The agricultural sector is one of the most important sectors of the Iranian economy that have various factors affecting its growth. Drought is one of the natural factors affecting this sector. Nowadays, knowledge is the main factor of growth, production and solving different problems in countries. Research and development (R&D) costs and the rule of law index are indicators of the knowledge economy. The purpose of this study was to investigate the impact of drought, R&D costs and rule of law on the growth rate of Iran′s agricultural sector. For this purpose, the casual Bayesian network method was used and data were collected for the period 2002-2017. The results showed that increasing the growth rate of R&D costs, increasing the rule of law index and placing Iran in wet conditions increased the agricultural water productivity and growth of this sector. Increasing R&D costs showed the best performance in improving the drought index. The highest probability of drought index was in the high condition with probability of 24.2% with 100% probability of R&D. The Rule of Law scenario showed the best performance in reducing water use in agriculture. So, by placing the rule of law probability in high position, the highest probability of growth of agricultural water use was in the low position with probability of 50.5%. Therefore, increasing R&D costs and targeting them, as well as enforcing laws and policies in various fields such as water resources can help increase water productivity in the agricultural sector and growth this sector.