Experimental design and evaluation of a peatland drainage system for forestry by optimization of synthetic hydrographs

Abstract

A ditch system was designed, constructed, and evaluated on a 1-km2 experimental plot in peat-covered wetland in Slave Lake Forest, Alberta, Canada, during 1981–1985. The purpose of the experiment was the development of techniques and procedures for the design of drainage systems for water-table control in large tracts of forested peatlands of northern Alberta, in order to enhance tree growth. The technical objective of the envisaged drainage systems is to prevent the water table from rising above a specified depth, or drainage norm νm, below the land surface, for a continuous time interval longer than a specified flood duration limit λm, during the growing season. The position of the water table and the rate and amplitude of its fluctuations in a ditched area depend on the environmental parameters (K, hydraulic conductivity; S*, field coefficient of water-table response to precipitation; z, depth to the first effectively impermeable stratum; P, pattern of daily rainfall) the design parameters (L, ditch spacing; u, ditch depth), and at later stages of drainage, the effect of the tree stand. The objective of the design is, therefore, to determine appropriate values of L and u as functions of the environmental parameters to be used in the construction of operational drainage systems. The central component of the design procedure is the optimization of synthetic hydrographs. The synthetic hydrograph is a graphical representation of calculated time series of the water table's fluctuations reflecting the effect of discrete rainfall events under specified drainage conditions. Synthetic hydrographs were calculated with the U.S. Bureau of Reclamation drain spacing formula adapted here for daily rainfall events and ranges of estimated environmental and assumed design parameters. An experimental drainage system was constructed with preliminary design parameters that were expected to satisfy the criteria νm and λm. In addition, 14 water-table observation wells and a rain gauge were installed and operated for two summers. Synthetic hydrographs were then computed using the actually implemented design parameters, the actual rainfall pattern and a fixed z value, leaving K and S* as the only unknown variables. Those K and S* values giving the best approximation of calculated to observed hydrographs were considered to represent the effective field values and were used in turn to calculate the final design parameters, again by hydrograph matching. Final values for K and S* for two subregions of the experimental plot were found to be K1 = 0.37 m/d, S1* = 0.13; and K2 = 0.26 m/d, S2* = 0.11. For subregion No. 2 a ditch spacing L = 25 m and ditch depth u = 0.9 m satisfy the stipulated drainage norm νm = 0.4 m and flood duration limit λm = 14 days, using P1962, which was the wettest year for the last 28 years in the region.