry season. The monthly average T s reaches the highest in July and the lowest in January. The T s values in the shallow soil layer are higher than those in the deep 115 soil layer in the wet season; conversely, T s in the shallow soil layer is lower than that in the deep soil layer in the wet season.
The maximum daily total PPT value is observed in October 2015 (121.0 mm d -1 ), and the maximum monthly total PPT is observed in June 2008 (336.7 mm month -1 ). The annual total PPT fluctuates from 732.9 mm yr -1 (2013) to 1322.8 mm yr -1 120 (2008), with an average value of 962.1 mm and a standard deviation of 179.3 mm ( Table 1). The PPT during the wet season ranges from 643.2 to 1171.9 mm, accounting for 75.9 % to 93.6 % of the annual total PPT. Because of the frequent drought influence, the PPT is below the 12-year average value for five consecutive years from 2010 to 2014.
The SWC values at different depths in the wet season are larger than those in the dry season, and their values also respond significantly to PPT. With the start of the rainy season (commonly occurring from late May to early June), the SWC rapidly increases and reaches its maximum. The daily average SWC values at depths of 4, 10, 20, 60, and 100 cm range from 0.07 to 0.73, 0.08 to 0.70, 0.19 to 0.59, 0.27 to 0.48, and 0.35 to 0.46 m -3 m -3 , with 12-year average values of 0.37, 0.38, 0.42, 0.40, and 0.42 m -3 m -3 , respectively.

Diurnal, seasonal and inter-annual variations in energy fluxes
The

Diurnal, seasonal and inter-annual variations in CO 2 flux
The monthly average diurnal variations in Fc from 2007 to 2018 are shown in Fig. 5b. For each month, the Fc has a noticeable diurnal cycle with positive values at night reaching their maximum at approximately 07:00 and negative values 150 during the day reaching their minimum at approximately 13:00. This finding indicates that the study area acts as a weak carbon source at night and a carbon sink during the day. The monthly maximal CO 2 release and uptake rates range from 1.46 to 4.54 μmol m -2 s -1 and from 2.59 to 16.62 μmol m -2 s -1 .
The seasonal and inter-annual variations in the daily and monthly average Fc values from 2007 to 2018 are shown in Fig.   6e and Fig. 7e, respectively. For each year, the Fc is higher during the transitional periods (between April and May, between

Conclusions
In this study, we report the variations in energy and CO 2 fluxes and their meteorological drivers at various temporal scales