Agricultural Water Management 165 (2016) 211–229 Contents lists available at ScienceDirect Agricultural Water Management jou rn al hom ep age: www.elsevier.com/locate/agwat Modeling water use, transpiration and soil evaporation of spring wheat–maize and spring wheat–sunflower relay intercropping using the dual crop coefficient approach Qingfeng Miao a , Ricardo D. Rosa b , Haibin Shi a,∗ , Paula Paredes b , Li Zhu a , Jiaxin Dai a , José M. Gonc ¸ alves b,c , Luis S. Pereira b a College of Water Conservancy and Civil Engineering, Inner Mongolia Agricultural University, Hohhot 010018, China b LEAF – Landscape, Environment, Agricultural and Food, Institute of Agronomy, University of Lisbon, Tapada da Ajuda, 1349-017 Lisbon, Portugal c Polytechnic Institute of Coimbra, College of Agriculture, Bencanta, 3045-601 Coimbra, Portugal a r t i c l e i n f o Article history: Received 16 July 2015 Received in revised form 13 October 2015 Accepted 27 October 2015 Available online 18 November 2015 Keywords: Basal crop coefficients Relay strip intercropping Yield advantage SIMDualKc model Hetao irrigation district Yellow River basin a b s t r a c t 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, K cb inter , which uses the daily values of the K cb 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 K cb 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 intercrop- ping systems were larger than those of the component sole crops, which led to significantly higher yields of intercropping relative to those of single crops. © 2015 Elsevier B.V. All rights reserved. 1. Introduction The Hetao irrigation district, located in the upper reaches of the Yellow River, in the Inner Mongolia Autonomous Region, is one of the three largest irrigation districts of China, with 0.570 Mha of irrigated land (Zhu et al., 2003; Xu et al., 2010; Yang et al., 2012). The main irrigation water source is a canal network supplied directly from the Yellow River at Dengkou. The crops growing sea- sons develop from April to October while the soil freezes for about 5 months, from November to March (Feng et al., 2005; Li et al., 2012). The average annual rainfall is about 150 mm, mainly concentrated ∗ Corresponding author. E-mail address: shi haibin@sohu.com (H. Shi). in summer (Xu et al., 2010; Yu et al., 2010), thus making that irriga- tion is essential for crop production. The major crops in the region are spring wheat (Triticum aestivum L.), maize (Zea mays L.) and sun- flower (Helianthus annuus L.) (Zhu et al., 2003; Du et al., 2011; Yang et al., 2012), which are cropped solely or in relay intercropped in alternate strips. Main challenges for sustainability refer to the need for adopting water saving irrigation (Pereira et al., 2003; Xu et al., 2010, 2011) while coping with soil salinity (Pereira et al., 2007; Wu et al., 2008; Yu et al., 2010). Intercropping is an ancient and traditional cropping system in China (Knörzer et al., 2009; Brooker et al., 2015) that still is widespread and is receiving increased attention in many irri- gated and water scarce areas (Zheng et al., 2010; Li et al., 2011b; Yang et al., 2011; Zhu et al., 2012; Fan et al., 2013; Gao et al., 2014). The interest for intercropping systems results from yield http://dx.doi.org/10.1016/j.agwat.2015.10.024 0378-3774/© 2015 Elsevier B.V. All rights reserved.