Seasonal Effects on Preferential Flow on Irish Grassland Soils
Gaelene Kramers1,2, Karl Richards1 and Nicholas M. Holden2
1 Teagasc, Environment Research Centre, Johnstown castle, Wexford, Ireland
2 UCD School of Agriculture, Food Science and Veterinary Medicine, University College Dublin
Earlsfort Terrace, Dublin 2, Ireland
gkramers@johnstown.teagasc.ie, kichards@johnstown.teagasc.ie, Nick.Holden@ucd.ie
Agricultural land in Ireland is dominated by permanent grassland (> 5 yrs grass growth) for both
grazing and silage. An important factor influencing water infiltration into grassland soils is preferential
flow through macropores. These can be biological macropores caused by soil fauna or the decay of
plant roots, or structural macropores caused by wetting and drying processes, freeze and thawing
cycles or subsurface erosion (Beven and Germann, 1982). Grassland in Ireland is usually ploughed
and reseeded every 7-10 years, if used for intensive livestock production. As only 2% of grassland by
area is reseeded each year, there is stable soil structure and biological activity, with rapid recovery
after disturbance in a well managed grassland where grazing is prohibited in the winter months and
other wet periods. The extent to which macropores occur in a soil under grassland will be influenced
by land management, climate and soil texture.
The Irish climate is characterized by mild temperatures with a wet winter and drier summer (Rohan,
1986). Grass growth and biological activity peak in the mild, wetter months of spring and slow down in
the warmer, drier months in summer. Preferential flow in grassland soils is usually initiated either at
the soil surface or in a near-saturated surface soil layer (Weiler and Naef, 2003). In Irish grassland
soils the water content of the surface root zone will normally be greater in spring than in summer
(Schulte et al., 2005). The seasonal difference in water content, evaporation, temperature and
biological activity in Irish grassland soils could have an impact on the extent of preferential flow
occurring at different times of the year. This study was designed to ascertain whether there is
seasonality associated with preferential flow in Irish grassland soils.
Using blue food dye (brilliant blue FCF), infiltration patterns will be studied on three soil types in spring
(March-April) and in summer (June-July). The three soils are a sandy loam, a loam, and a clay loam
which are all under permanent grassland. A continuous rainfall intensity of 4 mm/hr will be applied for
8 hours with a rainfall simulator in summer and in spring. The soil onto which the dye is applied will be
excavated and the infiltration patterns analysed using digital image analysis. The amount of large
macropores contributing to soil water flow as well as the actual infiltration patterns will be used to
determine whether there is any difference in the character of the infiltration between spring and
summer. The soils will be tested for water repellence during both experiments. Meteorological data
and soil temperature measurements will further enhance the dataset. Preliminary results will be
presented and discussed.
References
Beven, K. and P. F. Germann (1982). "Macropores and Water Flow in Soils." Water Resources
Research 18(5): 1311-1325.
Rohan, P. K. (1986). The climate of Ireland. Dublin, Meteorological Service, Ireland.
Schulte, R. P. O., J. Diamond, F. K., N. M. Holden and A. J. Brereton (2005). "Predicting the soil
moisture conditions of Irish Grasslands." Irish Journal of Agricultural and Food Research 44(1): 95110.
Weiler, M. and F. Naef (2003). "An experimental tracer study of the role of macropores in infiltration in
grassland soils." Hydrological Processes 17: 477-493.