This examination utilizes CROPWAT model to foresee water use in rainfed agriculture and simulated irrigation water prerequisite for tomato crop in the Ponnanaiyar basin, Vaiyampatti block, Tiruchirapalli district of Tamil Nadu. The harvest evapotranspiration (ETc) necessities by tomato crop were anticipated as 585.5 mm. The moisture deficiency at harvest was 17.6 mm of water which brought about an all-out yield decrease of 0%. Now, ETa would approach ETm and ideal tomato yield would be gotten with yield misfortunes anticipated at 0%. Yield gap examination uncovered that radiation, daylight and temperature are positive for tomato fruit production, however, the weighty reliance on precipitation makes the region truly helpless against dry spells.