Three monitoring stations were established: stations U1 and D1
in the receiving river were set at the upstream and downstream of
A10 site, respectively; station A1 in the drainage canal was set at
the outlet of A10 site (Fig. 1b and c). Drainage from small agricultural watersheds with intensive livestock production as well as A10 flows into the river at a few points between stations U1 and
D1.
The amounts of rainfall and the water levels in the drainage
were recorded at A1 at 5-min intervals with a self-registering rain
gauge (Model 674; Isco Inc., Lincoln, NE, USA) and with a self-registering pressure-type water gauge system (MC1100W; Sensor Technik Sirnach AG, Sirnach, Switzerland), respectively. Water
depths and velocities were measured once a week.
Water samples were collected weekly (at 12:00) at stations U1
and D1 from January to December 2006 and daily (at 12:00) at station A1 from January 2004 to December 2006.Weekly data in 2006
were used to compare the nutrient concentrations among the stations. In addition, when the rainfall was >4 mm h1, drainage
water was collected at A1 at 30-min intervals for 12 h from January
to July 2004 and at 15-min intervals for 6 h from August to December 2004 using an automatic water sampling device (6400 Portable Sampler; Isco Inc.). Data summarized at 30-min intervals for 6 h were used to compare rainfall events. Rainwater was collected
when there was a rain in 2006.