Automatic feedback control of precision irrigation scheduling allows the
determination of optimum soil water potential for crops, close determination of N
fertilizer requirements, and other measures of crop development and yield responses.
Soil water potential (SWP) was measured with granular matrix sensors (GMS;
Watermark Soil Moisture Sensors Model 200SS, Irrometer Co., Riverside, Calif.,
USA) at 0.2-m depth below onion and potato rows. Sensors were calibrated to SWP.
The GMS were connected to a datalogger (CR10X datalogger, Campbell Scientific,
Logan, Utah, USA) via five multiplexers (AM416 multiplexer, Campbell Scientific).
The datalogger was programmed to read the GMS in each irrigation treatment zone 4
to 8 times a day and irrigate each zone individually as necessary. Irrigations were
controlled by the datalogger signaling a controller (SDM-CD16AC controller,
Campbell Scientific) connected to solenoid valves for each zone. The pressure in the
drip lines was maintained at 69 kPa by a pressure regulator in each zone. The amount
of water applied in each zone was recorded by a water meter installed between the
solenoid valve and the drip tape. The irrigation criteria for onion (Allium cepa) and
potato (Solanum tuberosum) were determined for subsurface drip irrigation (SDI). N
fertilizer requirements may be reduced due to reduced N leaching under SDI.