Liquidity needs must be forecast by the financial institution. These needs are subject to the demands of the depositors along with the types of deposit. It is also subject to the interest and credit risk. Interest rate risk stems from the varying incomes from interest and varying expenses. The more risky the interest rate risk the less certain will be the ability to forecast the liquidity gap.
Credit risk is a function of diversification of assets.
You can use the Federal Reserve System of the U.S. as an example of central banking. Oversee Federal Reserve System, set reserve requirement, serve on FOMC, assist in setting discount rate. Banks and other depository institutions borrow from the Fed to meet reserve requirements. As banks overextend lending, they may be caught short on reserves. Federal funds and the discount window are ways banks may obtain the reserves to meet their requirements.