Linking Changes in Land Cover and Land Use of the Lower Mekong Basin to Instream Nitrate and Total Suspended Solids Variations

Abstract

Population growth and economic development are driving changes in land use/land cover (LULC) of the transboundary Lower Mekong River Basin (LMB), posing a serious threat to the integrity of the river system. Using data collected on a monthly basis over 30 years (1985–2015) at 14 stations located along the Lower Mekong river, this study explores whether spatiotemporal relationships exist between LULC changes and instream concentrations of total suspended solids (TSS) and nitrate—as proxies of water quality. The results show seasonal influences where temporal patterns of instream TSS and nitrate concentrations mirror patterns detected for discharge. Changes in LULC influenced instream TSS and nitrate levels differently over time and space. The seasonal Mann–Kendall (SMK) confirmed significant reduction of instream TSS concentrations at six stations (p < 0.05), while nitrate levels increased at five stations (p < 0.05), predominantly in stations located in the upper section of the basin where forest areas and mountainous topography dominate the landscape. Temporal correlation analyses point to the conversion of grassland (r = −0.61, p < 0.01) to paddy fields (r = 0.63, p < 0.01) and urban areas (r = 0.44, p < 0.05) as the changes in LULC that mostly impact instream nitrate contents. The reduction of TSS appears influenced by increased forest land cover (r = −0.72, p < 0.01) and by the development and operation of hydropower projects in the upper Mekong River. Spatial correlation analyses showed positive associations between forest land cover and instream concentrations of TSS (r = 0.64, p = 0.01) and nitrate (r = 0.54, p < 0.05), indicating that this type of LULC was heavily disturbed and harvested, resulting in soil erosion and runoff of nitrate rich sediment during the Wet season. Our results show that enhanced understanding of how LULC changes influence instream water quality at spatial and temporal scales is vital for assessing potential impacts of future land and water resource development on freshwater resources of the LMB.

Assessment of Hydrology and Sediment Yield in the Mekong River Basin Using SWAT Model

“…Beside the dam development in Mekong River Basin, climate change and land use change are also the crucial factors of sediment transport throughout the basin [69]. The instream sediment load and yield in the lower part of the Mekong River are likely influenced by the interaction between land use/land cover, rainfall-runoff, and anthropogenic activities within the basin [70]. Deforestation and agriculture expansion could change the landcover and alters basin erosion of the Mekong River [71].…”

Changing Land Use and Population Density Are Degrading Water Quality in the Lower Mekong Basin

“…However, our results differ from other highly urbanized tropical regions in Brazil [23] where urbanization was the strongest driver of nutrient concentrations in rivers, and nonintensive subsistence-level agriculture did not increase water nutrient concentrations. Most fertilizer use occurs in Europe and the United States, although fertilizers are increasing in Asia [39], which is increasingly replacing subsistence cultivations with intensive agricultural activities [17]. Interestingly, we found that NO 3 − and TN were positively correlated to agriculture (crops) but were negatively correlated to rice paddies, showing that traditional rice cultivation was not associated with an increase of nitrogen concentration in our sites.…”

“…Most forested areas are in Laos and Cambodia, although high rates of deforestation have recently occurred [9]. Lowland areas are dominated by rice cultivation [17]. Most people live in urban areas, with lower population densities in agricultural areas.…”

“…In Cambodia, the selected stations were Backprea (1), Kampong Thom (2), Kampong Toul (3), Kampong Thmar (4), Mekong (5), Neak Luong (6), Phnom Krom (7), and Tonle Sap (8). In Laos, the selected stations were Ban Hat Kham (9), Ban Keng Done (10), Houay Mak Hiao (11), Nam Houm Dam (12), Nam Ngum (13), Nam Souang (14), Se Bang Fai (15), Sedone Bridge (16), Thaleth Keokou (17). In Thailand, the selected stations were Ban Chai Buri (18), Ban Ku Phra Kona (19), Ban Som (20), Bad Tan Ton (21), Chiang Rai (22), Lam Dom Noi (23), Lam Pao Dam Site (24), Nam Kae (25), (Figure 1).…”

“…The degradation of water quality in the Mekong River has become a concern because of its importance for drinking, domestic uses, and agriculture, so water degradation could harm human, animal, and ecosystem health [16]. While some studies on water quality in the Lower Mekong have noted a decline of water quality [14][15][16], we found only one study that assessed the effects of land use change on total suspended solids (TSS) and dissolved inorganic nitrogen (NO 3 − ) [17]. A comprehensive study that assesses the effects of land use change on multiple water quality parameters in the Lower Mekong Basin is needed to provide reference conditions for water quality, including nutrients, dissolved oxygen, and total suspended solids, which are key parameters for driving ecosystem productivity.…”