Design and evaluation of modified screen net house for off-season vegetable raising in composite climate

Currently the use of conventional screen net houses for off-season vegetable raising in north India composite climate is not so effective and has many constructional and operational limitations like poor structural design, higher constructional cost, no greenhouse effect in winter and higher plant temperatures in summer. Similarly, the use of polyethylene sheet covered greenhouses also has problems like much higher constructional and operational costs and higher inside air temperatures in summers. In this study, modified designs of 500 m2 (one kanal) and 250 m2 (half kanal) screen net house have been presented particularly suitable for composite climate (where both winters as well as summers are harsh) as a replacement for conventional net house and polyethylene sheet covered greenhouse design. To make these designs low cost and more effective, low tunnels (covered with low density polyethylene sheet) have been designed and used in winter over the plant rows to generate localized greenhouse effect for faster plant growth. By doing so, average daily air temperature under the tunnels was raised about 9–10 °C above the open field air temperature. In this way, huge cost of covering the net house or greenhouse during winter with costly polyethylene sheet could be saved. Similarly, in extreme summer when the ambient air temperature exceeded 40 °C (during the fruiting stage of the crop) a 50% shade net was used inside the modified net house at 2.5 m height (instead of using active cooling system) resulting in 4–6 °C drop in the plant temperature. Experimental evaluation of the modified net house was conducted during winter and summer months of year 2007–08 (December to June) by growing brinjal crop and compared with conventional net house, polyethylene sheet greenhouse and in open field condition. It was observed that due to the combined effect of low tunnels (in winter) and shade net (in summer), the micro-climatic parameters like air temperature, plant temperature, solar radiation and light intensity remained within desirable range during different stages of crop growth resulting in 37.6% and 11.5% increase in the yield of brinjal crop as compared to conventional net house and PE covered greenhouse yield, respectively, at 31°N latitude. Economic analysis of the modified net house was also conducted and compared with the conventional net house and PE covered greenhouse of the same area. It was observed that 500 m2 area modified net house (coupled with low tunnels and shade net) produces highest yield and has the highest net present worth (Rs 3,35,324) and the lowest payback period (5.2 years) as compared to the conventional net house (Rs 2,04,712 and 6.5 years) and PE covered greenhouse (Rs 1,52,614 and 10.5 years). On the other hand for 250 m2 modified net house, the net present worth and payback period was Rs 1,20,417 and 9.7 years as compared to Rs 65,497 and 11.5 years and Rs 10,616 and 17 years for conventional net house and PE covered greenhouse.