Modeling water use, transpiration and soil evaporation of spring wheat–maize and spring wheat–sunflower relay intercropping using the dual crop coefficient approach

Intercropping is commonly used in the Hetao irrigation district, upper Yellow River basin, where the main crops are wheat, maize and sunflower. For a better use of land, water, radiation energy, and nutrients, spring wheat is traditionally intercropped with maize (W–M) and, more recently, with sunflower (W–S). Considering the need to reduce diversions of water for irrigation in Hetao, this study aims to assess crop water use of W–M and W–S intercropping systems in comparison to the same crops in monocropping. The dual crop coefficient approach was adopted with the SIMDualKc model, which allows partitioning crop evapotranspiration into crop transpiration and soil evaporation. A new model approach was tested to compute a combined basal crop coefficient for the intercropping, Kcb inter, which uses the daily values of the Kcb of the component crops together with the respective heights and fractions of ground covered by the crops. SIMDualKc was first calibrated and validated for wheat, maize and sunflower as single crops using field data of 2010–2012 and was later used with the new developed approach, which was validated with W–M and W–S field data of 2010–2011. The Kcb values calibrated for wheat, maize and sunflower compared well with literature and goodness of fit indicators have shown high accuracy of simulations used for calibration and validation. Results evidenced the importance of groundwater contribution in Hetao, and that soil evaporation is there low under surface irrigation. Moreover, it was found that actual evapotranspiration, crop transpiration, irrigation water use and groundwater contribution of intercropping systems were larger than those of the component sole crops, which led to significantly higher yields of intercropping relative to those of single crops.