Cowell said the global water technology market was worth around £23bn, while the UK's Environment Agency budget for flood defences was set to top £800m.
He used his first speech since taking the chair to say: "There is lots of scope to take advantage of new legislation. Drainage is often overlooked by clients but has such importance. Something that falls out of the sky allows you to diversify skills and take advantage."
Contractors and designers of large-scale projects and small gardens needed to cram up on latest laws and guidance, said Cowell, who replaced Richard Gardiner as BALI chairman.
These include the Code for Sustainable Homes and planning policy guidance on sustainable drainage systems, he told garden industry trade show Glee last week.
"There is so much information out there that could be utilised by each BALI member and each company. Sustainable urban drainage is not just block paving. It also includes water storage, swails, ponds and silt traps," he said.
Companies should consider closer collaboration with local authorities and engineers to become a closer member of a multidisciplinary team
"The market is constantly growing. It is a great opportunity to offer a unique selling point and entrepreneurial companies can design their own systems."