An important aspect of modeling a water system is understanding how it operates under a variety of hydrologic conditions. Natural variations in hydrology -- month to month and year to year -- can have major effects on the results of your scenarios.
WEAP has four methods for projecting the surface water hydrology over the study period: Water Year Method, Expressions, Catchments Runoff and Infiltration and Read From File Method. These methods may be used to project the inflow to every surface and groundwater inflow point in the system for every month in the study period. This includes river and tributary headflows, surface water inflows to river reaches, groundwater, local reservoir and other supply inflow.