Percolation Precipitation Abstract 70% of the population in Oregon lives above the seven major aquifer systems in the Willamette Valley. The seven primary hydrostratigraphic units include the High Cascade, Upper Sedimentary, Willamette Silt, Middle Sedimentary, Lower Sedimentary, Columbia River Basalt, and the Basement Confining. Precipitation recharges these units at an average rate of 22 in/yr between the high and lowland areas. The period of greatest rainfall occurs from November to April and allows for more water to infiltrate than is withdrawn from the regional aquifer systems. Summer months with evaporation and irrigation account for the most water use. Regional groundwater levels fluctuate by about 65 ft each year and are projected to decline with time as population in the valley increases and recharge varies with changing climate dynamics. Overview of the Willamette Basin l The seven primary hydrostratigraphic units include the High Cascade, Upper Sedimentary, Willamette Silt, Middle Sedimentary, Lower Sedimentary, Columbia River Basalt, and the Basement Confining all deposited within the basin (Figure 1). l The Willamette Basin is home to 1.9 million people, all in need of a continuous water supply. l Rain fall is the primary source of recharge to the system. l Water use, and climate effect the overall amount of ground water. l The future of the basin, water budgeting. Rainfall Recharge The Willamette Valley is a large scale watershed. Rainfall on the east side of the Coast Range, the west side of the Cascades and within the valley provide a system of recharge to the underlying water table. Rainfall is greatest in winter months. In Salem the annual rainfall is 79 inches during the months between December and March (Conlon et al. 2005). Figure 2 Figure 3 Shows the water system including precipitation for the year 2000 in the central portion of the basin. The amount of precipitation varies from season to season so understanding the exact amount of rainfall recharging the aquifers is difficult. Figure 4 shows the water cycle system with the influencing factors of ground water, precipitation, and percolation. Conclusions The Willamette Basin groundwater levels change over time based on varies factors. Factors may increase the level, rainfall and or surface water amounts, and factors may decrease the level, excessive withdrawal. Precipitation provided the most impute into the groundwater system, as seen in Figure 3, this occurs during the winter months. Humans play an important role in the use and recharge of water, and our impact has the potential to be negative if steps are not taken soon to reduce water wasting. References Cited Conlon, Terrence D, et al. Ground-Water Hydrology of the Willamette Basin, Oregon Scientific Investigations Report 2005–5168 (2005). 83 pages Oregon State University, 2004 Willamette Basin Explorer. Dept. home page. June 2004 May 27, Reed, Christopher W. “Town of Longmeadow 2006 Water Quality Report.” Longmeadow Water Department. 15 Apr May S.A. Water. The Water Cycle. 8 Feb May Woodward, Dennis G, Marshall W Gannett, and John J Vaccaro. Hydrogeologic Framework of the Willamette Lowland Aquifer System (1998). Groundwater Hydrology of the Willamette Basin, Oregon Prepared by: Brittnie Andrew Western Oregon University Spring 2008 ES473 Environmental Geology Human Help The current aquifer system is supplying the people within the valley with plenty of the water needed to sustain life, and then some. Over time based on the fluctuation of the groundwater levels more efforts are going to need to be put into the task of conserving water. However if efforts where made sooner conservation of water would be less costly in the future. Basic and small actions can be taken to help prevent water lose within the system. l Watering plants in the morning (also helps the plants). l Washing the car in the lawn if needed. l Collecting rainwater and storing it for future use. Figure 1 shows the general location of the Willamette value within Oregon as well as sub basins (OSU, 2008). Figure 2 Willamette River Portland Area (OSU, 2008). Water Use The Willamette Basin groundwater provides water to people, farming, and industries by means of wells. In 2000 the total amount of water that was pumped was about 300,000 acre-ft, of the total, 81 percent was pumped for irrigation, 14 percent for public supply, and 5 percent for industrial use (Colon et al., 2005). During the periods of little to no recharge, April through September in 2000, Figure 2, much of the stored water is consumed. Evaporation and transpiration are also factors in water use. Transpiration, best described as “tree sweat” accounts for the use of 28in/yr (Colon et al., 2005). This number as well varies from year to year, but that is based on the depth of saturation and the height of the water table. The water table levels fluctuate each year for varies reasons. l The amount of water withdrawn is greater than the amount of water imputed. More withdrawn during summer months then winter months. l Warmer climate means higher evaporation rates. l Building of more wells provides the potential for more water to be used. Figure 3 Water use and budget for Central Willamette Basin in 2000 (Colon et al. 2005). Figure 5 Cobble Mountain Watershed Reserve (Reed, 2008). Figure 4 A break down of the Hydrologic Cycle (S.A. Water, 2008).