Hi,  it looks like you may ...

Hi,  it looks like you may need to install some sub-surface drains to get the rainwater off the lands and into the channels.  This could be a porous pipe at every 10 to 20 m draining across the land into your channels, buried at about 600mm deep and wrapped in geotextile.  Alternatively you could dig trenches and pack them with gravel covered with river sand draining into the channels if you cannot get or afford porous pipes and geotextile. kind of like this

---------------------------------------------------------------------------------- I ↑

---------------------------------------------------------------------------------- I open channel towards tank

---------------------------------------------------------------------------------- I

subsurface drainage channels sloping towards open channel  →