Submitted to: United States Committee of Irrigation and Drainage Engineering Conference
Publication Type: Abstract Only
Publication Acceptance Date: July 1, 1997
Publication Date: N/A
Technical Abstract: Most of the irrigated land in Southern Idaho is a Portneuf silt loam varying in depth from 1 to 10 m over fractured basalt. Some studies have indicated annual overwinter losses from the soil profile of 50 to 150 kg/ha of nitrate, presumably by deep drainage. However, monitoring of groundwater and drainage water indicates little increase in nitrate concentrations. This study evaluated the downward movement of water and several anions (nitrate, Cl, and Br) at an irrigated field site over a 3-year period to determine the range of nitrate losses and possibly identify the causative mechanisms. Potassium bromide was incorporated into the top 20 cm of soil at the start of the study to serve as a conservative tracer. Soil sampling and soil solutions obtained from soil solution suction tubes at several depths to the bedrock monitored the downward movement of the anions. Soil water movement was determined with a soil water balance utilizing neutron meter data and evapotranspiration estimating techniques. The site was sprinkler irrigated sufficiently to provide some deep soil water drainage. Results indicate a rather slow downward movement of Br and nitrate. Nearly 36 months were required to move most of the applied Br from the sampled portion of the profile, even though drainage exceeded 300 mm. Water and nitrate movement out of the lower profile appear restricted even though a perched water table was never evident. Apparently the anions moved in the smaller soil pores, while most of the water moved in the larger pores via "bypass" flow in this soil.