Eichhornia crassipes (Mart.) Solms.is listed as one of the worst invasive alien plants that can be introduced to the new environment primarily by human activities. They have very high reproduction rates in favorable conditions, especially concentrated nutrients. Native plants habitats can be encroached by the density of Eichhornia crassipes mat and disappear in their environment. This study aims to study causes of the increase of Eichhornia crassipes by analyzing land use／land cover and water quality in Nong Bong Khai Non-hunting Area, Chiang Rai, Thailand. Then we used unmanned aerial vehicles (UAV) with 2 flight altitude levels (30 m, 90 m) for non-submerged aquatic vegetation mapping to study the distribution. The results showed that increases in Eichhornia crassipes resulted from high concentrations of nutrients in water. From 2009 to 2018, The land use ／ land cover reports indicated changes in land use type which area of field crop highly increased. The distribution map that was generated by applying a 90-m orthophoto from the UAV showed high covering area of Eichhornia crassipes by 13.82% of the lake area. Also, the vegetation map generated from a 30-m orthophoto showed the distribution and proportion of Eichhornia crassipes were more than those of other native plant species. This study showed that changes in land use／land cover around the lake affected water quality changes, which was a main cause of highly increasing rates of Eichhornia crassipes. Moreover, the orthophoto from the UAV at different altitude levels was suitable and useful for monitoring the distribution of Eichhornia crassipes and also other plant species.

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