Modeling the emergence of winter wheat in response to soil temperature, water potential, and planting depth

Seedling emergence is a critical stage in wheat (Triticum aestivum L.) establishment, particularly in dryland agriculture. Soil temperature, water potential, and planting depth are important factors influencing emergence. These factors can have considerable spatio-temporal variation, making it difficult to predict the timing and percentage of wheat seedling emergence. Our objectives were to: (1) measure the effect of soil temperature, water potential, and planting depth on seedling emergence of winter wheat; and (2) develop a population-based hydrothermal model to estimate time and percentage of winter wheat emergence planted at a given soil depth. The experiment was a 3 (wheat cultivars) × 3 (planting depths) × 5 (temperatures) × 5 (water potentials) factorial in a randomized complete block design (RCBD) with three replicates. Decreasing soil water potential from 0 to -1 MPa delayed emergence of Moro, Buchanan, and Rod by 13, 14, and 18 days after planting and prolonged the time to reach _maximum emergence by 19, 21, and 27 days after planting, respectively. Decreasing soil temperature from 25°C to 5°C delayed the average starting emergence of the three wheat cultivars by 19 days and delayed their average _maximum emergence by 25 days after planting. Increasing soil planting depth from 5 to 10 cm delayed starting emergence time for Moro, Buchanan, and Rod by 6, 4, and 7 days, while _maximum emergence was delayed by 12, 16, and 9 days after planting, respectively. Increasing the planting depth from 5 to 15 cm hindered the three cultivars from reaching 50% emergence. A hydrothermal time emergence model was developed that estimated observed emergence with 154 out of 207 data points having a root mean square error (RMSE) of ≤15%, whereas 19% of the simulated results did not exceed 19% RMSE and only 7% of the simulated data had RMSE greater than 20%. The simulations of the emergence model for _maximum percent seedling emergence (E_max) and number of days after planting to reach _maximum emergence (DAP_max) were compared with field data for three years and two cultivars (Rod and Moro). The emergence model gave reasonably good simulations of E_max and DAP_max for both winter wheat cultivars and in the three years of sowing dates.