Mole drain performance in a clay loam soil in Ireland

Abstract

There is uncertainty regarding the suitability of some soils for mole drainage. Soils with questionable potential for the formation of stable mole channels generally have <45% clay and >20% sand, and gravel mole drainage is an alternative. However, this technique is unattractive due to its relative cost (2014 price: €1500–2800 ha−1 versus €125–300 ha−1). The performance of conventional mole drains and gravel mole drains was compared on a clay loam soil in the south of Ireland over 12 rainfall events. The installation dates of mole drainage treatments were staggered (January versus July) to assess how soil moisture status during installation affected performance. Treatments were (A) undrained control, (B) mole drainage installed in January 2011 (suboptimal), (C) mole drainage installed in July 2011 (optimal) and (D) gravel mole drainage installed in July 2011. Gravel mole drains were consistently more efficient in removing excess water than mole drains. Mean total drain flow (DF) during rainfall events was greater (P < .05, standard error 0.56 mm) in gravel mole drains (7.4 mm) than any mole drainage treatment irrespective of installation conditions (B [3.8 mm] and C [4.2 mm]). Initially, DF volumes exceeded overland flow (OF) in all treatments (Events 1–4, mean total OF was 0.47 [±0.08] of mean total DF). From Event 5 onwards, this relationship changed with OF being greater than DF (mean total OF was 2.02 [±0.41] of mean total DF). Results showed that across treatment replicates, the water table depth was consistently shallower in control plots. Mole drains were effective for a period of approximately 2 years. Although gravel mole drains were more effective, they were also subject to significant deterioration during the study. Installation conditions had no effect on mole drain performance. The results suggest that on this site conventional mole drainage repeated every 2 years could provide adequate drainage at low cost.