The Ethiopian government is involved in the development of massive irrigation projects for sugar production in different parts of the country. One of these schemes is the Wolkayite sugar development project, located in the western zone of the Tigray region. It is planned to utilize the runoff of the Zarema River, which is a major tributary of the Tekeze river basin. To achieve this objective, a dam has been built across river Zarema, which would enable irrigation of more than 40,000 ha of land. The Sugar cane which is estimated to be cultivated around 40,000 ha of land is to be utilized as raw material for Welkit sugar factory which has a capacity of 24,000 TCD. The objective of the study is to develop operation modeling for the Zarema May Day Dam reservoir. The input data consider for the model are daily inflow, monthly irrigation, and monthly reservoir surface evaporation, and Zarema May Day Dam physical data. The reservoir operation modeling was done using Zarema river inflow source options. The high Simien mountain range ensures relatively high runoff in the river Zarema at the dam sit. The annual average runoff is around 1300Mm3. The seasonal flow regime is characterized by an important average monthly flow, particularly in July (169 m3/s) and August (225 m3/s), whereas the period from October to May exhibits a monthly average lower than the annual mean. The reservoir operation modeling has been developed using two cases of inflow and three different scenarios, which has the same operation zone but differ irrigation area in Ha. And also reservoir operation guide curve and each year reservoir pool level were established.