ABSTRACT: Fine-scale spatial variability of infiltration characteristics in‘nano-catchments’ during a rainfall event.

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Abstract

Current development practices are heavily altering infiltration rates and therefore aquifer recharge. Most recharge models rely on sparsely distributed monitoring stations to provide infiltration parameters, which are applied as representative values over large regions. In areas of complex terrain this could potentially create large bias in model results. To test this we measured infiltration characteristics during spring rainfall events at very fine spatial and temporal scales in two 'nano-catchments', sub-hectare bowl-like depressions with no surficial outlets, in southern Ontario: Glen Haffey (1,766m2, 4.5m relief) on the Oak Ridges Moraine, and Crawford Lake (6,233m2, 3.5m relief) on the Waterloo Aquifer. Saturated hydraulic conductivity KS was derived at each site using three sampling approaches. 1) By constant head permeameter with a single soil core 16.5cm diameter by 30cm. 2) Derivation by pedotransfer functions using three soil samples 5cm diameter by 5cm depth. 3) By direct measurement at five single ring infiltrometers. Mean KS at Glen Haffey ranged between 12.54 and 22.10 cm/hr across the methods, and mean KS at Crawford Lake ranged between 3.04 and 18.83 cm/hr. For each site we used the ProbeFusion mobile field data collection system developed in our lab to measure soil moisture ? over 15 cm and 30 cm depths at densities up to 200 points per day. Infiltration up to saturation FS was estimated at each point under varying rainfall intensities using the Mein and Larson (1973) model. Thiessen polygons were constructed around each data point and FS depth (cm) was calculated using the point's ? and the nearest KS, and a total FS volume (cm3) was aggregated for the bowl. The FS volume approach was used for the three KS estimate methods. For events up to 30 cm/hr, infiltration did not reach saturation for the dense ' ? method' whereas FS varied by a factor of 3 among the 'KS methods' at Glen Haffey and a factor of 10 at Crawford Lake. As rainfall intensity approached 60 cm/hr, FS from the ' ? method' and the five infiltrometers began to converge, but they were higher by a factor of 2 to 4 than the single permeameter or the three pedotransfer methods. The volumetric infiltration during a single rainfall cannot be fully represented by single point measurements, even at a sub-hectare scale. Regional infiltration process models can therefore have significant downward bias if they rely on sparsely distributed parameters.

Bibliography of Canadian Geomorphology