Nonpoint source (NPS) pollution contributed from free-range chicken farms (FRCF) is an important source
for total phosphorus loads in rural watersheds of southern Taiwan. The main objective of this study is to
quantify the impact of FRCF on water quality parameters. Different models were used; first, a distributed
rainfall-runoff model and water quality models were pre-selected. Their relevant parameters were then
calibrated and verified based on collected precipitation records, along with runoff and water quality data.
Since the raising cycle of the target chicken farm, located at Longchi Township in southern Taiwan, was
required for the estimation of NPS loads, three raising cycles per year were used in the case study. By
using water quality models together with relevant parameter values, the unit pollution loads per raising
cycle during the rainfall period were calculated. The simulated results indicate that the higher the rainfall
is, the higher the unit pollution load per raising cycle will be; confirming that the NPS pollution is highly
dependent on the change of seasons. The finding clearly indicates that the FRCF exerts high potential
threat to reservoir.