Comparative evalualtion of evapotranspiration parameters in a naturally ventilated polyhouse and open field

Abstract

Studies on the water requirement of horticultural crops in polyhouses are scarce and despite the water scarcity, irrigation is mainly scheduled according to farmer’s experience. Canopy development and management of some polyhouse horticultural crops is quite different from that outdoors. Differences in plant spacing, crop height and aerodynamic properties may affect the crop coefficient values. Moreover, the proportion of diffuse radiation in polyhouse is different from that outdoors. Thus it is questionable whether the standard crop coefficient values, determined experimentally outside polyhouse can be used directly to determine the evapotranspiration of the greenhouse crops. Complete data on meteorological parameters inside poly houses is very rarely obtained and it causes lot of limitations in applying indirect estimation methods of ET0 based on climatological data. Field experiment on the comparative evaluation of evapotranspiration parameters in a naturally ventilated polyhouse and open field was conducted in a naturally ventilated polyhouse and open field in the research plot of the Department of Irrigation and Drainage Engineering, in KCAET campus, Tavanur. In the study, Non-Weighing Mini-Lysimeters were used to determine evapotranspiration parameters and to develop crop-coefficient curves for Okra. Comparison with indirect methods was also done in order to assess the dependability of climatic data for evapotranspiration estimates. The study compares the data for open field and poly house conditions in order to quantify the effect of micro climatic variations. ETo estimated using climatological methods viz reduced pan, FAO -56 Penman-Monteith, FAO Blaney- Criddle and Thornthwaite were compared with measured lysimetric data using simple error analysis and linear regression. Out of all methods FAO-56 Penman-Monteith provides quite good agreement with evapotranspiration obtained by lysimetric data with a high correlation coefficient of 0.88 and 0.87 for polyhouse and open field conditions respectively. Studies on crop morphological parameters indicated that plant growth and yield parameters were not significantly different for polyhouse and open field conditions. Seasonal average ETo for polyhouse and open field were 4.74 and 5.02 mm/day. Seasonal average ETc values of Okra for polyhouse and open field conditions were 3.90 and 4.31mm/day. The calculated values of Kc for the initial, mid and late season stages were 0.58, 0.94 and 0.67 in polyhouse. Open field values were 0.61, 0.98 and 0.72 for different stages respectively. It was observed that polyhouse Kc values were lower than the open field. The variations in micro climate inside poly houses with reduced solar radiation and wind velocity combined with higher humidity and temperature may have contributed to lesser ET. The results implied that water requirement is lower inside the polyhouse compared to open field conditions. The results of this study can be used as a guideline in the computation of water requirement of poly house crops instead of depending on open field accepted values for Tavanur region. If the poly house irrigation is scheduled in such a way as to apply the right quantity of water enough to meet the evapotranspiration requirements of crops, considerable water saving can be achieved. The quality of produce is high, duration of crop season is longer and year round cultivation is possible inside poly houses which implies significant saving in water without compromising yield and product quality.