Coupling evapotranspiration partitioning with water migration to identify the water consumption characteristics of wheat and maize in an intercropping system
Introduction
Rainfed agriculture is conducted on 80% of the world's arable land and provides 60% of the world's food. It therefore plays an important role in meeting the needs of a growing population and ensuring food security (Biradar et al., 2009; Doll and Siebert, 2002; Postel et al., 1996). Climate change impacts are predicted to be harmful for rainfed agriculture in regions such as Mexico, China, India, Thailand, Spain, and other regions, which will face future yield reductions of up to 30% (Hernandez-Ochoa et al., 2018; Murray-Tortarolo et al., 2018; Valverde et al., 2015). Intercropping is a traditional sustainable planting system having advantages of increased production (Gao et al., 2009; Gou et al., 2017a; Miao et al., 2016) and improved yield stability (Raseduzzaman and Jensen, 2017). Intercropping is widely valued and has attracted increasing interest (Knörzer et al., 2009; Munz et al., 2014; Shah et al., 2016) as a strategy to deal with climate change (Bedoussac et al., 2015; Feike et al., 2012; Lin, 2010; Padovan et al., 2018).
There is some controversy as to water utilization in intercropping systems. Previously published research showed a water use advantage for the intercropping system, i.e., intercropping used less water than sole cropping to produce the same yield with the same management, or produced more yield for the same water use (Mao et al., 2012, Ren et al., 2018, Zhang et al., 2019). In some other cases there was a water use disadvantage with intercropping systems (Gao et al., 2009; Rees, 1986a, Rees, 1986b; Singh et al., 1988; Wang et al., 2015a). Wheat and maize are two of three staple food crops in the world, and the wheat/maize intercropping system is popularly used in China (Li et al., 2001a; Wang et al., 2015a), India (Sharma et al., 2017), and other places. However, previous studies have focused on the total water consumption of the entire intercropping system, and the water consumption of wheat and maize individually in the intercropping system was little known (Gao et al., 2009; Wang et al., 2015a, b). Quantifying the water consumption of each crop in the wheat/maize intercropping system expands our understanding of crop water use and how it might be better managed.
Assessing the water consumption of each component crop in the wheat/maize intercropping system can be facilitated by evapotranspiration partitioning. However, evapotranspiration partitioning of two component crops in an intercropping system is complicated and difficult, especially during the co-growth period (Adiku et al., 2001; Gao et al., 2013; Morris and Garrity, 1993). Unlike what occurs in the sole cropping system, soil water complementarity and competition usually occur in intercropping systems. Soil water varies spatially and temporally in an intercropping system, and a highly competitive crop can use water from the soil volume occupied by an adjacent crop (Chen et al., 2014; Ma et al., 2019). In the intercropping system, water competition occurs between adjacent crop strips. Soil water migration (ΔS, mm) was defined as the amount of the soil water movement between strips. Chen et al. (2014) reported that the migration amount was very large. If the water migration is not considered, water consumption will not be computed correctly for each component in the system. Therefore, it is necessary to evaluate the water migration amount between strips in intercropping systems, and this is also a precondition for correctly partitioning evapotranspiration in an intercropping system. The water balance method can be used to quantify evapotranspiration for the entire intercropping system (Gao et al., 2013; Wang et al., 2015b), but it is not applicable for distinguishing water consumption between intercrop components. Using crop models can be another approach for separating crop component evapotranspiration (Gao et al., 2013; Rafi et al., 2019; Tan et al., 2020). Different from the sole cropping system, the two or more crops used in an intercropping system will change the factors influencing evapotranspiration. These factors include canopy cover, light interception, soil temperature, etc. (Gou et al., 2017b; Li et al., 2017; Wang et al., 2017). In the intercropping system, the growth of the two crops is not synchronized, and the canopy structure changes greatly over time (Gao et al., 2014; Wang et al., 2015b). Because the boundary conditions are different, some model simulations are not accurate (Gao et al., 2013).
The sap flow meters and micro-lysimeters can be used to directly measure crop transpiration and evaporation in both sole cropping and intercropping systems (Gao et al., 2013; Wang et al., 2015b). However, the sap flow meter cannot be used throughout the entire growth period due to limitations imposed by insufficient plant size. Gong et al. (2007) evaluated evapotranspiration by the water balance method and directly measured evapotranspiration using sap flow meters and micro-lysimeters and confirmed that the two methods agreed very well. Consequently, this study tried to use the two methods to calculate the total evapotranspiration of each crop and evapotranspiration partitioning. Evaporation includes soil water evaporation and evaporation of canopy interception (Mitchell et al., 2009). Previous studies have demonstrated that evaporation of canopy interception can account for a large proportion of evaporation (Drastig et al., 2019; Jiao et al., 2018). However, previous studies on evapotranspiration partitioning often have only considered soil water evaporation while neglecting evaporation of canopy interception (Gao et al., 2009; Wang et al., 2015b).
The objectives of this study were to: (i) measure the amount of soil water migration between crop strips in order to assess water competition and complementarity in the intercropping system; (ii) investigate evapotranspiration of wheat and maize in a strip intercropping system; (iii) accurately separate crop transpiration and evaporation.
Section snippets
Site description
This study was done at the Institute of Water Saving Agriculture in Arid Areas of China, Shaanxi, China (34°20′N, 108°04′E; elevation 506m), during the 2014-2015 and 2015-2016 growing seasons. The station is located in a typical rainfed region, with a mean annual precipitation of 521 mm, of which about 50% occurs from June to September. The mean annual temperature is 12.9 °C, the mean annual sunshine duration is 2196 h, and the frost-free period is 213 d. The annual mean evaporation from a free
Comparisons of methods for estimating evapotranspiration
Evapotranspiration was estimated by the water balance method and the direct method. To reduce errors and fluctuations, we chose the cumulative value for continuous three–day periods. These periods were randomly selected when there was no rainfall at the beginning and end of the three–day periods. Therefore, ten sets of sole wheat evapotranspiration data and ten sets of sole maize evapotranspiration data were selected to compare the evapotranspiration measured by the two methods. The data showed
Water migration
In this study, there was large amount of water migration between wheat and maize strips in intercropping, and the water migration direction was from the maize strip to the wheat strip during the co-growth period and was from the wheat strip to the maize strip after wheat harvest (Tables 2 and 4). This indicated that complementary use of soil water resources in space and time occurred in the intercropping system, which increased the efficient use of water in the intercropping system (Willey, 1990
Conclusions
Evapotranspiration is the main pathway for water consumption in agricultural systems. Few studies have tried to separate the components of evapotranspiration intercropping systems because of the difficulty in doing so. In this study, the water balance method and direct method (sap flow plus micro-lysimeter plus canopy interception) were combined to quantify the water movement amount between wheat and maize intercropping strips. The water consumption of each component was accurately evaluated
Declaration of Competing Interest
The authors declared that they have no conflicts of interest to this work.
Acknowledgements
This work was jointly supported by the National Key R and D Program of China (2017YFC0403600, 2017YFC0403605), the National Natural Science Foundation of China (41571506, 41771316, 51579212), and the Key Science and Technology Innovation Team Program of Shaanxi Province (2017KCT-15). The authors would also like to thank the editor and anonymous reviewers for their valuable comments and suggestions, which substantially improved the manuscript.
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