~ 3494 ~ The Pharma Innovation Journal 2023; 12(2): 3494-3500 ISSN (E): 2277-7695 ISSN (P): 2349-8242 NAAS Rating: 5.23 TPI 2023; 12(2): 3494-3500 © 2023 TPI www.thepharmajournal.com Received: 17-12-2022 Accepted: 19-01-2023 S Vinitha Assistant Professor, Department of Agricultural Engineering, Pushkaram College of Agriculture Sciences, Pudhukkottai, Tamil Nadu, India A Mani Assistant Professor, Department of Soil and Water Conservation Engineering, Agricultural Engineering College and Research Institute, Tamil Nadu Agricultural University, Kumulur, Trichy, Tamil Nadu, India Corresponding Author: S Vinitha Assistant Professor, Department of Agricultural Engineering, Pushkaram College of Agriculture Sciences, Pudhukkottai, Tamil Nadu, India Estimation of crop evapotranspiration of drip irrigated okra crop by various surface canopy resistance model S Vinitha and A Mani Abstract In the study, the estimation of crop evapotranspiration was based on the Penman-Monteith model along with variable canopy resistance. The canopy resistance was calculated by using a modified climatological resistance weighed by a normalized soil water factor (F). Canopy resistance is considered as a variable parameter for each day. In the case, the daytime variation of canopy resistance was performed well with the standard FAO 56 Penman-Monteith method. During the crop season, The average surface canopy resistance ranged between 0 s m -1 and 776 s m -1 and the normalized soil water factor (F) ranged between 0.08 and 0.99. The largest values of r c were observed as 776 s m -1 on 44 DAS, which signified the lowest values of VMC 64.35 mm (14.3%) in the experiment. The surface canopy resistance was more when it had the lowest moisture content. The above stated approaches were compared on the basis of RMSE and coefficient of determination (R 2 ) values. Farias canopy resistance model was specified as best fitted with an RMSE and R 2 values of 0.99 and 0.97. The results show that if the micrometeorological data such as temperature, relative humidity, vapour pressure deficit, wind speed, net radiation and soil moisture are available then the Farias canopy resistance model is the best one for computing ET C in semi-arid region. The yield obtained at all the harvest were the maximum and minimum yield of 24 kg and 3 kg respectively. The total yield gained was 9.8 t ha -1 . Water use efficiency obtained were under standard FAO-56 PM method and Farias canopy resistance model as 27.80 kg ha -1 mm -1 and 39.23 kg ha -1 mm -1 respectively. During the crop cycle, the Farias canopy resistance model was reasonably performed well. It indicated with the limiting amount of water used by the crop which could be improved the economic yield of the crop. Keywords: Canopy resistance, farias model, normalized soil water factor, penman-monteith model, water use efficiency 1. Introduction In hydrologic cycle, Evapotranspiration (ET) is one of the largest components and it is defined as sum of evaporation from soil, plant and open water surface and transpiration through canopy. The estimation of crop evapotranspiration (ET C ) is essential in water resources planning and management, at the same time increase water use efficiency and yield of crop (Allen et al., 2011) [1] . Based on the crop coefficient K C approach, the crop water requirement is calculated. It represents crop specific water use. ET C is calculated using standard agro- meteorological data and a crop-specific coefficient K C considering relationship between atmospheres, crop physiology and farming practices (Allen et al., 1998) [2] . ET C is required for estimating crop water requirements, irrigation scheduling and water productivity. In addition to, the Penman-Monteith (PM) model using the fixed surface canopy resistance (r s ) of 70 s m -1 which is considered for computing crop water requirement. Most of the conditions, the r s can be varies with respect to microclimate characteristics of the boundary layer above the crop (Srivastava et al., 2018) [15] . In the ET process, the surface canopy resistance (r s ) is a physiological as well as aerodynamic parameter and which is a function of water potential and climatological variables (Alves and Pereira, 2000; Lecina et al., 2003) [3, 5] . For the last few years, many approaches were developed to determine the surface canopy resistance (r s ) based on the meteorological variables with the soil water status (Ortega-Farias et al., 2004; Li et al., 2014) [11, 6] . The estimation of variable canopy resistance is very important for improving the accuracy of evapotranspiration rate. The present study have been undertaken with the objective to estimate the surface canopy resistance by Farias model and compare with the FAO-56 Penman- Monteith method.