NO-TILLAGE EFFECTS ON INFILTRATION, RUNOFF, AND WATER CONSERVATION ONDRYLAND

Dryland crops produce limited amounts of residue, thus the water conse rvation benefits of no-tillage (NT) may be reduced. We compared infilt ration, runoff, and water conservation effects of NT with stubblemulch (SM) tillage at Bushland, Texas, in the semiarid southern Great Plain s. From 1981 to 1992, we cropped six field-sized (2 to 4 ha) graded-te rraced watersheds in a dryland, three-year, winter wheat (Triticum aes tivum L.)-sorghum [Sorghum bicolor (L.) Moench]-fallow (WSF) sequence. There were three pairs of watersheds in the sequence, each with NT an d SM treatments maintained on the same watersheds each year. Runoff wa s measured with H-flumes from each watershed beginning in 1984. Infilt ration differences were measured with a rainfall simulator in 1990 and 1991. We compared soil water storage during fallow to evaluate the wa ter conservation effectiveness of tillage treatments. Terminal infiltr ation rates were similar for both tillage systems; however, infiltrati on rates declined much more rapidly with NT than with freshly tilled S M, primarily due to surface sealing even though residue coverage excee ded 50% on NT. Cumulative infiltration after 2 h of simulated rainfall was 90% greater on SM than on NT for tests conducted during fallow af ter sorghum, and 26% greater during fallow after wheat. Infiltration w as greater on SM because tillage destroyed the consolidated surface cr ust, decreased bulk density, and increased surface roughness and depre ssion storage capacity. The first artificial rainfall application comp acted and smoothed the surface on the SM treatment; thus, infiltration during subsequent tests was similar for both tillage systems. Storm r unoff measured with H-flumes averaged 25.5 and 40.1 mm/year for eight cycles of WSF for SM and NT treatments, respectively, with most runoff occurring during fallow periods. Despite greater surface runoff from the NT system than from the SM system, NT management resulted in impro ved water conservation due to reduced evaporation. Total plant availab le soil water storage during fallow after sorghum was 18% greater with the NT treatment and 10% greater during fallow after wheat than for s oil water storage obtained with the SM treatment. More intensive cropp ing with less fallow appears possible on drylands by using NT manageme nt.