Sediment and phosphorus transport in irrigation furrows.

Sediment and phosphorus (P) in agricultural runoff can impair water quality in streams, lakes, and rivers. We studied the factors affecting P transfer and transport in irrigated furrows in six freshly tilled fallow fields, 110 to 180 m long with 0.007 to 0.012 m m-1 slopes without the interference of raindrops or sheet flow that occur during natural or simulated rain. The soil on all fields was Portneuf silt loam (coarse-silty, mixed, superactive, mesic Durinodic Xeric Haplocalcids). Flow rate, sediment concentration, and P concentrations were monitored at four, equally spaced locations in each furrow. Flow rate decreased with distance down the furrow as water infiltrated. Sediment concentration varied with distance and time with no set pattern. Total P concentrations related directly to sediment concentrations (r2=0.75) because typically >90% of the transported P was particulate P, emphasizing the need to control erosion to reduce P loss. Dissolved reactive phosphorus (DRP) concentrations decreased with time at a specific furrow site but increased with distance down the furrow as contact time with soil and suspended sediment increased. The DRP concentration correlated better with sediment concentration than extractable furrow soil P concentration. However, suspended sediment concentration tended to not affect DRP concentration later in the irrigation (>2 h). These results indicate that the effects of soil P can be overshadowed by differences in flow hydraulics, suspended sediment loads, and non-equilibrium conditions.