Hydraulic Conductivity of Central Peruvian Andean Peatlands

Hydraulic conductivity (k) strongly influences the movement of water through porous media, and is a key parameter quantified in hydrological studies. Peatlands, wetlands with organic soils, with different vegetation and hydrological regimes, may have distinct peat structure and levels of decomposition (humification). Head recovery tests are commonly used to calculate peat hydraulic conductivity. For slightly humified horizons the rigid soil theory provides high confidence estimations, but in more humified soil, estimations based on compressible media may be more accurate (Baird & Gaffney, 1994; Holden & Burt, 2003; Baird et al., 2005). Alpine peatlands in the central Peruvian Andes have high variability in groundwater level and support different vegetation types. To calculate the hydraulic conductivity of peatlands with different vegetation composition we performed a series of head recovery tests from slug withdrawal in piezometers completed at 50 to 150 cm depths at Nor Yauyos–Cochas Landscape Reserve. We calculated values of k using two approaches, a modification of the Bower & Rice equation (Cardenas & Zlotnik, 2002) assuming that peat acts as a non-compressible media, and the Brand & Premchitt (1982) model that assumes peat is a compressible porous media. Both approaches had similar results for the study sites with typical Andean cushion plant & pool structure dominated by the vascular plant Distichia muscoides. We measured k values between 5 x10^-5 to 3 x10^-7 m/s, with the slowest rates at the greatest depths. In areas where D. muscoides cushions were invaded by grasses we found strong divergence in the k values using both models. When the non-compressible media approach was used, k ranged from 6 x10^-4 to 2 x10^-8 and when a the model applied is that for compressible media it was 4 x10^-5 to 6x10^-8 m/s. This suggests that where the native cushion plant D. muscoides dominated the vegetation, soil humification was lower, and flow was predominantly Darcian and typical of non-compressible media. In contrast where the peatland has been invaded by grasses the increased soil humification and compressibility modified the time response of the head recovery process. Thus, soil compression is much greater in the grass dominated sites and clearly identifies that these ecosystems are in an advanced state of decomposition and collapse.