(Embargoed for release at 0001 GMT Monday Sept. 6)

LONDON, Sept 6 (Reuters) - Increasingly erratic rainfallpatterns related to climate change pose a major threat to foodsecurity and economic growth, water experts said on Monday,arguing for greater investment in water storage.

In a report by the International Water Management Institute(IWMI), experts said Africa and Asia were likely to be hardesthit by unpredictable rainfall, and urged policymakers andfarmers to try to find ways of diversifying sources of water.

The IWMI research estimates that up to 499 million people inAfrica and India could benefit from improved agricultural watermanagement.

"Just as modern consumers diversify their financial holdingsto reduce risk, smallholder farmers need a wide array of 'wateraccounts' to provide a buffer against climate change impacts,"Matthew McCartney, a hydrologist at IWMI, said in a statement.

"That way, if one water source goes dry, they'll have othersto fall back on."

The U.N. panel of climate experts has projected more extremeweather such as droughts, floods and heatwaves this century,caused by global warming.

The report said that, despite a great expansion inirrigation in recent decades in Asia, around 66 percent ofagriculture there is still dependent on rainfall.

In sub-Saharan Africa, the proportion is even greater at 94percent, it said. These are the regions where water storageinfrastructure is least developed.

The report cautioned against over-reliance on singlesolutions such as big dams, and said an integrated approachcombining large- and small-scale storage was a better strategy.

It suggested the use of water from natural wetlands, waterstored in the soil, groundwater and water collected in ponds,tanks and reservoirs.

"For millions of people dependent on rain-fed agriculture,reliable access to water can make all the difference betweenchronic hunger and steady progress toward food security,"McCartney added.

"Even small amounts of stored water, by enabling crops andlivestock to survive dry periods, can produce large gains inagricultural productivity and in the wellbeing of rural people."

The IWMI is funded by the Consultative Group onInternational Agricultural Research (CGIAR), a partnership ofgovernments, donors and international organisations.

It noted that, in response to increased demand for food andpower supplies, many developing country governments withfast-growing economies have recently invested in large dams.

The benefits of these projects in terms of storing water forcrop irrigation were clear, it said, "but so are the adversesocial and environmental impacts".

As examples of the value of small-scale storage options, thestudy cited field studies that have proven the effectiveness ofusing small planting basins to "harvest" water.

In Zimbabwe, such basins have been shown to boost maizeyields, whether rainfall is abundant or scarce. In Niger, theyhave led to three- or four-fold increases in millet yields. (Reporting by Kate Kelland; Editing by Kevin Liffey)