TOPOGRAPHIC EFFECTS ON SPRING WHEAT YIELDS AND WATER-USE

Water redistribution in a complex landscape needs to be quantified in order to determine field productivity. Landscape effects on water use and spring wheat (Triticum aestivum L.) yields were studied by monitoring two fields at each of two locations in North Dakota for 5 years. Four soil series at different positions within each field were monitored: the Zahl loam series (fine-loamy, mixed Entic Haploboroll) on hilltops and shoulder positions, the Williams loam series (fine-loamy, mixed Typic Argiboroll) on side slopes and hilltops, the Bowbells loam series (fine-loamy, mixed Pachic Haploboroll) on footslopes and toeslopes, and the Tonka silt loam series (fine, montmorillonitic, frigid Argiaquic Argialboll) in small undrained depressional areas. Topographic factors were calculated at each site by measuring the slope in four directions, 90-degrees apart, and adding the slopes together. If a slope was downward toward a site, it was considered positive, if a slope was upward toward a site it was considered negative. This number, if positive, would indicate that runon water would be added to the site and, if negative, water would be lost from the site due to runoff. Topographic factors were calculated 3, 6, 15, and 30 m from each site. As expected, wheat yields from the four soils were in the order Tonka > Bowbells > Williams > Zahl. Spring wheat grain yields ranged from 5.1 Mg ha-1 on Bowbells in 1985 to no yield in 1988 at the Underwood location. Spring wheat grain yields were generally correlated to total water use. When the topographic factor was added into the regression of yield vs. total water use, the coefficient of determination, R2, increased in the first 3 yr. The last 2 yr of the study were drought years and the topographic factor accurately reflected the lack of water redistribution in those years. Topographic factors measured 15 m from the site gave the highest R2 values.