Water purification – an essential service from forest ecosystems, and farming practices in the Pong River Basin, Northeast Thailand

Deforestation often results in biodiversity depletion, land degradation, and loss of forest ecosystem services such as water purification. This research examined the relationship between natural forest areas in the Pong River Basin (PRB), northeast Thailand, and water quality in the Pong River system, one of the country's most polluted rivers. Water parameters were measured in 15 tributaries where samples were collected 500 m upstream from the outlets, and 2–5 m from the streambanks during summer, rainy, and winter seasons in 2021. Land use and land cover (LULC) were classified by a supervised classification method in QGIS using LANDSAT8 satellite imagery and USGS SRTM data. Vegetative surveys were conducted near the sampling locations which included species richness and density. Water characteristics at all the sites were under Thailand's Water Quality Standard, except a level of biological oxygen demand (avg. BOD = 3.52 mg/L). The upper PRB, covered with more forested areas with higher tree densities along streambanks (∼43 % of the basin, 619 trees/ha), revealed better water quality including a higher concentration of dissolved oxygen (avg. DO = 7.57 mg/L) and a lower level of fecal coliform bacteria (avg. FCB = 642 MPN/100 mL). In contrast, the lower PRB having larger areas of farmlands and smaller forests (∼63 % and 23 %, respectively) yielded higher concentrations of FCB (avg. = 1,353 MPN/100 mL) and phosphates (avg. PO₄^3- = 0.34 mg/L). Positive correlations were found between the amount of forestlands (% of the sub-basin) with DO and nitrate concentrations (r = 0.468 and 0.409, p-value < 0.05, respectively), while agricultural areas were correlated with FCB (r = 0.414, p-value < 0.05). Amounts of forests determine water purity, while farmlands denoting dirtiness. These findings show that water purification is an essential forest ecosystem service, critical for stream health.