Graduate Theses & Dissertations

Modelling Monthly Water Balance
Water balance models calculate water storage and movement within drainage basins, a primary concern for many hydrologists. A Thornthwaite water balance model (H2OBAAS) has shown poor accuracy in predicting low flows in the Petawawa River basin in Ontario, so lake storage and winter snow processes were investigated to improve the accuracy of the model. Lake storage coefficients, represented by the slopes of lake stage vs. lake runoff relationships, were estimated for 19 lakes in the Petawawa River basin and compared on a seasonal and inter-lake basis to determine the factors controlling lake runoff behaviour. Storage coefficients varied between seasons, with spring having the highest coefficients, summer and fall having equal magnitude, and winter having the lowest coefficients. Storage coefficients showed positive correlation with lake watershed area, and negative correlation with lake surface area during summer, fall, and winter. Lake storage was integrated into the H2OBAAS and improved model accuracy, especially in late summer, with large increases in LogNSE, a statistical measure sensitive to low flows. However, varying storage coefficients with respect to seasonal lake storage, watershed area, and surface area did not improve runoff predictions in the model. Modified precipitation partitioning and snowmelt methods using monthly minimum and maximum temperatures were incorporated into the H2OBAAS and compared to the original methods, which used only average temperatures. Methods using temperature extremes greatly improved simulations of winter runoff and snow water equivalent, with the precipitation partitioning threshold being the most important model parameter. This study provides methods for improving low flow accuracy in a monthly water balance model through the incorporation of simple snow processes and lake storages. Author Keywords: Lake Storage, Model Calibration, Monthly Water Balance, Petawawa River, Precipitation Partitioning, Snow Melt
Groundwater Recharge in a Managed Forest on the Oak Ridges Moraine, Southern Ontario
Groundwater recharge was estimated and compared in two open grasslands, three mixed deciduous forest stands (100+ years in age), three young red pine plantations (27 ¨C 29 years in age) and two old red pine plantations (62 ¨C 63 years in age) on the Oak Ridges Moraine, southern Ontario, Canada. Recharge was estimated using a 1-d water balance with measured precipitation, throughfall, stemflow, snowpack water equivalent and soil water storage, and modelled evapotranspiration. Throughfall distribution beneath red pine canopies showed no consistent variation with distance from the tree boles. Old red pines were not major stemflow producers and although the young red pines showed a slight tendency to focus stemflow (focussing ratio > 1), the inclusion of focussed stemflow when calculating recharge at the stand scale made little difference. Conversely, sugar maple (the predominant species in the mixed deciduous stands) showed a strong tendency to focus throughfall proximal to tree boles and produce large quantities of stemflow, resulting in relatively high soil moisture contents and enhanced opportunities for recharge within ~ 0.5 m of tree boles. Inclusion of these focussed inputs resulted in a ~ 11 ¨C 18 % increase in stand scale recharge estimates. The interpretation of land cover control on recharge was complicated by variations in soil texture between sites. Soil texture and its influence on soil water storage capacity resulted in temporal variations in recharge, with sites exhibiting large storage capacities producing less recharge in the fall and greater recharge in the spring than sites with limited storage capacities. Recharge estimates for the entire study period or seasonal values for sites grouped on the basis of soil water storage capacities showed a general trend of increasing recharge in the order: old red pine ¡Ö young red pine ¡ú mixed deciduous forest ¡Ö open grasslands. The disparity between the red pine plantations and the other sites was driven in large part by greater modelled evapotranspiration in the red pine plantations. The similarity in recharge between mixed deciduous forests and open grasslands was the result of focused inputs and less soil evaporation and transpiration in the mixed deciduous forests compared to the open grasslands. The results of this study suggest planting red pine on grasslands on the Oak Ridges Moraine will initially decrease recharge and this decrease will continue as the red pines mature. However, as the red pine plantations are succeeded by mixed hardwood stands recharge will recover to that of the initial grasslands. Author Keywords: Groundwater Recharge, Land Cover Type, Oak Ridges Moraine, Stemflow, Throughfall, Water Balance

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