Modeling Planting Available Water For Winter Wheat In The Southern Great Plains.

Available transpirable soil water at planting (ATSWp) is critical in determining early-season winter wheat establishment in continuous wheat systems, and is a mandatory variable in crop models because it describes the initial moisture condition of the soil profile. This study aimed to predict ATSWp by simulating the soil water dynamics of the preceding summer fallow period. A soil water balance using daily weather data was calibrated for the summer fallow periods of 2009, 2010, 2011, and 2012 at Lahoma, OK (36.39°N, 98.11°W). To test the influence of initial available transpirable water at the beginning of the summer fallow (ATSWi) in predicting ATSWp, prediction of ATSWp was performed for the same four fallow periods by arbitrarily setting the ATSWi ranging from permanent wilting point (-1500 kPa) to field capacity (-33 kPa). We also predicted ATSWp for ten locations across the central-western part of Oklahoma by setting ATSWi as 0.5 times the soil’s available water holding capacity and compared these values to measured ATSWp. Depth of soil profile was 120 cm. Model performance was accurate for calibration during the four summer fallows in Lahoma, with differences between measured and simulated ATSWp ranging from 2 to 14 % (2.3 to 20.5 mm) and root mean square error ranging from 13 to 36 mm. Starting fallow soil moisture simulations using ATSWi either at field capacity or wilting point resulted in similar ATSWp. In most cases, 40 to 60 days were needed for the difference between simulation using ATSWi in the dry and the wet range to become within 5% of each other, suggesting an independency of ATSWp and ATSWi. Model predictions of ATSWp were acceptable for nine out of the ten locations. The use of arbitrary ATSWi resulted in strong correlation between measured and simulated ATSWp values (r² = 0.82) and differences in ATSWp ranging from 0 to 37 mm. Results support that ATSWp is independent of ATSWi; therefore, ATSWp can often be reliably simulated running a daily soil water balance using arbitrary ATSWi provided that simulations start at least 60 days prior wheat planting.