The affix ghañ is optionally used after the root grah 'to seize' when the word ava is in composition, the sense being 'withholding the rain'.,
The word 'optionally' is understood here. The word वर्षप्रतिबन्ध means the absence of rain, from some cause or another, when the season has arrived; as अवग्राहः or अवग्रहो देवस्य 'the draught of rain'.
Why do we say 'when meaning obstruction of rain'? Observe अवग्रहः पदस्य ।,