DWI PCAs

Project Overview for the EMPM Meeting of March 20, 2012

James N. Carleton, PhD

Rochelle F. H. Bohaty, PhD

Marietta Echeverria, Management Lead

Mohamed Ruhman, PhD

Shelly Thawley

Nelson Thurman, Senior Science Advisor

Katrina White, PhD

1
Background

• EFED uses percent crop area (PCA) in its 

drinking water assessments (DWA) to account for 

the fact that a drinking water watershed is not 

likely to be devoted entirely to agriculture

• PCAs were first developed based on 8-digit 

hydrologic units (HUC-8) 

– Process vetted through a Scientific Advisory Panel 

(SAP) 

– WQTT is in the process of finalizing updated PCAs 

based on most recent data and new crop groups

• Drinking water intakes (DWIs) specific PCAs are 

being developed for delineated watersheds

2
Drinking Water Watershed PCA Project

• Project Team:

– Rochelle Bohaty, Jim Carleton, Mohammed 

Ruhman, Shelly Thawley, Nelson Thurman, and 

Katrina White 

– Marietta Echeverria—management lead

• Goal: 

– To develop PCAs representative of delineated 

watersheds with surface water intakes serving 

community water systems (CWS)

• total agriculture

• crops or crop groups (i.e., vegetables, orchards)

3
Combining AgCensus and NLCD for Crop 

Distributions

• Use Ag Census for crop acreage, and National Land Cover Dataset 

(NLCD) Cultivated Crop layer for sub-county distribution

– Used in PCA calculations 

to apportion AgCensus

county crop acres to 

watersheds that cross 

county boundaries

– NLCD provides a 

distributional anchor 

for acreage

– Confines crop to 

cultivated areas 

within each county

– Doesn’t distinguish 

individual crops or crop 

groups within the NLCD

– Assumes uniform 

distribution of

individual crops 

across NLCD area
~12,000 Community Water Supplies (CWS) rely 

on surface water to serve ~210 million people 

4,840 DWI 

catchments judged 

“reasonable” by 

USGS

6,550 CWS 

sources suitable for 

watershed delineation

(streams, rivers, lakes, instream reservoirs)

5,450 CWS

sources not suitable for 

watershed delineation

(Great Lakes, infiltration 

galleries, aqueducts, offstream reservoirs etc.)

5,557 DWI 

watersheds delineated

(locations suitable for 

NHD+ delineation) 

Drinking Water Intake (DWI) Catchments

5
DWIs Without Delineated Catchment

– DWIs not tied to an NHD+

Possible Solution: Surrogate PCAs

6
DWI Delineated Catchment Size

• Of the 4,840 delineated basins

– ~1/3 are less than 10 miles2 

(6178 acres)

– ~1/2 are less than 50 miles2

– ~1/4 are greater than 1000 

miles2

7

0.00

0.25

0.50

0.75

1.00

0.001 0.1 10 1000 100000

Rel. Frequency

Watershed Area (sq. mi.)
Land Use/Cover: NLCD 2006

cultivated crop land

8
Land Use/Cover: NLCD 2006

cultivated crop land + pasture/hay

9
10

Land Use/Cover: NLCD 2006

+ DWI Watersheds
11

Land Use/Cover: NLCD 2006

+ DWI Watersheds
Limitations and Challenges

• DWI analysis is only as good as lowest 

resolved dataset

– AgCensus data resolved at county level

– Temporal variation in cropland data used for crop 

specific PCAs

• Transparency of analysis

– Security concerns

• EPA policy limits release of DWI locations 

12
Next Steps

April

2012

• Develop draft PCAs for total ag. and 9 crop 

groups

June

2012

• Validate GIS process and evaluate draft PCAs 

July

2012

• Finalize PCAs

November 

2012

• Finalize report and guidance document

13