Josef Tanny1, Shabtai Cohen1 and Yair Israeli2, 1Institute of Soil, Water & Environmental Sciences, Agricultural Research Organization, The Volcani Center, P.O.B. 6, Bet Dagan 50250, Israel
2 Jordan Valley Banana Experimental Station, Zemach, 15132, Israel
The use of screens and screenhouses is constantly increasing, especially in arid and semi-arid regions. One of the reasons for the wide use of screens is the potential increase in water use efficiency, which is a crucial environmental issue in such regions. Screenhouses modify the crop microclimate and thus may reduce the atmospheric water demand and lead to water saving. In Israel, much of the banana cultivation has moved inside screenhouses (Fig. 1) in order to save water and improve fruit quality. Therefore research was focused on measuring and estimating evapotranspiration (ET), namely, the water loss from the whole canopy to the atmosphere, of banana plantations in screenhouses, in order to optimize irrigation management.
In an attempt to illustrate the capability of screenhouses in reducing ET, a class-A evaporation pan was installed within a large banana screenhouse at the Zemach experimental station at a height of about 4 m, just above the plants and below the screen. A similar class-A pan was installed in a nearby open field, at the same height. Evaporation from the two pans was monitored over a period of 5 years and the results are shown in Fig. 2. The results show significant reduction in pan evaporation within the screenhouse, mainly during the months of May-October, where, on average, inside evaporation was about 60% of the outside.
In a study conducted in a large banana screenhouse near the Sea of Galilee in northern Israel, we have measured ET by an eddy covariance system (Tanny et al., 2006), which directly measures the flux of water vapour from the canopy into the atmosphere, and compared the measurements with two types of models. The first was the Penman-Monteith model of reference evapotranspiration (Allen et al., 1998) under external meteorological conditions, and the second was the modified ET model for internal screenhouse conditions, previously developed by
Möller et al. (2004). Figure 3 presents daily evapotranspiration over a period of 14 days in June 2005. The results show that the screenhouse model is close to the measurements. However, the model for reference ET under external conditions is significantly higher, illustrating the effect of the screenhouse in reducing evapotranspiration.
The results of Fig. 3 further show that irrigation (dashed horizontal line) was consistently higher than the actual crop water use. Note that this irrigation level was already about 70% of the irrigation supplied to open banana plantations in this region. Although irrigation was increased from JD (Julian day) 166, actual crop water consumption did not change, suggesting the possibility of water savings.
Sensitivity analysis was applied to the reference evapotranspiration model. The analysis showed that the main factors contributing to the reduced ET in the screenhouse were radiation and wind. Radiation reduction by the screen was the major factor in reducing the ET between 07:30 and 14:00, whereas wind speed reduction by the screen was most significant for the ET reduction during late afternoon (after 15:00). Air temperature and humidity modifications by the screenhouse had a much smaller effect on reducing ET.
Water use efficiency is an important agricultural parameter because it tells the farmer the expected yield per plant water consumption. Water use efficiency (WUE) can be defined as the ratio between yield and applied irrigation. In an irrigation trial conducted in a large banana screenhouse in the Jordan Valley, different levels of irrigation were applied (100%, 85%, 70% and 55%) and yield was measured for each treatment. The 100% irrigation level was based on that used for open banana plantations in the region. The results (Fig. 4) showed that at 85% irrigation the yield did not decrease as compared to the 100% level. Irrigation at 70% reduced the yield slightly but this reduction was not statistically significant. The lowest irrigation level of 55% did cause a significant reduction in yield. Hence, water use efficiency can be increased by about 20-30% by reducing irrigation when growing bananas in screenhouses in this region.
Our research group also examined the potential to increase water use efficiency by covering other crops with screens. Preliminary results in apple orchards demonstrated that an increase in WUE of about 15% is attainable. Future projects will study WUE of other orchard and vegetable crops covered with screens.
The research was funded by grants from the Chief Scientist of the Ministry of Agriculture, and from BARD – The United States-Israel Binational Agricultural Research and Development Fund.
Allen, R. G., Pereira, L.S., Raes, D. M., (1998). Crop evapotranspiration, guidelines for computing crop water requirements. FAO Irrigation and Drainage Paper 56, FAO, Rome, Italy.
Möller, M., Tanny, J., Li, Y., and Cohen, S., (2004). Measuring and predicting evapotranspiration in an insect-proof screenhouse. Agric. For. Meteorol., 127, :35-51.
Tanny, J., Haijun, L., Cohen, S. (2006). Airflow characteristics, energy balance and eddy covariance measurements in a banana screenhouse. Agric. For. Meteorol., 139, (1-2), 105-118.