The effect of water level fluctuation due to decreased precipitation on the non-submerged aquatic vegetation in Nong Bong Khai Non-hunting Area, Northern Thailand

The hydrology of a wetland largely influences its vegetation, which affects the value of the wetland to human and animal life. Water level fluctuation is one of many factors impacting wetland ecosystem diversity, and precipitation variability is one of the most influential drivers of plant species composition in wetlands. This study describes the effects of water level fluctuation caused by decreased precipitation on the non-submerged aquatic vegetation community in the Nong Bong Khai Lake, Nong Bong Khai Non-hunting Area, Chiang Rai, Thailand. We applied non-submerged aquatic vegetation distribution maps based on images from unmanned aerial vehicle (UAV) at two altitudes (30m and 90 m). Rapid decreases (from 1.0 m to 0.2 m) in the water level from 2018 to 2020 were influenced by lower than average precipitation during 2019 and 2020. Increased terrestrial area (dry land) caused by low water levels offered favorable conditions for terrestrial plant species productivity. The vegetation map generated from a 30-m orthophoto showed a decrease in native plant species and an increase in invasive plant species, most notably a rapid expansion of Mimosa pigra in low water level areas. The distribution map generated by applying a 90-m UAV orthophoto showed Eichhornia crassipes continuously increased during the period of low water level. Mapping revealed that the species declined in the area in March 2020 due to the destruction caused by a heavy hailstorm. Extensive water coverage by Eichhornia crassipes mats might result in low dissolved oxygen values, indicating an increase in pollution in the lake.