Effect of convection on the Penman–Monteith model estimates of transpiration of hot pepper grown in solar greenhouse
Introduction
Solar greenhouse industry is of great importance for local economy in the arid region of northwest China since the region has abundant light resource but limited precipitation and snow in winter. Such environment is highly suitable for developing solar greenhouse industry. However, limited water resources affect the sustainable production of greenhouse vegetables in the region. With development of irrigated agriculture and rapid population growth in the region, over-exploitation of water resources has led to gradually falling of groundwater table, shrinking of vegetation areas, soil salinization and desertification (Kang et al., 2004). Thus adapting best irrigation management practices to conserve water is of high importance for sustainable economic development. Since the soil surface is fully covered with clear plastic film (mulching) in order to reduce evaporation, irrigation water is mainly depleted by transpiration in the greenhouse, which is influenced by microclimate conditions such as solar radiation, high temperature, relative humidity, and vapour pressure deficit. Knowledge of transpiration over short-time intervals enables us to precisely evaluate the crop water requirements at different growth stages (Rouphael and Colla, 2004), thus provides a scientific basis for developing water management practices. Accurate measurement of transpiration is difficult due to temporal and spatial variations, thus models are often used to predict transpiration and to estimate crop water use (Villarreal-Guerrero et al., 2012).
The most common transpiration model for greenhouse crops was based on the Penman–Monteith (P–M) model (Monteith and Unsworth, 1990). The model was considered as a combination equation between the energy and mass transfer from canopy. The parameters of stomatal resistance and aerodynamic resistance in the P–M model were difficult to be directly measured and often estimated through their relationships with certain environmental variables within the canopy. Stomatal resistance has been related to solar radiation, air temperature, air humidity, vapour pressure deficit, carbon dioxide concentration, and soil water potentials (Berryman et al., 1994, Jarvis, 1976, Leuning, 1995, Tardieu and Simonneau, 1998, Tuzet et al., 2003). In a study on greenhouse-grown tomato, Stanghellini (1987) included solar radiation, vapour pressure deficit, air temperature and carbon dioxide concentration, while Jolliet and Bailey (1992) found that only the first two variables are significant. Many researches showed that a good representation could be obtained using only solar radiation for crops grown in greenhouse, e.g. cucumber (Yang et al., 1990), Ficus benjamina (Bailey et al., 1993, Zhang and Lemeur, 1992), geranium (Montero et al., 2001) and zucchini (Rouphael and Colla, 2004).
For the open field condition, aerodynamic resistance was generally parameterized through a model describing the turbulent transfer of water vapour between leaf and atmosphere, and using a logarithmic function of variables related to crop geometry characteristics and air speed (Brutsaert and Stricker, 1979, Monteith and Unsworth, 1990, Thom, 1975). The aerodynamic resistance in the model becomes infinity when air speed is close to zero. Air movement was generally slow within the solar greenhouse with natural ventilation. Thus the above model may be not suitable to use under greenhouse condition. As the eddy diffusion process transports both air and water vapour, aerodynamic resistance to the transfer of vapour can be expressed as a function of heat transfer coefficient that is related to non-dimensional groups, such as the Grashof and Reynolds number corresponding to free convection (due to buoyancy forces caused by temperature differences), forced convection (due to wind pressure), and mixed convection (free convection plus forced convection) of the air flow, respectively (Bailey et al., 1993, Kitano and Eguchi, 1990, Roy et al., 2002, Zhang and Lemeur, 1992). Given that heat transfer coefficient is often estimated differently with different types of convection regimes, understanding the air flow type and accurate estimation of heat transfer coefficients based on the convection types would have great impacts on estimation of aerodynamic resistance, in turn on accuracy of transpiration estimation in the greenhouse.
Previous studies showed that the P–M model can accurately estimate transpiration of geranium and F. benjamina grown in greenhouses assuming free convection (Montero et al., 2001, Zhang and Lemeur, 1992). Rouphael and Colla (2004) found that the P–M model assuming mixed convection performed the best with relatively higher coefficient of determination (R2) and the slope of regression between the observed and predicted transpiration closer to 1. However, previous studies did not explicitly distinguish the type of convection regime in greenhouse when the P–M model was used to estimate crop transpiration (Bailey et al., 1993, Montero et al., 2001, Rouphael and Colla, 2004, Zhang and Lemeur, 1992). In addition, most of the previous studies were conducted in temperate or maritime climate of Europe and Mediterranean areas (Bailey et al., 1993, Montero et al., 2001, Rouphael and Colla, 2004). Few studies on estimating transpiration have been conducted in solar greenhouses of northwest China where the air temperature and vapour pressure deficit in the solar greenhouse with natural ventilation system are relatively higher. With increasing popularity of the solar greenhouse industry in northwest China, accurate prediction of crop transpiration in the greenhouses is of great importance for developing water-saving management practices in the region with limited water resources. The objectives of this study were to study the patterns of convection regime in the solar greenhouse, and to evaluate the performance of the P–M model on transpiration prediction with aerodynamic resistance estimated by different methods according to air flow convection type in the solar greenhouse.
Section snippets
Experimental site and design
The experiment was conducted in a solar greenhouse at the Wuwei Experimental Station of Crop Water Use, Ministry of Agriculture, PR China. The experimental site is located in Wuwei city, Gansu Province of northwest China (N 37°52′, E 102°51′, altitude 1581 m). It is in a typical continental temperate climate zone with mean annual precipitation of 164.4 mm and pan evaporation of 2000 mm. The groundwater table is below 25 m. It is rich in solar radiation with mean annual temperature of 8.8 °C, mean
Microclimate conditions in the solar greenhouse
Table 1 shows the descriptive statistics of the daily averages of environmental variables during the study periods in the two growth seasons. The mean of the daily averages of all the environmental variables (especially radiation) were higher in 2011–2012 than those of in 2010–2011, except for the air temperature. The diurnal changes of environmental variables measured on a representative sunny day (April 1, 2011) are shown in Fig. 1. The maximum of solar radiation (Rs), net radiation (Rn), air
Conclusions
In this study, mixed convection dominated most of the daytime in the greenhouse for the two growth seasons, while pure free convection occurred at night and in the early morning. No pure forced convection was observed during the study period. The stomatal resistance was highly correlated with solar radiation and thus can be estimated from radiation measurements. Under pure free convection, the P–M model substantially underestimated the transpiration of hot pepper with h calculated by the
Acknowledgments
We gratefully acknowledge Dr. Laosheng Wu at University of California, Riverside for the advice and English editing of this study. Also we are grateful for the research grants from the National Natural Science Foundation of China (50939005, 51222905), the National High-Tech 863 Project of China (2011AA100502), the Ministry of Water Resources of China (201001061), China-EU Int’l Collaboration Projects (S2010GR0692) and Program of New Century Excellent Talents in University, Ministry of Education
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