การประเมินปริมาณไนเตรทและฟอสเฟตในแม่น้ำยมตอนล่างด้วยแบบจำลอง SWAT

Abstract

The objectives of this research are to evaluate the surface water quality data, nitrate-N concentrations (NO₃-N) and phosphate (PO₄³⁻) discharging along the lower Yom river and to assess the relative impact of point source and non-point sources, such as urbanization and agricultural activities, causing eutrophication on surface waters with adverse effects on aquatic ecosystems and human health using mathematical model “SWAT” for water quality. The Lower Yom River Basin is located in the north of Thailand which covered area 14.613.62 square kilometers. The study was divided into two parts: first part was the water sampling in the river, which were collected from 14 stations during 2012 - 2013 twice a year - in May (summer), and in August (rainy season) into SWAT model. The average temperature was 30.62 C and average pH was 7.6 The average concentrations of DO, BOD, (NO₃-N and PO₄³⁻ were 5.10, 3.50, 1.02 and 0.74 mg/l, respectively. The results revealed that water quality at Sawankalok District, Sukothai Province (YO07) and Wangching District, Phrae Province (YO09) were defined as a good quality properly used for the fishery purpose. In contrast, at Meuang District, Sukothai Province (YO06) and W03), the measured river quality were lower than the Thailand Surface Water Quality Standard and Classification. The second part was nitrate-N and phosphate modeling. This study Carried out calibration and validation using SWAT model in terms of streamflow, sediment, nitrate-N and phosphate concentration (Year 2000-2013) for 5 RID (the Royal lrrigation Department streamflow gauging stations. The model was well correlated with observed data in lower Yom River. The coefficient of determination (R²) values of runoff, sediment, nitrate and phosphate were 0.75 , 0.67 , 0.72 and 0.74 , respectively. Furthermore, this research showed that the observed and simulated water quantity and quality were not significantly different at the 95% confidence level. Additionally, this research simulated 3 scenarios as follows: 1) agricultural areas were transformed to paddy fields, 2) agricultural areas were transformed to corn area and 3) all communities along the river was transformed to urban areas having one point source per one community. The results of scenario 1 found that nitrate and phosphate were increased in all observatory stations. The nitrate and phosphate concentrations were increased on the average of 23.18 (0.76 mg/l) and 18.40 (0.70 mg/l), respectively. The simulated results indicate that paddy fields are the major pollution sources of nitrate and phosphate loading in Yom River Except for station Y.4 at Muang Sukhothai, it found that nitrate and phosphate were a highest increase on the average of 18% (0.21 mg/l) and 20% (0.22 mg/l), respectively. These concentrations derived from 3 scenarios exceeded the standard. Furthermore, nitrate And phosphate concentrations correspond to the high flow rates in lower Yom river and mostly occur in rainy season.