Changes in Infiltration Under Alfalfa as Influenced by Time and Wheel Traffic

Infiltration rates were measured for alfalfa, (Medicago saliva L., cv. WL514) subjected to treatments where wheel traffic was varied in terms of area covered and time of application on a Wasco sandy loam (coarse-loamy, mixed, nonacid thermic Xeric Torriorthent). Traffic treatments were (i) No-traffic, (ii) Preplant, MO Repeated, and (iv) traffic similar to what a grower would apply. Infiltration rates increased for all treatments, with increases being 240 % for treatments without harvest traffic and 140 % for treatments with har-vest traffic Increases in infiltration were related to decreases in stand density. Slight packing (traffic) applied before the soil was flood-irrigated in 1983 increased infiltration rates 20 % compared to flooding loosened soil (no traffic). Harvest traffic resulted in slower water movement in the soil. I NTHE IRRIGATED southwestern USA, a major effect of soil compaction is reduction in the infiltration rate, resulting in insufficient water being available for optimum crop growth (Goldhammer and Peterson, 1984). Entry of water into soil occurs by two processes: (i) piston-like flow through the soil matrix, and (ii) flow through macropores without displacement of ma-trix water (Thomas and Phillips, 1979). Bulk density and soil texture greatly affect piston-like flow of water through soil. Patel and Singh (1981) reported that if the bulk density in a coarse-textured soil was increased from 1.7 to 1.9 Mg m- 3, hydraulic conductivity decreased from 1.4 X 10- 3 to 5.5 X 10-' mm s Soil bulk density has little or no relationship to water flow through macropores. Macropores are formed in cropped soils as a result of decomposition of roots (Barley, 1954), wetting an