Water covers over 70% of the Earth surface and is a very important resource to people and the environment. Water pollution affects drinking water, rivers, lakes and oceans all over the world. This consequently harms human health and the natural environment. In this paper, a methodology based on Process Capability Indices (PCI) has been presented to control the pH value of dam's water for agriculture. Fuzzy Process Capability indices have been proposed for this aim. An application has been made for Kesikkopru Dam in Ankara, Turkey. The water stored in dam has been controlled for the suitability for irrigation.