Evaluation of the impact of natural and anthropogenic factors on river flow and groundwater in a Mekong River sub-basin, Laos

Author: Somphasith Douangsavanh

Douangsavanh, Somphasith, 2022 Evaluation of the impact of natural and anthropogenic factors on river flow and groundwater in a Mekong River sub-basin, Laos, Flinders University, College of Science and Engineering

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Water is essential for all living things on the planet, growing the economy and sustaining the world’s ecosystems. Human-made water infrastructure regulates about three-quarters of the world’s river networks to meet the socio-economic development demand. A common approach applied involves upstream regulation of major rivers. However, this can significantly change river flow regimes and stages downstream. Many studies have assessed the impact of the use of surface water for hydropower development via scenarios and estimation of environmental consequences on river flows, fisheries, aquaculture, and livelihoods. However, little is known of the impact of change in the river flow regimes, as consequences of anthropogenic factors, on surface water-groundwater interactions and groundwater systems. This thesis contributes to reducing this knowledge gap by studying these impacts and effects by comparing the pristine and post-dam conditions of the Nam Ngum River Basin, a major tributary of the Mekong River in Laos.

This research addresses the impact of human activities and climatic factors from the pre- to post-dam period on river flow regimes and connected groundwater systems. Specifically, the aims of this research are as follows: (1) evaluating the impacts of anthropogenic induced changes in water yield by observing the trends and driving factors; (2) assessing the impact of irrigation water, diverted from the surface water, on different water balance components (i.e., groundwater recharge, actual evapotranspiration, surface runoff, and interception); (3) evaluating the impact of river stages, as a consequence of hydropower reservoir operations, on downstream groundwater systems. The body of the thesis consists of five chapters: Chapter 1 provides a global overview of river regulation and describes the geographical context for the study area with problem statements, research aims and contribution of the PhD research, while the main part of the thesis is Chapter 2 to 4, which are written in the style of potential academic papers. Chapter 5 provides a summary of the main findings and conclusions from the three main chapters of the thesis as well as suggestions for the importance of future research.

The first part of this research (Chapter 2) examines the historically observed river flow patterns caused by anthropogenic and natural hydroclimatic drivers and investigates factors that significantly affect the river flow regime. This part also estimates how the river flow affects the water yield in a downstream basin (water productivity generated in the basin) from the pre- to post-dam period. Methods used are statistical trend analysis of relevant indicators such as GRACE total water storage, soil moisture, and actual evapotranspiration. The results show that the river flow was highly seasonal under pre-dam conditions, and the river was losing water to the groundwater in the dry season. However, in the post-dam period, the monsoonal peak flow decreased by 39%, while the flow increased by 120% in the dry season. Moreover, the river has become gaining year-round from 2001 onwards. It is concluded that the water infrastructure has a greater impact on the river flow regime than the climate-related factors. The annual and dry season water yield exhibit an increasing trend from the pre- to post-dam period.

The second part of the research (Chapter 3) evaluates the impact of irrigation water diverted from the river on groundwater recharge and other water balance components. This part aims to assess which portion of the groundwater recharge comes from irrigation schemes and will likely influence water yield in the downstream basin. The recharge and water balance components are quantified for the two different conditions: without irrigation schemes (pre-dam) and with irrigation (post-dam). The methodology is based on the WetSpass-M model. Estimated recharge is compared with assessments of the Water Table Fluctuation (WTF) method, showing an excellent agreement. It is concluded that irrigation has caused a relatively minor increase in groundwater recharge with an average of additional recharge (2012-2014) of 83 mm within the command irrigated areas and 6 mm for the basin-wide increase, resulting in a minimal influence on the water yield in the downstream part of the basin. The annual average recharge with irrigation schemes is assessed to be 444 mm, equivalent to 19% of the average annual rainfall.

The final part of the research (Chapter 4) investigates the impact of changes in river flow regime and river stages due to upstream reservoirs on surface water-groundwater interactions and groundwater systems in the downstream part of the Lower Nam Ngum River Basin. Elements studied are the river-groundwater budget, water table, and groundwater balance in the pre- and post-dam periods. The study develops an interpretive groundwater conceptual model, which focuses on demonstrating the status of groundwater systems when river stages are changed. The model is simulated for the pre- and post-dam conditions using the recharge estimated in the second part of the thesis, observed river stages, and aquifer properties obtained from well-tests. The result shows the status and change in river interaction. In the pre-dam period, the river and its tributaries were losing in the rainy season and gaining water in the dry season. However, they have become gaining year-round in the post-dam period, except for two tributaries upstream exhibiting slight changes from the pre- to post-dam period. The surface water-groundwater exchange has significantly declined in the post-dam period compared to the pre-dam period; the amount of water lost and gained by the river system has reduced by 53% and 23%, respectively. The total amount of water entering and leaving the groundwater system also declines by 22% compared to the pre-dam period. The increase in river stages in the dry season has raised the groundwater tables in the riparian and downstream areas in the post-dam period. Apart from a number of benefits from damming and regulating large rivers (i.e., energy production, flood and drought mitigation, and irrigation development), these research findings show the impacts of dam development on groundwater systems. The reduced surface water-groundwater interaction in the post-dam condition also has potential consequences for reduced hyporheic exchange and hence increased vulnerability of the surface water-groundwater quality and ecosystems in the lower basin.

Keywords: water yield, Mekong Basin, anthropogenic-induced changes, flow, groundwater-surface water interaction, Groundwater recharge, groundwater modelling, Lao PDR

Subject: Water Management thesis

Thesis type: Doctor of Philosophy
Completed: 2022
School: College of Science and Engineering
Supervisor: Okke Batelaan