Assessing the Impacts of Soil Conservation Practices on Surface Runoff and Water Quality Using an Agricultural Experimental Setup, Generalised Additive Mixed Models, and Hydrologic Modelling

Soil compaction and degraded soil structure can decrease water infiltration, increase surface runoff, and impact surface water quality. Soil conservation practices aim to protect soils from erosion and can restore soil physical and hydrological properties; however, their effect on restoring hydrological conditions at the field level is unknown. An agricultural experimental setup was established in 2020 in Saint-Lambert-de-Lauzon (near Quebec City, Canada) to assess the impact of soil conservation practices and soil compaction on surface runoff and water quality. The field site is composed of twelve 624 m² experimental plots in which the following are monitored: surface runoff and tile drainage, water quality (suspended solids, total phosphorus, nitrate and nitrite, dissolved metals), soil physical and chemical properties, and crop yields. The experimental design allows the comparison of four agricultural treatments: two compaction treatments (with and without soil compaction) and two conservation treatments (conventional and soil conservation agricultural practices). Each treatment is replicated three times. After a 3-year rotation cycle, analyses of monitored variables using Generalised Additive Mixed Models (GAMM) confirmed moderate but significant short-term capacity of soil conservation practices to reduce loads of suspended solids and nitrate and nitrite. Loads of total phosphorus were, however, not reduced significantly. Moreover, soil conservation practices were related to an unexpected increase in surface runoff during the spring flood. No effect was observed on soil properties and crop yields. An innovative methodological framework was explored to assess the long-term impacts of soil conservation practices on hydrology. The Soil and Water Assessment Tool model was set up and calibrated for each experimental plot to simulate water budgets and was run using restored soil physical conditions based on measurements conducted on surrounding unperturbed sites. Despite limitations in flow partitioning, simulations suggested that a restoration of soil physical properties could moderately reduce surface runoff at the plot scale in the long term. The study enhances understanding of local soil health, quantitative hydrology, and water quality processes, and demonstrates the potential of a new methodological framework to quantify the long-term benefits of soil conservation.