HR: 1400h
AN: H33B-04    [Abstracts]
TI: Influence of Antecedent Moisture and Rainfall Rate on the Hydrologic Response of and Nitrate Leaching from Intact Soil Monoliths
AU: * Lewis, M P
EM: mlewis@envmail.uwaterloo.ca
AF: Department of Geography, University of Waterloo, 200 University Ave West, Waterloo, ON N2L3G1, Canada
AU: Macrae, M L
EM: mmacrae@envmail.uwaterloo.ca
AF: Department of Geography, University of Waterloo, 200 University Ave West, Waterloo, ON N2L3G1, Canada
AU: Bourbonniere, R A
EM: Rick.Boubonniere@ec.gc.ca
AF: Canada Centre for Inland Waters, Environment Canada, 867 Lakeshore Road, Burlington, ON L7R4A6, Canada
AB: The transfer of excess nitrate from agricultural soil during and following rainfall events is a major environmental problem in many regions worldwide. Although field and modelling studies have linked discharge and nutrient export from catchments to spatial variables such as soil type and agricultural management practices, less is known about temporal variability in nutrient export. Several field studies have shown that the hydrologic response of catchments to storm events differs with variable antecedent soil moisture conditions as well as storm type/rainfall rate. Variable hydrologic responses are in turn, expected to affect the export of nutrients such as nitrate from agricultural landscapes. This research uses an experimental approach to characterize the combined influence of antecedent soil moisture and rainfall rate on runoff quantity and quality from agricultural soil. Two sets of experiments were performed on large, intact soil monoliths in a laboratory setting, where soil monoliths were subject to three moisture treatments (dry, moist, wet/field capacity) and two different rainfall simulations (2.5mm/h and 20.5mm/h). Under high rainfall rates, nitrate export for the event ranged from 1125mg m^-2 (wet soil) to 81.5mg m^-2 (dry soil) and ranged from 1053mg m^-2 (wet soil) to 429.1mg m^-2 (dry soil) under low rainfall rates. Antecedent moisture conditions had a greater effect on the depth of runoff and the total mass of nitrate leached than rainfall rate, with wetter soils producing more runoff (runoff ratios = 0.37, 0.33, respectively for the 2.5mm/h and 20.5mm/h events) and causing more nitrate export (1125 and 1053 mg m^-2 event^-1) than dry soil (runoff ratios = 0.033 and 0.105; nitrate export = 89.2, 429.1 mg m^-2 event^-1). The soil moisture response and the progression of the wetting front occurred earliest in wet soil and latest in dry soil, as illustrated by runoff volumes and soil moisture profiles recorded throughout both rainfall simulations. Patterns differed between high rainfall rates and low rainfall rates in terms of the flow- weighted export of nitate. Following high rates of rainfall, wet soil produced higher flow-weighted nitrate export (128.5mg m^-2 mm^-1), whereas dry soil produced higher flow-weighted nitrate (155.6mg m^-2 mm^-1) export at low rainfall rates. Moisture profiles showed a clear contrast in the progression of the wetting front between high and low rainfall rates. Matrix flow, utilizing a wide array of pore sizes, dominated during low rainfall rates at all three moisture levels and high rainfall rates with dry soil, whereas preferential transport appeared to dominate during high rainfall rates. Wet soils under high rainfall rates, which produced the most preferential flow, produced both the highest total mass of nitrate export and the highest flow-weighted nitrate export. In general, antecedent soil moisture had a greater effect on the total mass of nitrate leached than rainfall rates. The findings of these experiments will increase the understanding of the relationship between antecedent soil moisture, rainfall rates and leaching behaviour in agricultural soils and will help guide best management practices in the management of soil nutrients for agriculture.
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
DE: 0469 Nitrogen cycling
DE: 0496 Water quality
DE: 1865 Soils (0486)
DE: 1875 Vadose zone
SC: Hydrology [H]
MN: 2009 Joint Assembly