IRRIGATION UNIFORMITY OF LEVEL BASINS AS INFLUENCED BY VARIATIONS IN SOIL-WATER CONTENT AND SURFACE ELEVATION

Spatial variability in soils and surface elevation is an important consideration in the design and management of surface irrigated level basin systems. Field studies were conducted on a variable sandy loam soil in central Arizona to evaluate how the nonuniformities in in the antecedent soil water content (prior to irrigation) and field surface level affected the uniformity of infiltrated water for a level basin irrigation system. Relationships between the soil water content and the sand, silt and clay fractions were described for a 4.2-ha experimental site, which was separated into 12 basins and planted to wheat in 1985 and 1986. Measured water distribution uniformities (average low quater infiltrated depth/average infiltrated depth) were between 13-27% lower than that of an idealized uniformity of 98% where only infiltration opportunity time differences affect infiltrated depth uniformity. However, 0.51 to 0.68 of the variability in infiltrated depth could be explained differences in the antecedent soil water content (0-1.0 m soil depth), while as much as 0.23 of the infiltrated depth variability, and 0.60 of the variability in deep percolation, was explained by nonuniformities in surface elevation. For a properly designed level basin system on sandy loam, distribution uniformities of 85% or more should be attained when 85% of the basin elevation is within +/- 15 mm of the mean, provided the antecedent water content variability is on the order of 0.15 or less. However, those proposing to use level basins which have more extensive soil and water content variability should maintain a leveling precision, such that 100% of the basin is within +/- 15 mm to attain high distribution uniformities.