UBC Theses and Dissertations
Runoff generation and water erosion in the uplands of the Lower Fraser Valley Brown, Sandra J.
An understanding of runoff generation is a requirement for efficient erosion control and land management practices. This research is designed to investigate the processes by which runoff occurs on a Whatcom soil at an upland site in the Lower Fraser Valley. The objectives of this study are to summarize water erosion measurements, to determine soil hydraulic properties, to predict infiltration for typical rainstorms and to determine the mechanism of runoff generation. Rill, interrill and rainsplash measurements, and previous erosion measurements made at the study site are used to qualitatively assess the magnitude of water erosion. Soil loss is dominated by rill erosion and erosion rates are greatest from November to January. Runoff coefficients are relatively low (<26%), but erosion rates are anticipated to be in excess of 35-45 t ha⁻¹yr⁻¹. Soil hydraulic properties are measured using a low tension absorption technique described by Clothier and White (1981). The sorptivity tube device provides a simple method for obtaining S, λ* and K₋₂ Measured values of i and x* depend on t¹′² as expected from the constant-concentration absorption theory. Soil hydraulic variables and constant-concentration absorption theory are used to determine the soil-water diffusivity characteristics. Measured D(θ) functions for the field varied widely in a and b, especially for the lower horizons. The Van Genuchten (1980) θ(h) expression provides a good fit to the water retention data. D(θ) functions predicted from the soil-water retention curves and the conductivity at satiation are at least 1-2 orders of magnitude greater than the measured functions. Hysteresis effects may account for a 1 order of magnitude increase in D(θ), but the predictions are poor without matching at D(θ₅). The measured hydraulic properties are used to predict infiltration, runoff and drainage. Constant-flux infiltration theory is used to model soil moisture profiles for various rainfall events. High field moisture contents necessitate the inclusion of gravity effects during infiltration. Rainfall, runoff and soil loss measurements at the Mahal farm indicate that rainfall intensities <10 mm h⁻¹ cause considerable runoff and erosion. For rainfall intensities known to cause runoff and erosion, runoff is not predicted for most Ap horizon cores. Infiltration may be restricted at some sites by the lower horizon but runoff is not predicted at many sites. To evaluated soil moisture conditions between rainfall events, drainage is estimated from a unit gradient model. A rapid decrease in θ between rainfall events predicted by the unit gradient model, but θ measured at the Mahal farm suggests limited drainage between rainfall events. Rainfall, runoff and soil loss measurements suggest that factors other than those measured are contributing to the runoff observed in the field. Compaction, surface sealing and subsurface seepage may be factors influencing runoff and moisture conditions in the field. The high moisture contents observed in the field suggest the water table is perching on a layer of lower conductivity which in consistent with Dunne runoff. However, the measured infiltrabilities suggest Hortonian runoff and the resultes of the thesis are not conclusive with respect to the mechanism of runoff generation.