A case study of runoff and sediment yield in areas subjected to different forest thinning operations in a northern New Mexico forest

Forest thinning practices play a fundamental role in natural resource management and are used to reduce forest density, particularly in the southwestern United States. Increased forest density can significantly influence fuel loads and subsequent fire severity. Moderate and high fire severity may increase surface runoff and sediment yield during postfire rainfall events. This study was conducted to evaluate the effects of thinning treatments on runoff and sediment yield in northern New Mexico forests. Four cover types were tested to evaluate how they influenced runoff and sedimentation. The four cover types included control (nonthinned), interspace (between slash piles), lop-scatter (slash was scattered and burned), and pile (slash was piled and burned). In addition, each cover type was tested on two categories of slopes: mild slopes (<5%) and moderate slopes (5% to 20%). Rainfall simulations (~16 cm h^–1) for both dry and wet runs were used to measure runoff and sediment yield in 2015 and 2017. Results showed pile treatments had significantly higher time to runoff initiation than did control plots during dry runs. Time to peak runoff was significantly different in mild slope (38.1 min) than moderate slope (25.4 min) in the dry run. Wet run time to peak runoff was found to be greater under the pile treatment as compared to other treatments. Sediment yield was significantly greater on moderate slopes in pile and lop-scatter treatments as compared to control for dry runs. For wet runs, a significant effect was detected between slopes; moderate (greater) slopes produced greater sediment than mild (lesser) slopes. The pile treatments may provide beneficial impacts for watersheds despite the delayed peak runoff in response to rainfall on milder slopes. However, high sediment yield with runoff for pile thinning treatments in steeper slope locations will be important to include when evaluating management impacts on natural resources in the future. Overall, this case study shows that thinning activities in conifer forests of New Mexico have the potential to impact the hydrological function of the watershed and should be considered as part of an overall forest and watershed management strategy.