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This thesis is in four parts that report on the development of two procedures for the prediction of trafficability of tile-drained lowland soils in the Lower Fraser Valley of British Columbia. By these procedures one can predict trafficability either from soil water tension in the plough layer or from water table depth. A mathematical model developed in the Netherlands for the simulation of vertical non-steady flow of water in soils was tested by data collected in Spring from farmers' fields. The model predicted depth to the water table and, less accurately, tension in the top 15 cm in fields drained by tiles spaced at 30.5 m and 15.2 m. Cone penetration resistance (an index of soil strength) of soil in the field was found to be linearly dependent upon soil water tension between 0 and 100 - 175 cm of water. It was possible to predict the slope of this relationship for two mineral soils, but not for an organic soil. Trafficability tests with typical farm vehicles were carried out on one organic and two mineral soils at various degrees of wetness. Soil structure was significantly damaged after the first and third passes of the vehicles. The damage was greatest when the soil was near saturation. However, indices of structure could not be used per se as criteria for trafficable conditions. For each soil a relationship was established between soil strength and traction efficiency measured by wheelslip. A critical value of strength for trafficability was then obtained by using 20% wheelslip as a limiting value for traction efficiency. Reference to known strength-tension curves yielded critical tensions for trafficability. Soil strength was linearly dependent upon water table depth in Spring when evapotranspiration was small and when water table depth was less than 80 cm. Critical water table depths for trafficability inferred from this relationship were 53, 45, and 60 cm for Lumbum muck, Hallart silty clay loam (grassland), and Hallart silty clay loam (cultivated land), respectively. The effect of tile spacing on trafficability in Spring was assessed over a 2-year period. Evidence to support increased benefits due to the closer spacing was inconclusive in both soil types. When compared to undrained land the larger spacing of 30.5 m resulted in a significant increase in the number of trafficable days. Tile drainage was twice as important for the attainment of trafficable conditions in the muck as in the mineral soils. Two procedures for predicting trafficability of tile-drained farmland were described. Predictions by these procedures for two tile spacings in both soil types in Spring 1977 were within 3 days of one another and compared extremely well with dates on which measured water table depth and tension were considered adequate for trafficability. / Land and Food Systems, Faculty of / Graduate

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