Methodology for Calculating Water Supply for Irrigation from Reservoirs of Multifunctional Purpose (on the Example of China and Belarus

Reservoirs with multifunctional purposes are the most widely used as sources of water for irrigation. In China, a large number of irrigation systems have been created using reservoirs as sources for flood prevention, irrigation, industrial and agricultural water supply. In Belarus, the Soligorsk, Lyuban, Petrovich, Pogost and other multifunctional reservoirs are used for irrigation.  The useful (regulating) volume of a multifunctional reservoir consists of separate components calculated for each consumer in the basis of a hydrological and feasibility study. Thus, the useful volume of water intended for irrigation should provide the required irrigation capacity and capacity of the irrigation system. When determining the useful (regulating) volume of a multifunctional reservoir, the total water requirements for various purposes (water supply, irrigation, hydropower, etc.) are calculated. For each consumer, the required design supply is assigned and the appropriate method for determining water consumption is used.  Reservoirs intended for water supply count on 95–97 % availability of runoff, and for irrigation purposes – on 75–80 % availability in zones of moderate and excessive moisture. In arid, desert and semi-desert regions of China, where lack of water in dry years can lead to large crop losses, irrigation reservoirs are expected to be 90–95 % secure. The volumes and costs of water in the reservoir for irrigation purposes must cover the needs of plants in water within the established time frame for the year of estimated availability. The paper provides a formula for determining the weighted average irrigation rate and the volume of water required for irrigation. A methodology is presented for calculating the useful volume of a reservoir intended for industrial water supply and irrigation, for which flow regulation is calculated for two supply of water consumption. A method is proposed for determining the flow rate of entrained sediments for various stages of their movement. A formula is presented for the total volume of suspended and bottom sediments entering the reservoir.