Abstarct: This research aims to assess the effects of water consumption in paddy rice fields around Zayandeh-Rud, at the west of Isfahan, on the river water quantity. Values of actual evapotranspiration for rice (ETc) were calculated by FAO method and compared with the water used for irrigation between two hydrometry stations of Pol-Kalleh and Lenj calculated baxsed on balance equation. Meteorological data for Zarrin-Shahr, Mobarakeh and Felavarjan stations was firstly collected and verified by double mass test. Then, daily reference crop evapotranspiration (ET0) was calculated by different methods using Ref-ET computer program. baxsed on the best selected method of FAO-24 Penman annual values of ET0 for these stations were 1758, 1690 and 1466 mm, respectively,. ETc for rice was calculated by FAO-56 method, multiplying ET0 by a crop coefficient. The values of ETc for aforementioned stations were 1485, 1438 and 1224 mm, respectively, considering 200 mm for puddling. Total volume of rice water requirements was estimated to be about 77 Mm3 for the whole area of the paddy fields located between the two hydrometry stations, which is about 6630 ha. Considering irrigation efficiency of 50% in rice fields, this value is in agreement with the balance method (100 Mm3). Finally, change in cropping pattern around Zayandeh-Rud (form rice to other crops) was evaluated as a tool for optimal management of the river. Water requirements calculated for proposed crops of bean, sorghum, walnut, apple and grape were 739, 929, 1610, 1532 and 1411 mm, respectively. Results show that replacing rice with these crops will decrease water consumption of about 27, 15, 24, 29 and 40 Mm3/year, respectively. In other word, changing crop pattern from rice can provide a discharge of about 1.13 to 3 m3/s for river, annually. This water can be allocated to the environmental demand of the river.