Characterization of Summer Periods in Central-Western Oklahoma: Fallow Versus Double Crops.

Precipitation storage efficiency of summer fallow periods in central-western Oklahoma is low. Summer double crops following winter wheat are sometimes planted in an attempt to use summer rainfall more effectively. Nonetheless, summer double crops are often exposed to extreme water and/or heat stress. The objectives of this research were a) to characterize the climate of the summer period from June to October  and b) to calculate the probability of breaking even when planting double crops with different initial soil water contents after wheat.  Monthly and daily precipitation, reference evapotranspiration (ET_o), and air temperature data were analyzed for five locations across central-western Oklahoma. Aquacrop simulations were performed to predict soybean, grain sorghum, and sunflower double crop yields and evaporation (excluding transpiration) in 18 different scenarios. Average rainfall during the period of June-October was 413 mm whereas ET_o was 844 mm. July accounted for only 16% of summer rainfall but 52% of ET_o. During July one out of four days reached a temperature higher than 38 °C. Average non-productive evaporation during the growing season for all double crops and locations was 50% of growing season rainfall. In the case of summer fallow, non-productive water losses were at least 60% of summer rainfall. Due to high temperatures and evaporative demand, July is the most critical summer period when extreme heat and water stress are likely. Given our economic assumptions, Aquacrop simulations showed that soil water storage at double crop planting must be a minimum of 60% of the soil total available water capacity to ensure a 50% probability of breaking even when growing a summer double crop.