The effect of wetting history, botanical composition and depth on the specific yield of two common types of bog peat

Abstract

AbstractSpecific yield affects how much the water table rises and falls in response to rainfall, to evaporation and to seepage gains and losses. It also affects the aeration of the soil above the water table. Although widely measured in peat soils, the effect of wetting history on its value has not been investigated. Specific yield has been estimated in many studies by cutting peat cores into layers, and measuring how much water drains from the individual layers after they have been wetted. Specific yield, however, is a phenomenon of the soil profile and not subsections, so this method may not provide a reliable estimate of its value. Few studies have reported on the effect of botanical composition on specific yield, or at least have not controlled for the effect by keeping other peat properties like degree of humification constant. We addressed these questions by measuring specific yield in intact cores of peat in a series of laboratory experiments. Two common but contrasting types of Sphagnum peat were investigated that had similar degrees of humification: Sphagnum medium peat and Sphagnum pulchrum peat. We found that specific yield was highly variable, ranging between 0.16 and 0.62. Specific yield was not affected by wetting history, but was significantly different between the peat types, being on average 0.21 higher in the S. pulchrum cores. Specific yield did not vary with depth in the S. medium cores but declined linearly with depth at a rate of 0.018 per cm in the Sphagnum pulchrum cores. Finally, we found that drainage from the peat profile above the zone through which the water table falls is an important component of specific yield, contributing more than 66%–91% of its value in the S. pulchrum peat. Our results show that wetting history probably does not need to be accounted for when estimating specific yield, although further work on this potential effect is recommended. Our work highlights the importance of measuring specific yield using intact cores (field methods may also be appropriate) and suggests that many previous peatland studies may have underestimated its value.