# DWI PCAs

Project Overview for the EMPM Meeting of M arch 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, in

stream reservoirs) 

### 5,450 CWS

s ources not suitable for watershed delineation (Great Lakes, infiltration galleries, aqueducts, off

stream 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 miles 2 (6178 acres)

– ~1/2 are less than 50 miles 2

– ~1/4 are greater than 1000 miles 2

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

910 

# Land Use/Cover: NLCD 2006 + DWI Watersheds11 

# 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