Snow Catch and Soil Water Recharge in Standing Sunflower Residue

Sunflower (Helianthus annuus L.) is an economically viable crop for dryland rotations in the central Great Plains. Sunflower's ability to extract large amounts of soil water can depress subsequent crop yields unless significant soil water recharge occurs. The objectives of this study were to measure the effects of differences in standing sunflower stalk height and population on snow deposition and subsequent overwinter soil water recharge. Observations of snow depth and overwinter soil water recharge were made at seven sites near Akron, CO, over three winters. Sunflower stalks were laid flat after harvest, or left standing at approximately 18 or 28 in. Stalk population varied from approximately 10 600 to 26 100/acre. Silhouette factor (stalk height × diameter × population) ranged from 0 to 111 sq in./1600 sq in. Soil types in the study areas were Rago silt loam (fine, smectitic, mesic Pachic Argiustoll) or Weld silt loam (fine, smectitic, mesic Aridic Paleustoll). Increasing stalk height (and silhouette factor) increased snow catch and soil water content during the winter, but amounts of increase varied depending on wind speed and snow amount. With near-average snowfall amounts and number of drifting storms, soil water recharged linearly with increasing silhouette factor, thereby increasing stored soil water by nearly 5 in. where silhouette factor was 80 sq in./1600 sq in. Maintaining standing sunflower residue during the winter and spring following sunflower harvest can provide critically needed soil water recharge by increasing depth of snow deposition. Research QuestionSunflower offers central Great Plains growers an opportunity to diversify from the traditional winter wheat-fallow production system. Low soil water amounts after sunflower harvest require soil water recharge. The objectives of this study were to measure the effects of differences in stalk height and population on changes in snow depth and soil water during the winter and spring following sunflower harvest under the typically variable winter precipitation conditions of the central Great Plains. Literature SummaryStanding sunflower stalks have been shown to be effective at reducing wind speed near the soil surface. Approximately four snowstorms with significant drifting potential due to snowfall amount and wind speed occur per year in northeast Colorado. These drifting snow storms have the potential to interact with standing sunflower stalks to trap snow. Wheat residue has been shown to trap snow and increase soil water content over winter in Colorado, North Dakota, and Saskatchewan. Snow catch generally increases as wheat stubble height increases. Measurements in sunflower stalks in Saskatchewan and North Dakota showed snow depths ranging from 9 to 15 in., with melted water equivalent ranging from 2.8 to 3.5 in. Study DescriptionSeven sites with sunflower residue near Akron, CO, were observed during the winters of 1992–1993, 1993–1994, and 1994–1995. Sunflower stalks were laid flat after harvest, or left standing at either 17 to 19 in. or 26 to 29 in. Stalk populations varied from 10 600 to 26 100/acre. Snow depths were measured in the plot area following snow events. Soil water content was measured using a neutron probe and time-domain reflectometry. Applied QuestionsAre standing sunflower stalks effective at increasing snow catch and overwinter soil water content? For drifting storms, standing sunflower stalks can increase snow depth three to 10 times over areas with residues not standing, depending on the wind velocity associated with the snow storm and the silhouette factor of the standing stalks. The increased snow catch can result in increases in overwinter soil water content. For winters with approximately normal winter storm conditions for northeastern Colorado, producers could expect to see overwinter soil water increase by about 4 in. as the silhouette factor increases from 0 to 60 sq in./1600 sq in. (Fig. 1). Fig. 1Influence of sunflower silhouette factor on overwinter soil water change during three winters. RecommendationsProducers should leave sunflower stalks standing as tall as possible after harvest to potentially increase snow catch and overwinter soil water storage.