Orissa depends largely upon monsoon for its water resources. Southwest monsoon triggers rainfall in the state. About 78% of total annual rainfall occurs during the period from June to September and the balance 22% in the remaining period. In addition to seasonal availability, the rainfall in the state also shows spatial variation i.e. from about 1200 mm in southern coastal plain to about 1700 mm in northern plateau. This has resulted in causing droughts in some parts of the state and floods in some others. The long-term average annual rainfall in the state is of the order of 1482 mm. Under normal condition, the state receives annual precipitation of about 230.76 billion cubic metres (BCM) of water. Of the total precipitation, a part is lost by evaporation, transpiration and deep percolation and a part stored in the form of ground water reserve and the remaining appears as surface runoff. The groundwater reserve and surface runoff constitute the water resources of the state. The water resources scenario of Orissa and India are given in the table below.

The state is endowed with an extensive network of rivers and streams. As per an assessment made in 2001, the average annual availability of surface water is estimated as 120.397 BCM. Out of the above, the yield from its own drainage boundary is 82.841 BCM and inflow from neighbouring states through interstate rivers is 37.556 BCM. Considering the topography and geological limitations, 75% of the average annual flow can be utilized. Due to increasing demands for water for various uses, an attempt has been made to assess the availability of water by the year 2051. The assessment reveals that the surface water availability from its own drainage boundary remains more or less fixed but the inflow of surface water from neighbouring states will be reduced from 37.556 BCM to 25.272 BCM. The following table shows the assessed inflow of surface water pertaining to the years 2001 and 2051.