Nitrate in tile drainage of the semiarid palouse basin

Topographically heterogeneous agricultural landscapes can complicate and accelerate agrochemical contamination of streams due to rapid transport of water and chemicals to poorly drained lower-landscape positions and shallow ground water. In the semiarid Palouse region, large parts of these landscapes have been tile drained to enhance crop yield. From 2000-2004 we monitored the discharge of a tile drain (TD) and a nearby profile of soil water for nitrate concentration ([NO₃]), electrical conductivity level (EC), and water content to develop a conceptual framework describing soil nitrate occurrence and loss via subsurface pathways. Tile-drain baseflow [NO₃] was consistently 4 mg N L^-1 and baseflow EC was 200 to 300 μS cm ^-1. Each year sudden synoptic increases in TD discharge and [NO ₃] occurred in early winter following ∼150 mm of fall precipitation, which saturated the soil and mobilized high-[NO₃] soil water throughout the profile. The greatest TD [NO₃] (20-30 mg N L^-1) occurred approximately contemporaneous with greatest TD discharges. The EC decrease each year (to ∼100 μS cm^-1) during high discharge, a dilution effect, lagged ∼1 mo behind the first appearance of high [NO₃] and was consistent with advective transport of low-EC water from the shallow profile under saturated conditions. Water-budget considerations and temporal [NO₃] patterns suggest that these processes deliver water to the TD from both lower- and upper-slope positions, the latter via lateral flow during the high-flow season. Management practices that reduce the fall reservoir of soil nitrate might be effective in reducing N loading to streams and shallow ground water in this region.