The long-term exploitation of a geothermal field will, in most cases, cause pressure drop or drawdown in the system, creating a potential for cold-water inflow. The system may suffer subsequent cooling or changes in production characteristics. Chemical changes in geothermal fluids are often precursors of cooling, and chemical monitoring may thus give warning in time for preventive action. Monitoring programs have therefore been developed for different types of geothermal fields. The effectiveness of many of these has been demonstrated in both high-temperature and low-temperature geothermal exploitation.