Natural rim inflows refer to surface water inflows from the watershed into the CBP, such as Crab Creek watershed that is impounded in the Potholes Reservoir for use as irrigation water. Groundwater rim inflows are the subsurface inflows of groundwater from lands adjacent to the CBP. Seepage inflows from the river denote subsurface inflows into the CBP from the main stem of the Columbia River. The symbol ET is defined as evaporation losses (E) from moist soil and transpiration (T) losses of water from cropped plants as well as noncropped or native vegetation other than phreatophytes that extract water from the saturated zone such as open drains and wetlands. Groundwater rim outflows are subsurface flows from the CBP to adjacent lands, and groundwater outflows into river are subsurface accretions of water into the Columbia River. The above components of water flows are typically available only when an irrigation project has been subjected to detailed hydrological investigations and/or hydrological modeling.

Over decades, ±Δ Storage in Eq. (1) may be assumed to be zero, so that

The irrigation return flow (IRF) from the CBP into the Columbia River consists of spills from canals and laterals, surface irrigation drainage and groundwater outflow into the river. When data such as surface irrigation drainage and subsurface outflows into the river are not available, (as in the case at the CBP), the above mass balance equations may be used to obtain these flows as a closure term (i.e., by difference). For the case of the CBP, the principal missing data are surface irrigation drainage for surface water outflows into the river as