Calculations on Annual Discharges of Water from the Columbia Basin Project
Gauged data are available on water withdrawals from Lake Roosevelt that serve as the principal supply water to the three irrigation districts in the Columbia Basin Project (CBP). In contrast, the total discharge from CBP that returns back into the main stem of the Columbia River is not measured or estimated. An attempt is hereby made to calculate irrigation return flows through an annual mass (volume) balance on water (Tanji and Kielen, 2002). The annual mass balance on water from an irrigation project is defined as:
Volume Water Inflows – Volume Water Outflows = ± Δ Storage
If the control volume (system of interest) for CBP includes both the vadose and saturated zones of the CBP, Eq. (1) expands to
(Surface Water Inflows + Subsurface Water Inflows) – (Surface Water Outflows + Subsurface Water Outflows) = ± Δ Storage
For a comprehensive mass balance on water in the CBP, the components of inflows and outflows may include:
Surface Water Inflows = Irrigation Water + Precipitation + Captured natural rim inflows
Subsurface Water Inflows = Groundwater Rim Inflows + Seepage Inflow from River
Surface Water Outflows = Crop ET + Non-crop ET + Precipitation E&ET + Reservoir evaporation + Irrigation Canal & Lateral Evaporation + Drain Canal Evaporation + Operational and Lateral Spills + Surface Irrigation Drainage into River
Subsurface Water Outflows = Groundwater Rim Outflows + Groundwater Outflows into River + Phreatophyte ET
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
Water Inflows = Water Outflows
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
well as groundwater outflow into river for subsurface water outflows.
Fortunately, the Water Supply, Use and Efficiency Report regarding the Columbia Basin Project is available from Montgomery Water Group, Inc. (1997). This report, however, does not contain all the water flow components identified in Eqs. (3) to (6), and therefore a more simplified water balance is utilized taking into consideration only the major components of water flow. The rationale for the simplification and the neglect of certain flow components is as follows:
Annual average precipitation in the CBP is only about 10.1 inches, much of which is lost through ET, and hence precipitation and precipitation E&ET may be neglected.
Groundwater rim inflows into the CBP and rim outflow from the CBP as well as seepage from the Columbia River into the CBP are difficult components to estimate and herein assumed to cancel each other.
Noncrop ET or ET from native vegetation is assumed to be small as compared to crop ET and, because of low annual precipitation, ET from phreatophytes is also assumed to be small.
If one accepts the above assumptions and simplifications, the annual mass balance on water in the CBP may be rearranged to
(River Withdrawal + Captured Natural Rim Inflows) – (Crop ET + Reservoir Evaporation + Canal & Lateral Evaporation + Operational and Lateral Spills) = (Surface Irrigation Drainage + Groundwater Outflows into River)
Appendix Table C-1 contains the annual mass balance on water for the CBP from 1975 through 1994. Column J gives the combined surface irrigation drainage and groundwater outflow into the Columbia River, the closure term. In this mass balance it is not possible to separate out groundwater outflow from surface irrigation drainage. The latter could be monitored comparatively easily but not the former. The ratio of irrigation return flow to total inflow averages 0.30 (30 percent) of supply water. This also means consumptive water use (evaporated to the atmosphere) is 70 percent because ΔS is assumed to be zero.
The irrigation return flow ratio for the CBP is similar to those of irrigation districts in California, for example, Glenn-Colusa Irrigation District in the Sacramento Valley (0.29) and Panoche Water District in the San Jaoquin Valley (0.31) (Tanji, 1981) and Imperial Irrigation District in the Imperial Valley (0.36) (Kaddah and Rhoades, 1976).
Kaddah, M. T., and J. D. Rhoades. 1976. Salt and water balance in Imperial Valley, California: Soil Science Society of America Journal 49:93-100.
Montgomery Water Group, Inc. 1997. Water supply, use and efficiency report, Columbia Basin Project: 47 pp. plus figures and tables.
Tanji, K. K. 1981. California irrigation return flow case studies: ASCE Journal of Irrigation and Drainage Division 107(IR2):209-220.
Tanji, K. K., and N. C. Kielen. 2002. Agricultural drainage water management in arid and semi-arid climates. FAO Irrigation and Drainage Paper 61, Food and Agriculture Organization of the United Nations, Rome, Italy.