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Visual drainage assessment: A standardized visual soil assessment method for use in land drainage design in Ireland

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan www.asabe.org

Citation:  2016 10th International Drainage Symposium Conference, 6-9 September 2016, Minneapolis, Minnesota  .(doi:10.13031/IDS.20162487020)
Authors:   Pat Tuohy, James Humphreys, Nicholas Holden, James O’ Loughlin, Brian Reidy, Owen Fenton
Keywords:   Drainage design, drainage systems, site-specific, visual soil assessment

Abstract. In Ireland, the implementation of site-specific land drainage system designs is usually disregarded by landowners in favor of locally established ‘standard practice‘ land drainage designs. This is due to a number of factors such as - a limited understanding of soil:water interactions, lack of facilities for the measurement of soil physical or hydrological parameters and perceived time wastage and high costs. There is a need for a site-specific drainage system design methodology which does not rely on inaccessible, time-consuming and/or expensive measurements of soil physical or hydrological properties. This requires a standardized process for deciphering the drainage characteristics of a given soil in the field. As an initial step, a new visual soil assessment method, referred to as visual drainage assessment (VDA), is presented whereby an approximation of the permeability of specific soil horizons is made using seven indicators (water seepage, pan layers, texture, porosity, consistence, stone content and root development) to provide a basis for the design of a site-specific drainage system. Across six poorly drained sites (1.3 ha to 2.6 ha in size) in south-west Ireland a VDA-based design was compared with (i) an ideal design (utilizing soil physical measurements to elucidate soil hydraulic parameters) and (ii) a standard design (0.8 m deep drains at a 15 m spacing) by model estimate of watertable control and rainfall recharge/drain discharge capacity. The VDA method, unlike standard design equivalents, provided a good approximation of an ideal (from measured hydrological properties) design and prescribed an almost equivalent land drainage system in the field. Mean modeled rainfall recharge/drain discharge capacity for the VDA (13.3 mm/day) and ideal (12.0 mm/day) designs were significantly higher (P<0.001, s.e. 1.42 mm/day) than for the standard designs (0.5 mm/day), when assuming a design minimum watertable depth of 0.45 m.

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