Uncertainty in the net hydrologic flux of calcium in a paired-watershed

harvesting study

LTER ASM Estes Park, COSeptember 2015

John Campbell, Ruth Yanai, Mark Green, Genen Likens, Craig See, Amey Bailey, Don Buso, Daqing Yang

Paired watershed studies

W6W5

• Watersheds are unreplicated

• It’s difficult to find suitable replicate watersheds and expensive

• Uncertainty analysis can be used to report statistical confidence

Ca response to harvesting

HarvestHarvest

Data courtesy G.E. Likens

Sources of uncertaintyPrecipitation • Interpolation model• Collector efficiency• Gaps in volume• Chemical analysis• Unusable chemistry

Stream water• Watershed area• Stage height-discharge• Gaps in discharge• Chemical analysis• Interpolation model

May 1956

Precipitation interpolation method

W1 W2 W3 W4 W5 W6 W7 W8 W9

Ann

ual p

reci

p. (m

m)

1340

1360

1380

1400

1420

1440

1460

1480

1500

1520ThiessenKrigingIDWSplineRegression

Uncertainty = 0.6%

Watershed area

W6

Uncertainty = 2.3%

Streamflow

Monte Carlo approach

Chemical value

Net Hydrologic Flux

Etc.

Calculation

Ca response to harvesting

Contributions to Uncertainty

Chemical analyses

Uncertainty = 1.0%

• Precision describes the variation in replicate analysis of the same sample

• At Hubbard Brook, one sample of every 40 is analyzed four times 

Acknowledgments

Calcium data were obtained through funding from the A.W. Mellon Foundation and the NSF, including LTER and LTREB.

Amey BaileyIan HalmNick GrantTammy WoosterBranda Minicucci

Gaps in streamflow

• 7% of streamflow record is gaps• 65% due to the cart recorder (50% clock)

Uncertainty = 7.9%

Easier said than done…

• Difficult to identify sources of uncertainty

• Difficult to quantify sources

• Multiple approaches to uncertainty analysis

• No single answer

Water year (June 1)1960 1970 1980 1990 2000 2010

Net

hyd

rolo

gic

flux

(kg

ha-1

yr-

1)

-24

-21

-18

-15

-12

-9

-6

-3

0

W6 (reference)W5 (harvested)

Source of excess Ca in W5

• Dissolution of calcium oxalate, which is common in plant tissue and is known to accumulate in forest soils (Bailey et al. 2002).

• Dissolution of nonsilicate minerals, such as calcite and apatite, which are more rapidly weathered than silicate minerals (Hamburg et al. 2003).

Source of excess Ca in W5