Writing prompt Grade 12 Science
Shifting Snowmelt Timing in the Verdigan Basin
A grade 12 explanatory writing prompt using model output from a fictional mountain basin to explain why earlier snowmelt shrinks summer streamflow.
Objectives
- Students will explain how the phase of winter precipitation and the timing of melt control when water reaches a river.
- Students will use model output to support a written explanation of declining summer streamflow.
- Students will distinguish a change in total annual precipitation from a change in the timing of runoff.
Vocabulary
- Snowpack
- The snow that piles up in the mountains over the winter. As it melts in spring and summer, it acts like a giant water tank feeding rivers and reservoirs.
- Baseflow
- Baseflow is the water that keeps a stream flowing even when it has not rained for a while. It comes from groundwater slowly seeping into the streambed.
Also: snow water equivalent, freezing level, runoff, cryosphere
Teacher overview
The Verdigan Basin is a fictional mountain watershed whose river supplies a small utility called the Alderfell Water District. The district holds almost no reservoir storage, so its summer supply depends on snow that fell months earlier and melts slowly through the warm season. This prompt gives students output from a fictional regional climate model and asks them to explain, in writing, how a basin can keep nearly all of its annual precipitation and still lose nearly half of its late-summer river flow. The explanation has to name a mechanism, since the point of the task is the physical chain that connects winter air temperature to a July streamflow measurement.
Prompt
Background. Snow is a form of water storage. When winter precipitation falls as snow, it stays on the ground as a snowpack, and hydrologists measure how much water that snowpack holds as snow water equivalent, or the depth of liquid water the snow would make if it all melted at once. The snowpack releases that water gradually once air temperatures climb above freezing, which is why a mountain river can keep flowing through a rainless summer. Rain behaves differently. Rain that falls in January runs off within days.
The freezing level is the elevation where air temperature reaches the freezing point. When winter air warms, the freezing level rises, and a larger share of the basin receives rain instead of snow. Warmer spring air also starts the melt earlier and drives it faster.
The table below shows output from a fictional climate and runoff model for the Verdigan Basin. The baseline column describes the modeled period from 1990 to 2019. The projection column describes the modeled period from 2040 to 2069 under one warming scenario. All values are model output for this fictional basin.
| Modeled quantity | 1990-2019 baseline | 2040-2069 projection |
|---|---|---|
| Mean annual precipitation | 41 inches | 40 inches |
| Share of precipitation falling as snow | 62 percent | 44 percent |
| Mean November-March air temperature | 29 degrees F | 34 degrees F |
| April 1 snow water equivalent at the basin's high-elevation snow course | 26 inches | 17 inches |
| Date of peak river flow | May 28 | May 6 |
| Annual runoff | 84,000 acre-feet | 82,000 acre-feet |
| December-March runoff | 9,000 acre-feet | 19,000 acre-feet |
| July-September runoff | 21,000 acre-feet | 12,000 acre-feet |
Writing task. Write an explanatory essay for the Alderfell Water District's board of directors. Explain why the July-September flow of the Verdigan River falls by nearly half in the projection even though annual precipitation and annual runoff barely change. Your essay must trace the full physical chain from winter air temperature to a July streamflow reading, and it must use at least four numbers from the table as evidence. Do not argue for or against any policy. The board has asked for an explanation of the mechanism, not a recommendation.
Planning Organizer
Fill in every row before you draft. Each row corresponds to one scored part of the rubric.
| Planning step | Your notes |
|---|---|
| The question in one sentence | |
| Link 1: what warmer winter air does to the freezing level and to the form precipitation takes | |
| Evidence for link 1 (numbers from the table) | |
| Link 2: what a smaller snowpack and an earlier melt do to the timing of runoff | |
| Evidence for link 2 (numbers from the table) | |
| Link 3: why summer flow depends on stored snow rather than on summer rain | |
| Evidence for link 3 (numbers from the table) | |
| The apparent contradiction and its resolution: annual runoff holds steady while summer runoff drops | |
| Technical terms you will define for the board | |
| Closing sentence stating what the model output does and does not show |
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Exemplar & Rubric
Exemplar response
The Verdigan Basin model projects almost no change in how much water the basin receives each year, yet it projects a sharp drop in how much of that water arrives in late summer. Annual precipitation moves only from 41 to 40 inches, and annual runoff moves only from 84,000 to 82,000 acre-feet. July-September runoff, however, falls from 21,000 to 12,000 acre-feet. The basin keeps nearly all of its water but loses the storage that used to hold water until summer, and that storage is the snowpack itself.
The chain begins with winter air temperature. Mean November-March temperature in the model rises from 29 to 34 degrees F. That shift lifts the freezing level, the elevation at which air reaches the freezing point, so a larger fraction of the basin sits below it during storms. Precipitation that would once have fallen as snow at those elevations falls as rain instead. The model shows the snow share dropping from 62 percent to 44 percent of total precipitation.
The consequence appears in the snowpack. April 1 snow water equivalent at the high-elevation snow course falls from 26 inches to 17 inches, so the snowpack enters spring holding about a third less water in frozen form. Rain that falls on the lower basin during winter storms does not wait for spring. It runs off within days, which is why modeled December-March runoff more than doubles, from 9,000 to 19,000 acre-feet. Water still reaches the river, but it reaches the river in January rather than July.
Warmer spring air then advances what remains of the melt. The modeled date of peak flow moves from May 28 to May 6, three weeks earlier. A snowpack that begins melting earlier and melts faster is exhausted earlier. By late July there is little snow left at any elevation to feed the river, and the basin receives little summer rain to replace it. The river falls back toward baseflow, the slow groundwater contribution that continues after surface runoff ends, and baseflow alone cannot supply the volume the district drew in past summers.
This resolves the apparent contradiction in the table. Annual runoff is a yearly total, and a yearly total does not show when the water arrives. The same quantity of water passing a gauge in a different month is a different resource for a utility with no reservoir. Alderfell's demand peaks in the same weeks that the projected supply falls, and the model indicates those weeks now follow an earlier and smaller melt. One limitation deserves mention: these are results from a single fictional model run under one warming scenario, so they describe a direction of change rather than a guaranteed volume in any particular year.
Rubric
| Criterion | Emerging | Developing | Proficient | Advanced |
|---|---|---|---|---|
| Mechanism | Restates that summer flow drops without naming a cause | Names warming as the cause but skips intermediate steps | Traces a complete chain from winter temperature through precipitation phase, snowpack, melt timing, and summer flow | Traces the complete chain and explains why each step follows physically from the one before it |
| Use of model output | Cites no numbers, or cites numbers that do not support the point being made | Cites fewer than four values, or cites values without connecting them to a step in the chain | Cites at least four values from the table, each attached to the step it supports | Cites at least four values, attaches each to a step, and compares baseline with projection to show the direction of change |
| Resolving the timing paradox | Does not address the steady annual runoff | Notes the steady annual total without explaining it | Explains that a stable annual total can hide a shift in the month water arrives | Explains the shift and states why timing rather than volume determines what a utility without storage can use |
| Scientific writing conventions | Technical terms are used incorrectly or left undefined | Some terms defined; organization is uneven | Terms such as snow water equivalent, freezing level, and baseflow are defined and used correctly in an organized sequence | Terms are precise throughout, transitions carry the causal chain, and the closing states the limits of a single model run |
Differentiation
Support
Provide the chain as three labeled boxes on a handout, one for precipitation phase, one for snowpack and melt timing, and one for summer river flow, and ask students to place each table value in the box it belongs to before drafting. Give a sentence frame for each link, such as "Because winter air temperature rises from ___ to ___, the freezing level ___, which means that ___."
Extension
Ask students to add a paragraph identifying which single row of the table would be most useful to measure directly each year, and to justify the choice by explaining what that measurement would tell the district that the other rows would not.
ELL supports
Pre-teach snow water equivalent, freezing level, runoff, and baseflow with a labeled cross-section of a mountain basin showing snow above the freezing level and rain below it. Allow students to complete the Planning Organizer in phrases and to talk through the causal chain with a partner before writing it.
Extensions
Have students compare this modeled basin with a real river gauge record from a mountain watershed in their region, using published streamflow data, and write a short note on whether the observed date of peak flow shows any trend over the period of record. Discuss why a single gauge record makes trend detection harder than a model comparison does.