The affix ṇamul comes after the verb pur 'to fill' when the word so formed expresses a measure of rainfall and optionally the long ū of this pur is elided.,
Thus गोष्पदपूरं or गोष्पदप्रं वृष्टो देवः 'it rained filling up small puddles (lit. the impression made by the foot of the cow on the soil)'. सीतापूरं or सीताप्रं वृष्टो देवः 'it rained filling up all furrows'.
Why have we used the word अस्य 'of this pur' in the sutra ? The long ऊ of पूर् is to be elided, and not the long ऊ, if there be any, of the upapada. Thus मूषिकाबिलपूरं or मूषिकाबिलप्रं वृष्टो देवः । Here the long ऊ of मू is not shortened.,