The Nashwaak Experimental Watershed Project: Analysing effects of clearcutting on soil temperature, soil moisture, snowpack, snowmelt and stream flow

Abstract

A forest disturbance such as clearcutting affects local climate conditions by affecting surface reflectance, amount of snow catch, amount and timing of snowmelt and stream water discharge, soil moisture, soil temperature and soil frost. In this paper, snowpack water equivalents, soil moisture and soil temperature are simulated for pre- and post-harvest conditions at the Nashwaak Experimental Watershed Project. This Project involved a paired watershed experiment, with one of the forest-covered basins cut by way of a conventional harvesting operation, and the other basin left as a control. The computer simulations involved the application of a forest hydrology model (ForHyM) and a soil temperature model (ForSTeM). Both models used monthly amounts of rain and snow and mean monthly air temperatures records as input. Forest-atmosphere energy balances were calculated for year-round conditions. In this report, special attention was given to the energy balance when the ground is covered with snow. Doing so generated a process-oriented approach for calculating snowmelt throughout the winter and at the beginning of spring. Year-round model simulations for stream discharge were compared with pre- and post-harvest discharge observations. Also compared were simulated and observed snowpack water equivalents. Simulated results were in good agreement with field observations, thereby supporting the general calculations made for pre-and post-harvest soil moisture, temperature and frost conditions. Altogether, it was found that clearcutting produced positive as well as negative contributions to the water budget of the harvested basin. Positive contributions were likely due to reduced evapotranspiration. Negative contributions were likely due to reduced snow and fog water catch. Increased ground-level insolation advanced the snowmelt season for the cut basin by about two weeks.