SOLAR RADIATION AND IRRIGATION OF GLASSHOUSE TOMATOES
Sde Nitzan is a 5-year old settlement consisting of 60 family farming units, each with 2 single dunam (O.1 hectare) glasshouses. The glasshouse is of the Venlo type produced by the Voskamp Co. of Holland. It is a ten-span structure constructed from steel and aluminium with a concrete foundation, and covered with clear glass. There is no heating and ventilation is done naturally through roof and side windows opening to the east and west. The glasshouses are aligned on a N-S axis.
The solar radiation was measured by a Kipp solarimeter whose signal was fed to a digital integrator. Concurrently, the daily irrigation totals of the crop were recorded. For the purpose of this paper, 2 growing seasons are considered, 1974–75 and 1975–76. A single tomato crop was grown for each of the 2 seasons. The first season, seeds were sown on 6 October and for the second, 19 September. Irrigation was done daily and independently of the radiation information.
The data were reduced to weekly totals and then to a daily average for the week. A regression equation was calculated of the daily average irrigation vs. the global radiation outside the glasshouse.
The regression equation has a significant y - offset term. The implication of this term is that below a certain level of outside radiation, no irrigation is required. One possible explanation for this is a varying transmission factor of the glasshouse. That is, for low solar ray angles of incidence such as in the winter or early or late in the day, the transmission factor may decrease to such an extent that there is little energy for evaporation.
In summary, I think it may be said that there is an excellent correlation between the water requirements of a tomato glasshouse crop and the outside global solar radiation. This year, at present, an experiment is being conducted by the Volcani Institute and the the Southern Development Project, comparing the yields and plant characteristics from tomato plants being irrigated automatically according to solar radiation to tomato plants irrigated conventionally. In addition, the net global radiation inside the glasshouse is being measured at various times of the day and season.
This work was done in conjunction with the Agricultural Meteorology Department of the Volcani Institute which in addition supplied the instrumentation.