Girja Sharan and T. Madhavan
A program was initiated in 1998 to develop technology to improve water and land productivity in Kutch, a vast semi-arid and hot region in north-west India. Greenhouse cultivation was identified as basic approach. A new experimental facility was designed consisting of a greenhouse coupled to ETHE in closed.loop with added provisions for shading, natural ventilation and supplementary evaporative cooling via the foggers.
ETHE was placed in a trench 20 m long, 6 m wide and 3 m deep and back-filled with excavated soil. The greenhouse, a single span saw-tooth (20 X 6 X 3.5 m) structure was erected directly above. ETHE provided 40 air changes per hour. There are three continuous (closable) vents - two laterals along base of long sides, one near top of taller wall. A retractable cover with 60 % shading was provided on top over the cladding. There are 39 overhead foggers placed overhead. The facility was installed in late 2001, at Kothara (j 23° 14 N, l 68° 45 E) and investigations carried out to determine (a) the extent to which new facility improves yields, extends cropping season, conserves water compared to open- field in the area, and (b) the extent to which environmental control is achieved.
By using control measures in sequence and in conjunction, it was possible to crop the greenhouse over a span of ten months (July to April), long enough to raise two crops. Hybrid tomato has been raised three times, with mean single crop yield of 62 t / ha, and crop water use 245 mm. Additional 50 mm water was used in supplementary cooling. Yield was nearly two times that of open-fields and water used (for irrigation and cooling) less than half. Natural ventilation along with top shading was effective till the end of February, limiting greenhouse temperature to 34°C. Subsequently, ETHE and foggers were operated. With adult tomato crop inside (4 plants / m2), operating the ETHE (vents closed, top shaded) for six hours with evaporative supplement from foggers restricted greenhouse temperature to 37 . 38°C.
Water used by foggers, 100-108 liters over the day was one third to one fifth of what fan and pad would need to service a facility of this size. ETHE used 20-24 kWh over a day, about 25% more than estimated for fan and pad system. It was found economical to crop till the end of April or fist week of May and keep the house closed till the end of June. In heating mode - ETHE was able to heat the greenhouse easily from 9°C to 22-23°C in half hour in the cold winter nights and keep it at that till morning. The new facility appears promising for improving yields, making better use of water and extending the growing season in this hot semi-arid region. Work is ongoing, to find ways to reduce installation cost of the ETHE and to develop a more easily scalable design than the present one.
