Agricultural Water Management 117 (2013) 93–105 Contents lists available at SciVerse ScienceDirect Agricultural Water Management j ourna l ho me page: www.elsevier.com/locate/agwat Dual crop coefficient modelling applied to the winter wheat–summer maize crop sequence in North China Plain: Basal crop coefficients and soil evaporation component Nana Zhao a , Yu Liu a , Jiabing Cai a , Paula Paredes b , Ricardo D. Rosa b , Luis S. Pereira b,∗ a State Key Laboratory of Simulation and Regulation of Water Cycle in River Basin, China Institute of Water Resources and Hydropower Research, Beijing 100038, China b CEER-Biosystems Engineering, Institute of Agronomy, Technical University of Lisbon, Tapada da Ajuda, 1349-017 Lisbon, Portugal a r t i c l e i n f o Article history: Received 29 June 2012 Accepted 15 November 2012 Available online 12 December 2012 Keywords: SIMDualKc model Basal crop coefficients Ratio evaporation to evapotranspiration Microlysimeters TDR Soil water balance a b s t r a c t The dual crop coefficient (K c ) approach to estimate crop evapotranspiration (ET c ) separately considers soil evaporation (E) and plant transpiration (T) by computing a soil evaporation coefficient (K e ) and a basal crop coefficient (K cb ), respectively, with K c = K e + K cb . This approach may be more precise than the single K c approach particularly when the crops incompletely cover the ground. The SIMDualKc model, which is adopted in this study, is an irrigation scheduling simulation model that uses a daily time-step for performing two separate soil water balances, one for the soil evaporation layer from which K e is computed, and the other for the entire root zone, thus allowing to compute the actual K cb adjusted to the soil moisture conditions (K cb adj ). The standard K cb is corrected to the climate, crop density and height. Two years of field experimental data relative to winter wheat and summer maize were used for model calibration and validation using soil water content data observed with time-domain reflectometry (TDR) in a silt loam soil. Field data also include E measured with microlysimeters placed along the crop rows. The calibration procedure consisted in adjusting the basal crop coefficients, the soil evaporation parameters used to compute K e , and the soil water depletion fraction for no stress (p) to achieve the best fit of the observed soil water content data. The calibrated K cb values for winter wheat were 0.25 for the initial and the soil frozen period, 1.15 for the mid-season and 0.30 at harvesting. For the summer maize, the initial, mid season and end season K cb were respectively 0.2, 1.10 and 0.45. Model results have shown a good agreement between model predictions and field observations of the soil water content of both crops, with root mean square errors of estimates (RMSE) of about 0.01 m 3 m -3 for both the calibration and validation. The modelling efficiency EF and the index of agreement d IA were larger than 0.96 and 0.99, respectively, thus indicating good performance of modelling with SIMDualKc. Model estimates of E using Ritchie’s approach were compared with microlysimeter data; for winter wheat a RMSE = 0.37 mm d -1 was obtained, while for maize RMSE of 0.45 and 0.49 mm d -1 were obtained for both years of observations. Results for soil evaporation allow confirming the appropriateness of using Ritchie’s model to estimate soil evaporation of a cropped soil. E averaged 124 mm for wheat, representing 29% of ET c , and 146 mm for summer maize, i.e. 41% of ET c . In conclusion, results show that the model is appropriate to simulate the soil water balance adopting the dual K c approach and may be further used to develop improved irrigation schedules for the winter wheat–summer maize crop sequence in North China. © 2012 Elsevier B.V. All rights reserved. 1. Introduction The main crops in the North China Plain are irrigated winter wheat followed by summer maize, generally rainfed. Numerous studies refer to this crop sequence and various approximations were developed to assess the respective crop coefficients (Liu et al., 1998; Liu and Pereira, 2000; Kang et al., 2003), to develop ∗ Corresponding author. Tel.: +351 213653339; fax: +351 213653287. E-mail address: lspereira@isa.utl.pt (L.S. Pereira). appropriate irrigation scheduling programmes (Liu et al., 2000; Li et al., 2005, 2011; Shang and Mao, 2006), to assess impacts and feasibility of deficit irrigation (Zhang et al., 1998, 2003), to evaluate water saving practices (Pereira et al., 2003; Zhang et al., 2003, 2006; Fang et al., 2010), or aiming at a better understanding of processes relative to crops water use and saving (Xu and Mermoud, 2003; Zhang et al., 2003; Hu et al., 2005; Liu et al., 2006; Cai et al., 2009). Generally these studies adopted the single time averaged crop coefficient (K c ) approach to estimate crop evapotranspiration (ET c ) and often used experimentation involving the collection of weather, soil and crop data. Soil water balance models to 0378-3774/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.agwat.2012.11.008