Despite the prominence of the view that cooperation in water resources management is desirable, investment and operational decisions for specific projects are often made without full consideration of potential downstream impacts. This study estimates the economic cost of sector specific uncoordinated water resource development and operation in Tana sub-basins and Upper Beles using the potential abilities of hydro-economic optimization models. Methodologically, the study comparatively analyzing the ideal situation in which different water using sectors fully coordinated to maximize overall economic benefit of the limited water resource in Tana sub-basin. And the more realistic case where all decision-makers act independently and non-cooperatively with an objective of maximizing their sector specific economic benefit. To investigate the opportunity cost of un-coordinated management under different hydrological flow condition, deterministic hydro economic model with one year planning horizon covering 46 years (19602005) of historical flow were used. Analysis were made for three future structural development scenarios to evaluate the sensitivity of cost of un-cooperative management with level of water resource development. Model output showed that on average uncoordinated reservoir management costs the system by 166, 345 and 525 MEBr (3.5, 6.0 and 9.5 %) under scenario one, two and three respectively treating minimum environmental and social demands as a constraint. The magnitude of coordination benefits generally increases with low water availability and with inflow variability, ranging from no cost during years of high water availability to more than 50% during the driest year. On other hand as compared to sector specific management, there found a decline of 18,580, 25,700 and 31,748 ha of dry season irrigated area in Tana Sub-basin, in return additional 104, 145 and 225 GWh/year from Tana-Beles hydropower with fully satisfying Upper Beles irrigation demand for three development scenarios respectively. Even though the study showed sectoral collaboration generates more benefit and saves high economic loss during water stress years, assuming perfect hydrological flow and static economy influences the result. A small change in economic value of crop or hydropower may completely reverse the water allocation so as the economic value collaborated management. In order to get better estimate, it is recommended that future study should consider future economical dynamics and stochastic approach.