African governments should recognise customary rights to water for millions of small farmers who have been sidelined or "criminalised" by permit systems created during the colonial era, said a report published on Monday.

Restrictive permit systems in Kenya, Malawi, South Africa, Uganda and Zimbabwe have left more than 100 million people without access to enough water, according to the report by the Sri Lanka-based International Water Management Institute (IWMI).

Those countries should "decolonise statutory water law through a hybrid approach", according to the report.

A hybrid system that recognises both existing permits and includes customary laws would improve water access for small farmers, said Barbara Schreiner, a co-author of the report released at a conference in Libreville, Gabon.

"This is critical for expanding smallholder irrigation," said Schreiner, who is executive director of the South Africa-based advocacy group Pegasys Institute.

"(It) will lighten the administrative burden on the state, while making formal legal access to water more equitable."

About half of sub-Saharan Africa governments recognise customary rights to water, according to the Washington-based think tank World Resources Institute, but most are restricted to home use and limited farm irrigation.

In the five countries surveyed, researchers found that laws restrict small farmers to irrigating less than one acre of land without a permit, said Barbara van Koppen, a co-author and principal researcher at IWMI.

Recognising customary rights would help small farmers expand their agricultural production through irrigation, she said.

"Customary water rights should be recognised as well as other statutory permits to encourage small users to invest (in irrigation)," Koppen said by phone from Pretoria.

She said state agencies mandated to license water use have concentrated on large commercial users and failed to reach small farmers.

While many countries promoted water permits as the best practise in the 1990s, poorly-funded state agencies mandated with this task could not reach millions of small farmers, leaving many at risk of prosecution, the report said.

Recognition of customary water rights would also help strengthen land rights for a majority of communities whose land is still untitled across Africa, said Timothy Williams, IWMI's director for Africa.

About 90 percent of rural land in Africa is undocumented, according to the World Bank, with most of it held under customary law.

"In many parts of Africa water right are tied to land rights and the two are not separable. To be able to use water in many parts of Africa you need to have land," Williams told the Thomson Reuters Foundation by phone.

"Regulating water rights therefore implies also policing land rights for millions of farmers. They end up being unintentionally criminalised because they can be prosecuted for using water without permits."

This piece was written by Kevin Mwanza and was originally posted on the Place website.