Investors entering 2016 who are sensing uncertainty or a possible downturn in the stock market may wish to allocate at least part of their investment portfolio to safer, income-oriented investments. One possible choice is money market funds. Money market funds are not the same as a money market account. Money market accounts are more or less high-yield savings accounts, while money market funds are mutual fund investments made in various debt securities that may include government and corporate debt.

Money market funds are not FDIC-insured and carry a risk level commensurate with the creditworthiness of the debt issuers and the fund's specific debt securities. However, despite some risk, money market funds are still generally considered a very safe investment. Since the Federal Reserve Bank's decision in December 2015 to raise interest rates should boost money market fund returns, the beginning of 2016 may be an opportune time to consider money market funds for your investment portfolio.

The following are five of the top-ranked and most-promising money market funds for 2016. Because of the relatively low returns offered by money market funds, fees are critically important in determining an investor's real return on investment. None of the funds detailed below charge 12b-1 fees or sales loads. Investors should consider the total picture of yield offered, risks and fees when selecting a money market fund.

1) Fidelity Money Market Fund - Premium Class

The Fidelity Money Market Fund - Premium Class was launched in 1989. With $5.6 billion in assets under management (AUM), the fund invests in U.S. dollar-denominated money market securities, both domestic and foreign, as well as repurchase agreements and reverse repurchase agreements. Certificates of deposit and financial company commercial paper make up more than half the portfolio. The fund's expense ratio is 0.28%. The seven-day yield is 0.26% and 10-year total return for the fund is 1.33%.

2) American Century Capital Preservation Fund

American Century launched the Capital Preservation Fund in 1972. The fund has $2.4 billion in assets. With the primary investment goals of maximum safety and liquidity, this fund is almost exclusively invested in U.S. Treasury bills and bonds. The expense ratio is a very low 0.04%. The seven-day yield is 0.01% and 10-year total return is 1.03%.

3) Vanguard Federal Money Market Fund Investor Class

The Vanguard Federal Money Market Fund, with $4.8 billion in portfolio assets, aims for income and capital preservation through a conservative investing strategy of holding short-term U.S. government securities and repurchase agreements. This fund, launched by Vanguard in 1981, has a relatively low expense ratio of 0.11%. The seven-day yield is 0.23% and 10-year return is 1.27%.

4) Fidelity Government Money Market Fund

The Fidelity Government Money Market Fund seeks the highest possible level of income consistent with capital preservation. The fund was launched in 1990 and has $45 billion in total assets. Typically, the fund's assets are at least 99% invested in either U.S. government securities, cash or fully collateralized repurchase agreements. The fund's expense ratio is 0.15%. The seven-day yield is 0.01% and 10-year cumulative return for the fund is 1.21%.

5) Schwab Cash Reserves Fund

The Schwab Cash Reserves Fund, launched in 2004, aims for high current income within the bounds of capital preservation and liquidity. The fund invests its $40 billion in assets under management in high-quality, short-term money market securities, including U.S. Treasury bills, commercial paper, repurchase agreements, certificates of deposit and various other debt securities from both domestic and foreign issuers. The expense ratio for this fund is 0.16%. The seven-day yield is 0.07% and 10-year return is 1.19%.