Saturday, 15 July 2006

Runoff, Sediment Loss, and Aggregate Stability Under Center Pivot Irrigation.

Gary A. Lehrsch and D. C. Kincaid. USDA-ARS, 3793 N 3600 E, Kimberly, ID 83341-5076

Sprinkler irrigation often causes runoff and erosion. Relatively high water application rates under center pivot outer spans usually exceed the infiltration rates of medium-textured soils, leading to runoff and sediment loss, particularly from sloping fields. We hypothesized that incorporated fresh (i.e., stockpiled) or composted dairy manure would reduce runoff and sediment loss, possibly by improving surface soil structure. Our field study had four treatments, plus a duplicated non-amended control, in each of six blocks, with each block situated under a different span of a lateral-move sprinkler system, simulating a center pivot. In October 1999, we incorporated 29 or 72 Mg ha-1 of dry manure or 22 or 47 Mg ha-1 of dry compost to a depth of 0.15 m into Portneuf silt loam (Durinodic Xeric Haplocalcid) on slopes from 0.8 to 4.4%. In Spring of 2000 and 2001 before irrigating and Fall of 2001 after irrigating, we collected surface soil (0-5 mm) from bed tops to determine aggregate stability. We applied 22 to 45 mm of water at an average application intensity of 28 mm h-1 to 6.4- x 36.6-m field plots six times in 2000 and twice in 2001. Additional non-monitored irrigations were made as needed to produce maize (Zea mays L.) silage each year. Using small trapezoidal flumes, we measured runoff rates and collected 1-L runoff samples at 15- to 30-minute intervals to determine sediment loss for each monitored irrigation. In general, neither amendment nor rate significantly affected runoff, sediment loss, or aggregate stability. At the highest amendment rate each year, however, runoff from compost-treated plots tended to be greater than from control plots while runoff from manure-treated plots tended to be less. As sediment loss increased, silage yield decreased at an increasing rate, possibly a consequence of water stress due to the loss of the higher water-holding Ap horizon, particularly from steeper slopes. Sediment loss decreased exponentially as aggregate stability increased. Stable surface aggregates were less prone to breakdown by sprinkler droplet kinetic energy, and consequently produced fewer aggregate fragments and primary particles, susceptible to detachment by sprinkler droplets and transport in runoff. As expected, plot slope greatly affected runoff and sediment loss. From slopes ≥2.5%, runoff per irrigation averaged 14.2% of the water applied, three-fold greater than from flatter slopes. Similarly, sediment loss per irrigation from slopes £2.5% averaged 244 kg ha-1, compared to 11 kg ha-1 from flatter slopes. Sediment loss and runoff were affected more by slope than by manure or compost.

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