Beginning And Ending. Poem by gnrao rao

I fell on the mountains,
I evaporated from the water-bodies,
I became dark clouds.
I dashed against the top mountains,
Outside and up was cool,
I fell as drop by drop.
All drops joined as a team,
and became streams,
then streams joined,
and turn into rivers,
I flowed up and down,
Filling up the ditches,
over the sand and mud,
I became muddy.
then you stored me in a dam,
the dam's doors were opened,
I flowed through the canals,
Then I filled up the lake,
From the lake again,
through the canals,
I irrigated the fields,
then I drained myself
Either in the sea
Or ocean,
Waiting to evaporate.
Sometimes I pour heavily,
Some times as hail stones,
Sometimes I don't fall at all.
Every one digs down the earth,
to find me.
Who am I?