By Haiying Tao, Assistant Professor
Soil water recharge and storage is important in the inland Pacific Northwest because approximately 70% of the region’s precipitation occurs between October and March. Of this 70%, approximately 35% is in the form of snow. Trapping more snow can increase soil water storage for the growing season and also provide insulation to protect plants from winterkill.
In the unique, hilly topography of the Palouse, ridge tops and south-facing slopes generally retain the least amount of snowpack and valley areas retain the most. This variable snow distribution is mainly caused by wind induced snow drifting and snowmelt runoff. These differences in depth and duration of snowpack can cause substantial spatial variation in soil-water availability. In addition, the excessive water in areas with heavier snow accumulations can cause nitrogen loss from leaching, runoff, and denitrification.
The redistribution of snow by wind and water can be substantially reduced by leaving crop residue standing and practicing no-till or undercutter tillage. At all topographic locations, no-till retains more soil water with less spatial variation in snow depth than conventional tillage. Compared with chopped residue left on the soil surface, standing crop residue such as wheat and sunflower stubble is more effective not only for reducing wind speed and evaporation, but also for increasing snow catch. In no-till, snow catch generally increases as stubble height increases.
One research project, conducted in the Palouse near Pullman, Washington, compared a no-till field with a conventionally tilled field during multiple snow events and after snowmelt in spring. Compared with the conventionally tilled field, no-till ridge tops with standing wheat residue at 3.5 to 13 inches tall retained 3.9 to 4.7 inches more snow. Similarly, no-till south-facing slopes with standing wheat residue at the same height retained 3.9 to 5.5 inches more snow. By spring, the no-till field stored 2.4, 1.1, and 0.5 inches more water in the 5-foot soil profile at ridge tops, south-facing slopes, and valley locations, respectively, compared to the conventionally tilled field. Another project, a long-term study in Saskatchewan, Canada, concluded that leaving 35 to 47 inch-wide strips of standing stubble residue, about 16 to 24 inches tall every 20 feet, trapped 1.6 times more snow than shorter stubble at 12 inches tall.
Tall standing stubble, achieved by harvesting with a stripper header and leaving stubble at full-crop height, can conserve water and reduce residue decomposition rates. Tall standing stubble is especially important for sparse stands. A 4-year study conducted in a low-rainfall area near Ralston, Washington, found that this practice can reduce soil surface temperatures and also slow soil surface wind speeds to less than one half of the average. The stripper header-managed winter triticale stubble also preserved greater amounts of soil moisture and resulted in more uniform soil moisture conditions in the 0 to 3-inch seed zone.
These improved moisture conditions allow for timely planting and establishment of fall-seeded canola. Another 5-year study in Fort Collins, Colorado, also concluded that tall standing residue provides numerous benefits, including increased snow trapping; decreased decomposition rates, wind speeds, weed pressure, and soil temperatures during the fallow period; and substantially reduced within-field variation in snow cover and water storage.
These two articles discuss strategies of soil and water conservation in Pacific Northwest region: Crop Residue Management To Reduce Erosion and Improve Soil Quality and Soil and Water Challenges for Pacific Northwest Agriculture.
For questions, comments, or more information, please contact Dr. Haiying Tao (email@example.com), Assistant Professor in the Department of Crop and Soil Sciences at Washington State University.