The municipal bonds that help finance a major portion of the nation’s water supply may be riskier than investors realize because their credit ratings do not adequately reflect the growing risks of water shortages and legal battles over water supplies, according to a new study.

As a result, investors may see their bonds drop in value when these risks become apparent, and water and electric utilities may find it more expensive to raise money to cope with supply problems, the study warned.

Looking at significant water bond issuers across the southern part of the country, the report concluded that Wall Street’s rating agencies had given similar ratings to utilities with secure sources of water and to those whose water sources were dwindling or were threatened by legal battles with neighboring utilities.

Among the seven cities and agencies examined in the report, Los Angeles and Atlanta were identified as the ones whose water systems faced the greatest risk in the years ahead.