Peat bogs receive water only from precipitation (input); it can be stored within the peat deposit or leave the peatland. This flow of water can be expressed by the following equation:
Water Input - Water Output = Water Storage
Water output in natural peatlands is dominated by evaporation, which accounts for more than 80% of water losses during the summer season. Small amounts of water are lost by surface or subsurface runoff. The difference between water input and output corresponds to the water that is stored in the peat deposit. Peat extraction affects water output and water storage. In currently harvested sites, drainage becomes more important and water storage decreases substantially. Thus, storing more water (limiting loss) is an important objective of peatland restoration.
Recent studies show that blocking drainage ditches can be very effective in limiting loss of water by runoff. The use of straw mulch in peatland restoration is also effective in reducing loss of water by evaporation. However, the loss of the acrotelm, subsidence following drainage and the decomposition of peat resulting from its exposure to air greatly reduce the water storing capacity of peat deposits in harvested peatlands. Hence, this situation must be balanced by keeping as much water as possible in restoration sites, specifically by building berms. It has been demonstrated that peat deposits swell following rewetting associated with restoration, suggesting that part of water storage capacity can be recovered in the short term.