describe California’s water supply as a system of connected stores and flows, analyze real reservoir and snowpack data, and explain how one change to the surface can trigger feedbacks in other Earth systems.
Instruction
The phenomenon. Every year on or near April 1, the Department of Water Resources (DWR) measures the Sierra Nevada snowpack, because April 1 is usually when it holds the most water. On April 1, 2015, after four dry years, the statewide snowpack held about 5% of its April 1 average. Surveyors stood on bare grass where there should have been meters of snow. Eight years later, in the spring of 2023, the snowpack was more than twice the average. California’s water supply can swing from famine to flood in a few years. Why, and what happens to the rest of the system when it does?
Earth’s systems. Earth scientists divide the planet into four interacting systems: the geosphere (rock, soil and sediment), the hydrosphere (all water: oceans, rivers, lakes, groundwater and ice), the atmosphere and the biosphere (living things, including people). The water cycle runs through all four. Water evaporates from the Pacific, winter storms carry it inland, and it falls as rain and, in the mountains, as snow. The snowpack is a natural reservoir: it stores winter precipitation and releases it as meltwater in spring and summer, just when farms and cities need it most. DWR estimates that, on average, the snowpack supplies about 30% of the state’s water.
Built reservoirs. Dams add a second store. Shasta Lake, the largest reservoir in California, holds up to 4,552,000 acre-feet (an acre-foot is 1,233 m3, the water that covers an acre one foot deep). Lake Oroville holds up to about 3,537,600 acre-feet.
Worked example: reading reservoir data. DWR’s monthly records (CDEC) show Shasta holding about 2,662,000 acre-feet in April 2015: 2,662,479 / 4,552,000 ≈ 58.5% of capacity. In September 2021, at the end of another drought, it held 1,074,000 acre-feet, only 23.6%. In April 2023 it held 4,430,000, or 97.3%. Oroville fell to 787,600 acre-feet (22.3%) in September 2021 and stood at 3,461,000 (97.8%) in April 2024. Notice the seasonal pattern too: between April and September 2024, Shasta’s storage fell by about 1.6 million acre-feet as water was released for farms, cities and river flows.
Feedbacks. A feedback is a chain in which a change in one system causes changes that loop back. A snow-poor winter starts several.
- Snow and sunlight: fresh snow reflects most sunlight; bare ground absorbs it. Less snow means warmer ground and air, which melts the remaining snow sooner: a reinforcing (positive) feedback.
- Surface water and groundwater: when rivers and reservoirs run low, farmers and towns pump more groundwater, lowering the water table.
- Groundwater and the geosphere: as water is pumped from clay layers, the clay compacts and the land surface sinks, reducing the aquifer’s ability to store water in future wet years.
Your questions. Good questions for this unit connect systems: Where does the water go when the snow melts early? How much water is underground? Why does the ground sink, and can it rise again?
Formative check
Work through these before moving on. They are not graded — they tell you, and your teacher, whether the standard below has landed yet.
In a year when the snowpack is very small, what do you expect to happen to groundwater levels in the Central Valley?
Simulation & tools
Snowmelt in a Sierra river
Open the USGS page for the American River at Fair Oaks (site 11446500). Set the graph to show daily discharge for the last full water year (October 1 to September 30). Record the month of the highest flow, the highest discharge in cubic feet per second, and the flow in late September. Then write two sentences: which months carry snowmelt, and how would an early melt change the shape of the graph?
No device? Use the printed hydrograph from the teacher and read the same three values.
Open U.S. Geological Survey ↗ · U.S. Geological Survey · Public domain (U.S. federal)
Shasta holds up to 4,552,000 acre-feet. In September 2021 it held 1,074,000 acre-feet, which is % of capacity; in April 2023 it held 4,430,000, which is %.
Why is less snow on the mountains an example of a reinforcing feedback?
Draw a system model of California’s water with at least six stores (ocean, atmosphere, snowpack, reservoirs, rivers, groundwater) and arrows for the flows between them. Mark the flows people control. Circle one feedback and explain it in two sentences. Then write two investigable questions for the class board.
How confident are you that you can analyze California water data and explain how one change can cause feedbacks in other Earth systems?
Practice
Work these on paper or in your notebook, then open Check your answer. Aim for all of Fluency and Application; try at least one Challenge.
Printable version: this unit’s practice workbook (PDF)
Fluency
Build speed and accuracy with the core skill.
- Lake Oroville holds up to 3,537,577 acre-feet. In September 2021 it held 787,600 acre-feet (CDEC). What percentage of capacity was that?
Check your answer
Answer: 22.3% - Shasta Lake (capacity 4,552,000 acre-feet) held 2,662,479 acre-feet in April 2015. What percentage of capacity was that?
Check your answer
Answer: 58.5% - Convert Shasta’s full capacity of 4,552,000 acre-feet to cubic kilometers (1 acre-foot = 1,233.48 m3).
Check your answer
Answer: about 5.61 km34,552,000 × 1,233.48 m3; 1 km3 = 109 m3.
Application
Use the skill in context. Show your reasoning.
- Shasta held 4,364,000 acre-feet in April 2024 and 2,766,000 in September 2024 (CDEC monthly values). How much storage was released or lost over the summer, and what fraction of capacity is that?
Check your answer
Answer: 1,598,000 acre-feet, about 35% of capacity - Snow 1.5 m deep has a density about 0.3 times that of water (a typical spring value). How deep would the water be if it melted (its snow water equivalent)?
Check your answer
Answer: 45 cm1.5 m × 0.3 = 0.45 m. - Explain the snow-albedo feedback in a dry winter and say whether it is reinforcing or balancing.
Check your answer
Answer: Less snow exposes darker ground, which absorbs more sunlight and warms, melting the remaining snow sooner; the change amplifies itself, so it is reinforcing.
Challenge
Stretch problems. Expect to think before you write.
- Trace a chain of at least four linked changes, across at least three Earth systems, that starts with a snow-poor winter and ends with the land surface in the San Joaquin Valley sinking. Name the system at each step.
Check your answer
Answer: Atmosphere: little snow falls. Hydrosphere: less snowmelt reaches rivers and reservoirs. Biosphere (people): farms pump more groundwater. Hydrosphere: water tables fall. Geosphere: clay layers compact and the land subsides.
Review
Keep earlier skills sharp.
- Compare a magnitude 5.5 earthquake with a magnitude 4.0 earthquake. About how many times larger is the ground-motion amplitude, and about how many times more energy is released?
Check your answer
Answer: amplitude ≈ 31.6 times; energy ≈ 178 timesEach whole step in magnitude is 10 times the amplitude and about 32 times (101.5) the energy. - A sample starts with 200 g of an isotope whose half-life is 8 years. How much remains after 32.0 years?
Check your answer
Answer: 12.50 g32.0 years is 4 half-lives; halve 4 times.