Surface Water

Abstract

AbstractThis chapter assesses surface water changes due to climate change and human activities, by particularly examining runoff and streamflow. Changes in the hydrological cycle due to climate change and human intervention can lead to diverse environmental impacts and risks. Fresh water is the agent that delivers many of the impacts of climate change on society. As the major component of freshwater systems, surface water has been significantly altered across basins in terms of spatial and temporal characteristics. The comprehensive understanding of the current status of surface water in the LMRB, such as the distributions and patterns of runoff changes across the Lancang-Mekong River Basin was completed through the high-resolution river network extraction and sophisticated hydrological models. Significant but different trends were found in the seasonal and annual runoff from the LMRB due to different reasons. Over the period of 1971–2010, the annual streamflow shows a general downward trend due to the continued enhancement of human activities. Runoff in the dry season is found to increase faster than the mean annual runoff. As for the spatial distribution, significant trends in streamflow were observed mainly in the middle basin and east of the lower basin. Superimposed on the substantial seasonal cycles is the noticeable lake shrinkage in recent years, especially the Tonle Sap Lake. Evidently decreased inundation was found in most years in the recent two decades from 2000 to 2018. An evident decreasing trend in runoff caused by climate change in the high correlation zone of the Tonle Sap Lake, mainly due to the precipitation decreasing, indicates that climate change contributed to the decrease in water level in the Tonle Sap Lake in addition to human activities. In addition to the decreases in the runoff, streamflow and water level in the Tonle Sap Lake, a significant (p < 0.05) downward trend in the baseflow was also found from 1980 to 2007. Unlike the historical changes in runoff, previous studies projected with high confidence an increasing trend for streamflow in the LMRB, regardless of the climate forcings and models used. However, the flow regime is highly susceptible to a variety of drivers, e.g., dam construction, irrigation expansion, land-use change and climate change. Substantial changes are expected in both annual and seasonal flow, along with a generally increasing trend. Although hydropower development exhibits a limited influence on total annual flows, it has the largest seasonal impact on streamflow, with an increase in the dry season and a decrease in the wet season, by outweighing those of the other drivers.