Modelling soil erosion due to natural rainfall in Manitoba
Wright, Charles Glenn Ralph
Two water erosion sites, representing two soil types, a Gretna clay and Leary sandy loam, were monitored in 1991 and 1992. Data collection commenced in mid-April and ended in mid-September. Each site had four standard Universal Soil Loss Equation (USLE) erosion plots measuring 22.13 by 4.57 m on a uniform 9% slope. At each site four continuous crop-management system were represented: fallow, corn, wheat and alfalfa. Soil losses were measured using a l% Coshocton runoff sampling wheel. The rainfall-runoff erosivity factor (R) was measured with a tipping bucket rain gauge fitted with a digital recording device. Crop canopy and mulch cover measurements were taken weekly to determine the crop management (C) factor value. Soil erodibility (K) was calculated by dividing measured soil losses by the corresponding R factor value. In addition, antecedent soil moisture levels and runoff flow rates were monitored. The sediment sampling systems, runoff recorders and rain recording systems were tested to determine their reliability. Experimentally derived USLE factor values were compared to calculated USLE factor values for each site. Field testing of the sediment sampling systems and rain recording systems showed that, for the most part, soil losses and rainfall intensity and amounts were being measured accurately. However, high intensity segments occurring within some rainstorms occasionally exceeded the capacity of the rain recording systems. These 'cloud bursts' often accounted for a significant portion of total storm rainfall and produced large soil losses. Measured K values did not compare well with the calculated USLE nomograph K values for the experimental soils. Measured average annual R values from both sites were about 1117 MJ mm ha-1 h-1 which compared favourably to the value of 1160 MJ mm ha-1 h-1 obtained by Wall et al. (1982) for Winnipeg Manitoba. A comparison of USLE C values proved to be impossible due to fundamental differences between field measurements and the measurements required for determining USLE C factor values. Observed soil losses, soil loss ratios and soil erodibility values were extremely variable and were dependent upon rainfall characteristics, plot surface morphology and antecedent soil moisture levels. Multiple regression was used to establish relationships estimating soil losses and soil erodibility from soil and rainfall characteristics. Since these relationships had low coefficients of determination, it was concluded that more research is needed in order to develop field measurement techniques which will be useful in helping to describe the observed variability in individual storm soil losses.