Global urbanisation identified as the landscape change most responsible for water-quality deterioration over last 20 years

In recent decades the world’s landscapes have experienced profound changes, mainly due to increases in agricultural production and urbanisation – which have affected nearly three-quarters of global land. Alongside these changes there have been losses in natural landscapes such as forests and wetlands. 

Landscape change is a major contributor to the deterioration of water quality. There has been plenty of research in this area at a regional level, but little has been done globally to understand the impact of the different changes in landscape and the role of environmental factors such as climate, seasons and type of water body. 

This study examined over 20 000 peer-reviewed publications dating from 1976 to 2022 and identified 625 studies that used statistical and machine-learning methods from 63 countries. These studies covered different types of water bodies and used a variety of water-quality measures such as acidity, total phosphorus and total nitrogen which are indicators of eutrophication, chemical oxygen demand (COD, which measures organic pollution) and heavy metal pollution. 

From each study researchers analysed the correlation between changes in landscape and water-quality indicators and then summarised these correlations across all the studies. They further analysed the role of environmental factors such as climate zones, water-body types, seasonality and latitude in the relationship between land-use change and water quality. 

Before 2007 most studies were from Northern America and Europe, but in 2008 and 2020 there was an increase in the quantity of studies, especially in China. Around half of studies investigated flowing water such as rivers and streams, a third studied areas of land that drained into specific water bodies (watersheds) and 10 per cent studied static water such as lakes and reservoirs. 

Five of the seven types of landscape composition showed significant impact on water quality – urban lands, agricultural lands, forests, wetlands and grasslands. Analysis of the shifts in urban land and forest coverage showed that both were correlated with water quality. Increases in urban land coverage were linked with contamination of water – as measured by the levels of total dissolved solids and metal ions. Urbanisation was also associated with a reduction in dissolved oxygen in the water, which is essential for the survival of fish and other aquatic organisms.

In contrast an increase in forest coverage was negatively linked with contamination and positively linked with improved dissolved oxygen. However, overall, the contaminating effects of urbanisation outweighed the purification of water by increased forest coverage. Increases in agricultural lands contributed to nutrient pollution by nitrogen and phosphorus from fertiliser run-off, whilst ploughing and irrigation led to increased solids in the water. 

Changing seasons contributed to the impact of urbanisation on water quality – where the wet season produced more run off and more pollutants in water, and the dry season resulted in less dilution and a higher concentration of contaminants. Latitude was an influential factor in forestry’s ability to effect change – where the benefits of forest cover for water quality seem to gradually decrease at increased latitudes (a greater distance from the Equator).

Analysis showed that changes in wetland coverage overall did not have a significant impact on water quality. However, this may be because the coverage of wetlands was small across the whole landscape. Numerous small-scale experiments have proved the role of wetlands in removing pollutants from local water bodies. 

The analysis did show that wetland coverage in cold areas and high latitude regions was associated with high levels of chemical oxygen demand (COD) in water, indicating the presence of organic matter pollution. This could be partly due to cold temperatures restricting organic matter decomposition.

Overall, the study indicates that the impact of land-use change on water quality has been intensifying since the 1990s and that negative effects from contamination and pollution are driven by urbanisation and agricultural land changes.

The researchers say that increasing forest cover can have a restorative effect, particularly at low latitudes, and this could form part of a land-use-change management strategy that includes water protection through nature-based solutions. The insights on how latitude, climate zones and seasonality can influence the impact of landscape change on water quality could inform strategies that are appropriate to local regions, including in Europe. For example, they could guide decisions on where to plant forestry to improve water quality and which type of tree species would perform this role best according to how they grow in different seasons. 

The study also demonstrates that there are still gaps in research on the impact of land-use change on water quality, particularly in developing countries and regions in Africa and South America.  

The researchers emphasise the regional inequality revealed in the sampled studies and call for more emphasis on global water equity and environmental justice, as the world experiences intense climatic change.

Source:

Shi, X., Mao, D., Song, K., Xiang, H., Li, S., Wang, Z. (2024) Effects of landscape changes on water quality: A global meta-analysis. Water Research 260: 121946. 

To cite this article/service:

“Science for Environment Policy”: European Commission DG Environment News Alert Service, edited by SCU, The University of the West of England, Bristol.

Notes on content:

The contents and views included in Science for Environment Policy are based on independent, peer reviewed research and do not necessarily reflect the position of the European Commission. Please note that this article is a summary of only one study. Other studies may come to other conclusions.