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. Water pollution can also affect the crops. So, water pollution is an important issue for humanity. Therefore, the control of irrigation water is a necessity. In this paper, a methodology based on process capability indices (PCIs) has been presented to control the levels of pH, dissolved oxygen ( DO) and temperature ( T) in dam's water for irrigation. Fuzzy PCIs have been proposed for this aim. The fuzzy estimates of (C) over cap (p) and (C) over cap (pk) are obtained for pH, DO, and T based on Buckley's interval estimation approach and based on fuzzy specification limits. An application has been made for Kesikkopru Dam in Ankara, Turkey. In this paper, Buckley's approach is re-arranged to obtain a triangular fuzzy membership function because it cannot be obtained from Buckley's approach in some situation.