Seasonal variation and controlling factors of evapotranspiration over a “floating blanket” wetland in southwest China

Abstract

Beihai wetland is a special type of alpine marsh in the southeast of the Qinghai-Tibet Plateau, with plants like a blanket floating on the water surface permanently. Long-term datasets with eddy covariance (EC) technique of evapotranspiration (ET) from the wetland ecosystem are still not too much. Based on EC measurements from 2016 to 2020, we investigated the variations of ET and its controlling factors over the Beihai wetland. Our results showed that ET was mostly influenced by the net radiation (Rn) from half-hourly to monthly timescales as a result of sufficient water supply at the observation site. In addition, the cloudiness was also a key factor controlling ET on daily scale by regulating radiation. On overcast and cloudy days, cloudiness suppressed ET primarily due to the decrease in the direct radiation (Rdir). While under sunny conditions, the increased diffuse radiation (Rdif) by cloudiness was conductive to increasing the canopy conductance (Gs) and therefore ET. On annual scale, the annual ET ranged from 762.3 to 990.5 mm, and the annual precipitation was 1,210.4–1,780.2 mm during 2016–2020. The annual ET changed by around 30 %, while the annual precipitation changed by 47 %. There was a significant negative relationship between the annual ET and the annual precipitation. More annual precipitation in 2016 with more precipitation days and greater precipitation intensity than that in other four years resulted in less annual ET. Furthermore, the fraction of water surface was another factor controlling year-to-year variation of ET. There was a larger fraction of water cover in current site (37.3–38.4 % in 2017–2020) than that in the original site (29.7 % in 2016). Annual ET tended to increase as the fraction of water cover increased.