Chlorophyll content in plant leaves is an essential indicator of the growth condition and the fertilization management effect of naked barley crops. The soil plant analysis development (SPAD) values strongly correlate with leaf chlorophyll contents. Unmanned Aerial Vehicles (UAV) can provide an efficient way to retrieve SPAD values on a relatively large scale with a high temporal resolution. But the UAV mounted with high-cost multispectral or hyperspectral sensors may be a tremendous economic burden for smallholder farmers. To overcome this shortcoming, we investigated the potential of UAV mounted with a commercial digital camera for estimating the SPAD values of naked barley leaves. We related 21 color-based vegetation indices (VIs) calculated from UAV images acquired from two flight heights (6.0 m and 50.0 m above ground level) in four different growth stages with SPAD values. Our results indicated that vegetation extraction and naked barley ears mask could improve the correlation between image-calculated vegetation indices and SPAD values. The VIs of ‘L*,’ ‘b*,’ ‘G-B’ and ‘2G-R-B’ showed significant correlations with SPAD values of naked barley leaves at both flight heights. The validation of the regression model showed that the index of ‘G-B’ could be regarded as the most robust vegetation index for predicting the SPAD values of naked barley leaves for different images and different flight heights. Our study demonstrated that the UAV mounted with a commercial camera has great potentiality in retrieving SPAD values of naked barley leaves under unstable photography conditions. It is significant for farmers to take advantage of the cheap measurement system to monitor crops.

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An estimation based on a mathematical model to predict hourly evapotranspiration (ET) rates that occur inside a plant factory system was made using the Stanghellini model. The Stanghellini model is considered more appropriate for estimating the rate of ET inside the soilless culture of greenhouse tomatoes. The model requires some climatic data (e.g., solar radiation, air temperature, relative humidity, and wind speed) and plant growth parameters (leaf area index) as inputs. In this study, the observed data were obtained from an experimental greenhouse located at the Ehime University, Japan. The ET rate of tomato (Lycopersicon esculentum Mill.) crop was measured using a weighing method. Accurate determination of ET is essential to precisely compute crop water use and to assist growers for applying good irrigation management. The results showed that solar radiation and vapor pressure deficit are important factors driving the ET rate. The model's output showed good results for determining the ET rate and depicted crop water requirements on an hourly basis.