Sphagnum farming provides a sustainable wet land use alternative for drained peatland agriculture. Since 2011 Sphagnum has been cultivated on formerly drained bog grassland at Hankhauser Moor in north-west Germany. The site has been rewetted and is equipped with an automatic irrigation system which controls the inflow and outflow of water. We used measurements and models to determine the amount of irrigation water needed to keep the water table just below the Sphagnum surface throughout the year. In winter there was a water surplus, whereas in spring and summer evapotranspiration exceeded precipitation, creating a water deficit. Next to evapotranspiration, horizontal seepage was a major cause of water loss. Modelling indicated that the amount of irrigation required to make up the water deficit in the dry hydrological year 2013 (November 2012 to October 2013) amounted to 359 mm (3,588 m³ ha-1). To compensate for the average water deficit over the 20-year period 1993–2013, 160 mm of water would be needed annually (i.e. 1,600 m³ ha-1 yr-1), but the maximum water deficit accumulated during that period was much higher, at 6,363 m³ ha-1. In relative terms, both evapotranspiration and seepage losses will decrease with increasing size of the rewetted area because drained surroundings enhance water losses from the wetter Sphagnum farming site, both as evapotranspiration due to advection (oasis effect) and as horizontal seepage due to steepening of hydraulic gradients. For successful Sphagnum farming the water demand must be considered and an appropriate water supply must be guaranteed.
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