Evaluation of reference evapotranspiration models and determination of crop coefficient for Momordica charantia and Capsicum annuum

• Temperature based models tend to overestimate ET values.
• The Penman and Rohwer models perform better than the Penman Monteith model.
• Kcmid of bittergourd was lower than the referenced FAO adjusted values.
• Kcmid and Kcend of chili were lower than the referenced FAO adjusted values.

Studies on water balance and irrigation at agricultural areas require accurate values of reference evapotranspiration (ETo). This study was conducted in the agricultural farm in the Modern Agriculture Centre in Kluang, Malaysia, to determine the crop coefficients of bittergourd (Momordica charantia) and chili (Capsicum annuum) by choosing the best ETo model. The experiment was conducted for two different crop cycles between October 2013 and May 2014. An automatic weather station was installed to record weather parameter at 30 min interval. Twenty six ETo models which were classified into four different groups were employed. The performance of the models was evaluated using Class A pan evaporation data from Kluang weather station. Eight statistical tests were used to assess and rank the accuracy of these 26 models. The ET values from the best ETo model of each group were then modeled with weather variables using multiple regression technique. Crop coefficient (Kc) curves were developed as the ratio between actual crop evapotranspiration measured by minilysimeters and the ET values of the best model. The temperature based models tend to overestimate observed pan ET values, thus were not recommended at this site. Results of the mass transfer based Penman model show comparatively better ETo estimates among others. The total water requirement for bittergourd for the whole growing period is 153 mm while chili recorded 229 mm. The Kc values for the bittergourd were 0.58, 0.88 and 0.69 while for chili were 0.58, 0.95 and 0.73 for the initial, mid and end growth stages, respectively. The present results show very similar mid and end season chili Kc values with a study from Ghana while the mid and end season Kc of bittergourd is quite similar with a study in Florida, USA using cucumber. The values obtained can help farmers to determine the water requirement of these vegetable crops so that proper irrigation can be applied according to its growth stage and weather condition.