Crop coefficient and evapotranspiration of grain maize modified by planting density in an arid region of northwest China
Introduction
Crop evapotranspiration (ET) is of importance in irrigation management and water allocation (Kang et al., 2002). It is affected by many factors, e.g. weather parameters, crop characteristics, irrigation scheduling and field management (Allen et al., 1998, Kang et al., 2003). It is also affected by planting density (Allen et al., 1998, Allen and Pereira, 2009). Higher planting density increases the radiation intercepted by the plant canopy and reduces the radiation at the soil surface (Papadopoulos and Pararajasingham, 1997), but the increased radiation absorbed by plant canopy can lead to intensify soil water consumption (Reicosky et al., 1985). The rate of transpiration (T) tends to increase with the increase of planting density due to the well exposed leaf area at the top of plants (Papadopoulos, 1985, Papadopoulos and Pararajasingham, 1997). Chen et al. (2010) found that higher plant density increases the T of winter wheat but decreases soil evaporation (E) in the North China Plain. Eberbach and Pala (2005) also reported that higher planting density can result in higher ET and lower E.
The Priestley–Taylor method (Ding et al., 2013, Priestley and Taylor, 1972), Penman–Monteith method (Agam et al., 2010, Monteith, 1965, Utset et al., 2004), Shuttleworth–Wallace method (Shuttleworth and Wallace, 1985, Teh et al., 2001), and Clumping method (Brenner and Incoll, 1997, Domingo et al., 1999, Zhang et al., 2008) are often used to estimate ET in the field. But ‘crop coefficient (Kc) × reference evapotranspiration (ET0)’ is a useful and convenient method to estimate ET (Allen et al., 1998, Doorenbos and Pruitt, 1975, Kang et al., 2003). Kc can be calculated by different methods (i.e. the single and dual crop coefficient methods) (Allen et al., 1998, Jensen et al., 1990). In the single crop coefficient method, the effect of crop transpiration and soil evaporation are combined into a single crop coefficient. The dual crop coefficient method has two coefficients, i.e. the basal crop coefficient (Kcb) to represent primarily the transpiration component of ET, and the soil evaporation coefficient (Ke) to describe evaporation from the soil surface (Allen et al., 1998, Rosa et al., 2012, Zhao et al., 2013). As soil evaporation may fluctuate daily as a result of irrigation or rainfall, applying Kc that expresses only the time-averaged effects on ET, is more useful and convenient than computing a daily Kc based on a dimensionless ‘stress’ coefficient (Ks), Kcb, and Ke (Allen et al., 1998, Allen and Pereira, 2009).
The Kc is chiefly affected by the amount, type, density and height of vegetation under the assumption that the ET0 accounts for nearly all variation caused by climate factors (Allen and Pereira, 2009). The Kc tends to decrease with the decrease of leaf area or plant density (Allen and Pereira, 2009, Qiu et al., 2013). Allen et al. (1998) found that during the mid-season stage of crops, the vegetation nearly covers the soil and varies with planting density, the Kc values should be adjusted by a factor (Acm) depending on the actual vegetation development (The Allen method). Allen and Pereira (2009) proposed a density coefficient (Kd) to estimate both basal and average Kc. In the dual Kc coefficient method, the basal crop coefficient (Kcb) should be formalized using the estimated Kc during the mid-season stage (at peak plant size or height) for full vegetation (Kc full), the minimum Kc for bare soil (Kc min) and Kd (Kcb method). The single crop coefficient (Kcm) was similarly estimated and was adjusted using a Ksoil representing background evaporation from wet soil (Kcm method).
At present, there are many studies about the effects of planting density on yield and water productivity (Chen et al., 2010, Eberbach and Pala, 2005, Griesh and Yakout, 2001, Salah et al., 2008). But only fewer studies have been conducted to assess the response of Kc and ET to planting density (Allen et al., 1998, Allen and Pereira, 2009, Daisuke et al., 2011, Qiu et al., 2013). The method proposed by Allen et al. (1998) considered an adjustment for crops at the middle season stage that the soil is usually nearly completely covered by the vegetation. The equations proposed by Allen and Pereira (2009) is only applied to the mid and eventual late season stage. Whether these formulas are applicable to the grain maize with different planting densities in the arid region of northwest China should be further studied.
As the planting area of grain maize in the arid area of northwest China is rapidly developed, understanding the response of ET and Kc of grain maize to planting density is important in improving the irrigation management in the region with limited water resources. Thus the objectives of this study were to (1) evaluate the effect of planting density on crop coefficient and crop evapotranspiration of grain maize; (2) develop a relatively simple and accurate method with density ratio (Kdensity) to calculate Kc and ET, and (3) compare the accuracy of estimated ET using the Allen, Kcm, Kcb and Kdensity methods, so as to validate whether Kdensity method had better performance in calculating ET under different planting densities or not.
Section snippets
Experimental site
The experiment was conducted in two consecutive years during April to September in 2012 (first season) and April to September in 2013 (second season) at Shiyanghe Experimental Station of China Agricultural University, located in Wuwei City, Gansu Province of northwest China (37°52′ N, 102°50′ E, altitude 1581 m). The site is in a typical continental temperate climate zone with mean annual precipitation of 164.4 mm, mean annual pan evaporation of 2000 mm. Average groundwater table is below 25 m,
Environmental variables at different growth stages of grain maize
Average daily environmental conditions at different growth stages of grain maize is shown in Table 1. In the two growing seasons, average daily Rs was 259.7 and 210.0 W m−2, average daily Ta was 18.7 and 19.2°C, average daily RH was 25.6 and 52.2%, average daily u2 was 0.76 and 0.56 m s−1, total P was 129.4 and 72.4 mm, and total reference evapotranspiration (ET0) was 595.2 and 549.0 mm, respectively. Environmental conditions varied in the two seasons, daily averaged Rs, P and ET0 in the first season
Conclusions
Higher planting density increased crop evapotranspiration (ET) and crop coefficient (Kc) significantly and decreased soil evaporation (E) and evaporation coefficient (Ke) slightly, and the differences of ET, E and Kc among different planting densities were mainly caused by the difference of leaf area index (LAI). The basal crop coefficient (Kcb) and Kc increased significantly until approximately a LAI of 3 m2 m−2, but it increased slightly when LAI was larger than 3 m2 m−2. The Allen method
Acknowledgements
We are grateful for the research grants from NSFC (51321001), the National High Technology Research and Development Program of China (863 Program, 2011AA100502), and The Discipline Innovative Engineering Plan (111 Program, B14002).
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