2.5. Rainfall simulators
Portable rainfall simulators (Humphry et al., 2002) fitted with
plastic tarps to provide a wind screen were used to apply preirrigation
and runoff producing events to plots. Rainfall simulators
were equipped with a single Spraying Systems Fulljet HH50WSQ4
nozzle at a height of 3.0 m, were centered over plots, and had an
operating nozzle pressure of 28 kPa to yield a rainfall intensity of
70 mm h−1 (Humphry et al., 2002) with droplet size distribution
and velocity similar to natural rainfall. The water source for this
study was provided by filtered (5-m), deionized ground water
with temperatures ranging from 21 to 27 ◦C. Before pre-irrigation
and runoff collection events, the water was deionized (≥50 k
resistance; to simulate rainfall water quality) using cation and
anion exchange columns. Additional filtration was provided by an
in-line filter (5-m), located on each simulator upstream of the
pressure regulator to prevent foreign particles from clogging the
regulator and nozzle. Surface runoff was collected in metal gutters
at the down slope edge of each plot and pumped from the collector
bucket to a 190 L plastic barrel, which was lined with a clean plastic
liner. Gutters in all plots were sealed with a bentonite/soil mixture
and were washed of soil and clay before the rainfall simulations were initiated. During the experiment, runoff collection gutters
were covered to exclude water that had not fallen on the plot
surface. Total surface runoff volume was then measured, and the
bulk solution was stirred so that a proper subsample (1.9 L) could
be collected for laboratory analyses.