Sprinkler Irrigation Effects on Infiltration and Unsaturated Hydraulic Conductivity.

Sprinkler irrigation alters soil hydraulic properties both at and below the soil surface, yet its effects are not well characterized, particularly for the first irrigation of a tilled soil.  We evaluated the effects of a single sprinkler irrigation on infiltration and near-surface hydraulic conductivity measured under tension in a structurally weak, recently roller-harrowed Portneuf silt loam (Durinodic Xeric Haplocalcid). The experimental design was a randomized complete block with two treatments (pre- and post-irrigation) and four replications.  We used two half-circle spray heads to apply 127 mm of water at 70 mm h^-1 in one irrigation to duplicate 1 x 2 m plots.  Unconfined (three-dimensional) infiltration rates at steady-state were measured at potentials of ‑55, ‑35, then ‑15 mm of water before and about 10 days after irrigation.  Irrigation increased surface bulk density (0 to 34 mm) by 18% and increased the proportion of water-filled pores by 35% in the wet soil under the infiltrometer.  At ‑15 mm water potential, the unconfined infiltration rate was 53 mm h^-1 before, but 16 mm h^-1 after irrigation.  At ‑35 and ‑55 mm, irrigation decreased infiltration by 68%.  Irrigation also decreased infiltration nearly 5-fold through near-surface pores with diameters from 0.55 to 0.86 mm.  Irrigation tended to decrease hydraulic conductivity by 48%, on average, at each measured potential.  Sprinkler droplet kinetic energy consolidated unprotected soil and greatly reduced tension infiltration.  Our findings provide data needed to model hydraulic properties and will help producers manage infiltration and avoid runoff during a growing season when surface properties change as recently tilled soils are sprinkler irrigated.