Before and after gauge study, 118 square kilometres, 14 water years

Hydrology research poster example

Runoff ratio at one catchment gauge before and after thirty per cent reforestation, laid out as a four column A0 landscape board. Each column takes one stage of the argument, so a reader moves left to right from the question to the limits without doubling back.

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WU
Fourteen years at one gauge: runoff ratio before and after thirty per cent reforestation
B. Ellery ¹, H. Nakagawa ¹
1 School of Environmental Engineering, Windermere University
Abstract
A 118 square kilometre headwater catchment was planted to native forest over three years, taking canopy cover from 22% to 52% of the area. A single stream gauge at the outlet has a usable record covering seven complete water years before planting and seven after. Mean annual runoff ratio, the depth of streamflow divided by the depth of rainfall, fell from 0.31 before to 0.26 after, a relative fall of 16%. The change is concentrated in autumn and winter and is close to zero in summer. In depth terms mean annual streamflow fell from 263 mm to 224 mm, about 4.6 gigalitres a year off a catchment that received slightly more rain after planting than before it, at 861 mm against 848 mm.
One gauge, one catchment
This is a before and after study at a single gauge, which is the weakest design in catchment hydrology and the only one available here, because the planting was a land management decision rather than an experiment and no paired catchment was instrumented before it began. Two things make the comparison worth reporting anyway. The rainfall record at the same site shows no trend across the fourteen years, with annual totals of 848 mm before and 861 mm after, so the fall in runoff ratio is not a fall in rain. And the planting was large, fast and mapped: 30% of the catchment area went from grazing to closed canopy inside three years, with a planting record accurate to the hectare. A reader should still treat the result as one catchment observed rather than as an effect measured.
REFERENCES
1.Nakagawa, H. and Ellery, B. (2025) Before and after designs at a single gauge. Journal of Catchment Hydrology, 44(1), 40 to 66.
2.Oduya, K. (2024) Seasonal signatures of afforestation in temperate headwaters. Water Resources Practice, 28(3), 215 to 240.
3.Villiers, S. (2026) Rating history and record quality screening. Windermere Hydrology Notes 2026-02.
METHODS
Catchment. 118 square kilometres of headwater, canopy cover 22% before planting and 52% after.
Gauge. One continuous recorder at the outlet, rating curve checked by gauging twice a year.
Period. Seven complete water years before planting and seven after, 14 in the comparison.
Measure. Annual runoff ratio: streamflow depth divided by catchment rainfall depth, both in mm.
Rainfall. Four rain gauges in the catchment, Thiessen weighted; no trend across the fourteen years.
PARTICIPANT FLOW
Years of gauge record available
21
Gauge moved or rated poorly
4
Years with a complete record
17
Years inside the planting period
3
Years used in the comparison
14
Seven before planting and seven after
7
16%
lower annual runoff ratio after planting
0.31 before against 0.26 after, seven water years each side
r
Pre
Post
00.080.160.240.32
Figure 1. Mean annual runoff ratio (r): Pre is the seven years before planting, Post the seven after.
Source: outlet gauge and four rain gauges, 14 complete water years.
Results
Annual runoff ratio averaged 0.31 in the seven years before planting, with a range of 0.27 to 0.36, and 0.26 in the seven after, with a range of 0.22 to 0.30. The two ranges overlap in a single year, which is what a fall of this size at a single gauge looks like. Seasonally the pattern is clear. Autumn ratio fell from 0.24 to 0.17, winter from 0.46 to 0.38, spring from 0.33 to 0.29 and summer from 0.14 to 0.13. The autumn fall of 0.07 is the largest of the four and the summer fall of 0.01 is inside the year to year noise. Rainfall averaged 848 mm before and 861 mm after, so the catchment received slightly more rain and yielded less water. In depth terms, mean annual streamflow fell from 263 mm to 224 mm, a loss of 39 mm a year across 118 square kilometres, which is about 4.6 gigalitres. Peak flows behave differently from totals: the mean annual maximum daily flow fell by 9% against 16% for the annual ratio, so the change is mostly in the baseflow and the small events rather than in the floods, which is consistent with an interception and transpiration mechanism rather than with a change in the soil store.
Reading a single gauge
The seasonal pattern is the part of this result that carries information. A forest uses more water than pasture mostly through interception and transpiration, both of which bite hardest when the soil is wet and the canopy is intercepting steady rain, which in this catchment is autumn and winter. A fall concentrated in those two seasons and absent in summer is what the mechanism predicts; a uniform fall across all four would have pointed at the gauge or the rating instead. That is as far as a single catchment can be pushed. The design cannot separate the planting from any other change over fourteen years, and two are known: a farm dam licence was issued in the eleventh year, and three kilometres of road were sealed in the ninth. Neither is large enough to account for 39 mm a year on its own, and removing the two affected years from the post planting mean moves the ratio from 0.26 to 0.265.
Conclusion
Runoff ratio fell 16%, from 0.31 to 0.26, after 30% of the catchment was planted, with the loss concentrated in autumn and winter. Rainfall did not fall over the same period, so the catchment received slightly more rain and yielded 39 mm a year less water.
Runoff ratio 0.31 before planting, 0.26 after
Autumn fell 0.07, summer fell 0.01
Rainfall 848 mm before and 861 mm after, no trend
Mean annual streamflow down 39 mm, about 4.6 gigalitres
Data and access
The gauge is operated by the state water authority and the record was supplied with its rating history, which is what allowed four years to be excluded on quality grounds rather than on judgement. The planting areas came from the land manager own mapping, which is accurate to the hectare and dated by planting season. No funding was received for this analysis.

Block by block

What each block on the board is for, in the order a reader walks it.

Title band, two authors and a QR code
The title gives the record and the change: fourteen years at one gauge, runoff ratio before and after thirty per cent reforestation. Two authors share one school affiliation, with the QR code at the right of the band.
Abstract
A 118 square kilometre headwater catchment planted from 22% to 52% canopy. Seven complete water years each side of the planting. Mean annual runoff ratio fell from 0.31 to 0.26, a relative fall of 16%, concentrated in autumn and winter and near zero in summer.
One gauge, one catchment
The design caveat stated plainly, with the two things that make the comparison worth reporting: no rainfall trend across the fourteen years, and a planting that was large, fast and mapped to the hectare. The block still tells a reader to treat it as one catchment observed.
Methods as a labelled protocol
Five entries: the catchment area and canopy change, one continuous recorder with the rating curve gauged twice a year, seven water years each side, runoff ratio as streamflow depth over rainfall depth, and four Thiessen weighted rain gauges with no trend.
Record screening flow
Twenty one years of gauge record available, 4 dropped where the gauge moved or the rating was poor, 17 complete, 3 falling inside the planting period, 14 used in the comparison and seven of those on each side of the planting.
Key number and figure
A key number block reads 16% lower annual runoff ratio after planting, with the detail line giving 0.31 against 0.26 and the seven water years each side. The figure plots the two means as Pre and Post, keyed in the caption.
Results, discussion and references
Annual ranges, the four seasonal ratios, the rainfall figures and the 39 millimetre streamflow loss worth about 4.6 gigalitres. The discussion reads the seasonal pattern as the mechanism and names the two other known changes, with three references and a data access note.

What makes this board work

The weakest design is named first

The second column opens by saying this is a before and after study at a single gauge, which is the weakest design in catchment hydrology. A reader who is told that up front trusts the two arguments that follow it.

Rainfall is ruled out with a number

848 millimetres before and 861 after, with no trend across the fourteen years. The catchment received slightly more rain and yielded less water, which is the one sentence that keeps the result standing.

The seasonal split does the mechanism work

Autumn fell 0.07 and summer 0.01. A fall concentrated where the soil is wet and the canopy is intercepting is what interception and transpiration predict; a uniform fall would have pointed at the gauge or the rating instead.

Questions people ask

What goes on a hydrology research poster?

The catchment and its area, the gauge and its rating history, the record length and how years were screened, the measure, the change with its seasonal structure, and the alternative explanations. A single gauge study has to spend more of the board on limits than on the result.

How do I screen a record on a poster?

Use a flow. This board shows 21 years of record, 4 dropped for a gauge move or a poor rating, 17 complete, 3 inside the planting period and 14 used, split seven before and seven after. That answers the obvious question in five boxes.

Why use a four column layout?

Because it maps to the argument. Column one holds the question and the method, column two the design caveat, column three the figure and the results, and column four the interpretation and the limits. A reader can stop after any column and still have something whole.

What type size does an A0 landscape poster need?

Body text here is near 24 pixels on a 2340 by 1656 canvas, about 12 millimetres printed, readable at a metre. Four narrow columns mean short lines, which reads well, but check that no column falls below about 45 characters a line or the text starts to look ragged.

Can I print this at 300 DPI?

The export doubles the working canvas, so A0 landscape leaves at 4680 by 3312 pixels, about 100 DPI at full size. That is the working resolution for a board read from a metre away, and the export produces no bleed, crop marks or CMYK.

How do I adapt this to my own catchment?

Retype the title, abstract and five method entries, edit the flow node labels and counts, and change the two bar figure values. The key number block takes a value, label and detail line, and the acknowledgements block has been retitled here as data and access.

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Sources

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