The ability to efficiently and accurately manage irrigation for greenhouse crops is of increasing interest for commercial growers for both water use efficiency and crop health. Current irrigation methods vary depending on the crop and grower preference. In many commercial greenhouse operations, irrigation scheduling is typically based on the experience of the grower by “lifting and touching,” using qualitative, rather than quantitative, information such as using scientific sensors to monitor the water status of crop or growing substrate. Also, there are growers using methods such as timer-based scheduling, which may not be synchronized with crop water demand. Some growers have also begun to use sensors to monitor vapor pressure deficit (VPD) and photosynthetically active radiation (PAR) in addition to gravimetric methods to assist in making irrigation decisions depending on the crop of interest (e.g., crops such as tomato, pepper, and cucumber). Solar radiation integrals have also been used to set thresholds for irrigation in plant productions using soilless growing substrates (Jovicich et al., 2007). Recently, substrate moisture sensors have also been tested for providing additional information and to assist growers in making irrigation decisions. This kind of system initiates irrigation at a user-identified substrate VWC(s) (Belayneh et al., 2013; Chappell et al., 2013). Soil moisture sensor-based systems are promising; however, there is not enough information on how many sensors are adequate for a given greenhouse area, crop type, or greenhouse condition. It is important to position enough sensors throughout a greenhouse to ensure that over- or under-watering of plants in certain locations is not occurring. It is well known that greenhouse microclimate conditions, such as air temperature, relative humidity, VPD, and solar radiation, can influence plant evaporation rate, water demand, and eventually irrigation frequency and amount. Microclimatic variations within a greenhouse can be caused by structural elements of the greenhouse, such as overhead pipes, support beams and poles, circulatory fans, ventilation screens, heaters and other equipment, as well as external weather, which may cause uneven solar radiation to reach all plants within a greenhouse. Ventilation is also important as the internal airspeed influences the humidity and temperature of the air in the greenhouse (Boulard et al., 2004). Because solar radiation is considered to be the largest component of energy input to a plant canopy, variation in light distribution to plants is an important factor to consider as well (Jovicich et al., 2007; Körner et al., 2007; Stanghellini, 1988). Microclimate variation in a greenhouse can lead to uneven water usage within a crop. When this variation in water usage is not accounted for when irrigating, areas of under- and over-watering may develop, which can reduce plant growth through drought (Gindaba et al., 2005) or flooding stress (Olivella et al., 2000).
Many different efforts have been made to reduce microclimate variations within a greenhouse. For example, models of greenhouse climates have been examined recently to provide insight into the improvement of greenhouse design and management, and to provide more homogenous climate conditions in a greenhouse. The Penman–Monteith model (Monteith, 1965) has been adapted in some instances for modeling greenhouse climates, rather than open field conditions (Morille et al., 2013; Qiu et al., 2013; Seginer, 2002; Vanthoor et al., 2011). However, there is little quantitative data on the magnitude of the microclimatic variations existing in most of the commercial greenhouse types. These types of data are in need, especially when deciding which irrigation technologies to use. Apparently, more research is in need to quantify microclimatic variations within greenhouses and also to test whether soil moisture sensor-based systems are suitable for monitoring and controlling irrigation of greenhouse crops with the currently existing microclimatic variations.
The objectives of this study were to 1) determine and demonstrate variations in microclimate of various greenhouses using evaporation rate as an integrated indicator, 2) quantify variation in growing substrate VWC and correlate them with plant performance in two greenhouse crop production systems, and 3) determine whether sensor-based irrigation systems are suitable for monitoring and controlling irrigation of crops under greenhouse conditions.
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