Effects of land cover change on evapotranspiration in the tropical Lancang-Mekong River Basin from 2001 to 2020

Abstract: Land cover types have changed dramatically in the tropics as human activity is ever increasing in recent years. These changes can cause great impacts on regional water security. Therefore, it is a high demand to accurately quantify the effect of land cover change on evapotranspiration (ET) for a better understanding of the mechanism of the water cycle under global warming. This study aims to investigate the effects of land cover change on the ET in the tropical Lancang-Mekong River Basin (LMRB) from 2001 to 2020. Firstly, the land cover data was reclassified and we corrected the unreasonable change types. The land cover product (MCD12Q1) was then evaluated using high spatial resolution images with Google Earth Pro. Secondly, the ET product (MOD16) was assessed using a total of 10 eddy covariance observation sites. Pearson's correlation coefficient (r), Root Mean Square Error (RMSE), Mean Absolute Error (MAE), and Mean Relative Percentage Error (MRPE) were also used to analyze the land cover changes and ET trends in this region. Finally, a dynamic analysis was developed to accurately quantify the effect of land cover changes on ET water consumption, where the impacts of climate change were excluded. The results show that: 1) The MCD12Q1 performed better with an overall accuracy beyond 82%, the forests and cropland of which were 90.5% and 89.4%, respectively. The RMSE values of MOD16 on the 8-day and monthly scales were only slightly larger than 1 mm/day. Therefore, the two products (land cover and ET) can be expected to analyze the ET changes in the study area. 2) The changing area accounted for 24.7% of the total. There was a degradation trend of the overall vegetation, where the conversion areas of the forest to shrubs and shrubs to cropland accounted for 61.2% of the changing area. 3) The trend analysis showed that there was an increasing average ET of 5 mm/a in the entire region. 4) A significant difference was observed in the annual average ET of each land cover type. Generally, the annual average ET of the forest was higher than that of shrubs, and the annual average ET of shrubs was higher than that of cropland. 5) The major types of land cover change caused a total decrease of 27.89 billion m3 of water consumption, whereas, climate change led to an increase of 19.10 billion m3 of ET water consumption. 6) Although there was a decrease of ET in the land cover change area, there was no significant influence of the land cover change area on the increasing ET. In general, the vegetation degradation resulted in a decrease in ET and water consumption, indicating the conversion of forests to shrubs and shrubs to cropland from 2001 to 2020. A better understanding of the water cycle response to global change can provide useful knowledge to effectively monitor the water resources security and the allocation of land and water resources in the tropical LMRB.