Long-term changes in pollutant load outflows and purification function in a paddy field watershed using a circular...

Area of Science:

• Environmental Science
• Hydrology
• Water Quality Management

Background:

• Long-term water quality and hydrology data were collected over 8 years and 7 months in a paddy field watershed.
• A circular irrigation system was utilized, with annual rainfall varying significantly between 1270 and 2226 mm.
• Irrigation water amounts inversely correlated with rainfall, indicating adaptive water management practices.

Purpose of the Study:

• To investigate the long-term water quality and hydrological dynamics in a paddy field watershed.
• To quantify the annual outflow loads of total nitrogen (T-N), total phosphorus (T-P), and chemical oxygen demand (COD).
• To determine the factors influencing pollutant loads and the watershed's purification capacity.

Main Methods:

• Continuous monitoring of water quality parameters and hydrological data over an 8-year, 7-month period.
• Analysis of annual rainfall, irrigation water usage, and river flow.
• Calculation of annual outflow loads for T-N, T-P, and COD, considering irrigation and non-irrigation periods.

Main Results:

• Annual outflow loads ranged from 13.6 to 75.3 kg ha(-1)yr(-1) for T-N, -3.55 to 2.21 kg ha(-1)yr(-1) for T-P, and -24.7 to 48.5 kg ha(-1)yr(-1) for COD.
• Negative T-P and COD loads indicated a net purification function within the watershed.
• Hydrological water volumes were the primary driver for T-N and COD load variations, and partially for T-P.

Conclusions:

• The paddy field watershed exhibits a significant purification capacity for phosphorus and COD, and a partial capacity for nitrogen.
• Hydraulic retention time is a key factor enabling pollutant purification within the watershed.
• Water management strategies should consider the interplay between rainfall, irrigation, and pollutant transport for effective watershed management.