Scope of
Work
Funding Source
Funding provided by the Texas State Soil and Water Conservation
Board as part of a Clean Water Act Section 319(h) Grant.
Background
The Arroyo Colorado flows through Hidalgo, Cameron and Willacy
Counties in the Lower Rio Grande Valley of Texas into the Laguna
Madre. As a result of low dissolved oxygen levels, the tidal
segment of the Arroyo Colorado (2201) does not currently support
the aquatic life use designated by the State of Texas and described
in the Water Quality Standards. This has been the case for every
303(d) List prepared by the State since 1986. There have
also been concerns about high nutrient levels in this stream as
documented on every 305(b) Assessment prepared by the
State since 1988. The watershed was originally modeled by TCEQ in
1999 using the HSPF model. This model indicated that a 90%
reduction in nitrogen, phosphorous, oxygen demanding substances and
sediment was necessary to meet the dissolved oxygen criteria at
least 90% of the time during the critical period of March through
October. In 2003, the TCEQ directed staff to collect additional
data and increase the sophistication of the TMDL analysis to reduce
uncertainty and to better characterize the watershed and then to
reassess needed loading reductions. This project will help with the
reassessment of the needed loading reductions by simulating current
loadings using the SWAT model.
The need for this project is substantiated in A Watershed
Protection Plan for the Arroyo Colorado Phase I and
specifically in the volume Components Addressing Agricultural NPS
Pollution. This project utilizes information generated and compiled
through TSSWCB CWA §319(h) project 06-10 Arroyo Colorado
Agricultural NPS Assessment and quantifies load reductions
achieved through TSSWCB CWA §319(h) projects 05-10 Education of
BMPs in the Arroyo Colorado Watershed and 05-12 WQMP
Implementation Assistance in the Arroyo Colorado
Watershed.
Goals and Objectives
The project will consist of using a computer modeling software
(SWAT model) and a geographic information system (GIS) to simulate
the current sediment, BOD and nutrient loadings in the Arroyo
Colorado watershed. The SWAT model will be used to quantify the
sediment and nutrient loadings in the watershed. TAES-Blackland
will conduct the model simulations.
Meteorological, in-stream flow, wastewater flow and loading, GIS
and measured water quality data will be compiled along with
information on the type and extent of management measures
implemented for both agricultural and urban areas in the watershed.
Examples of GIS data that may be used are SSURGO (Soil Survey
Geographic) and CBMS (Computer Based Mapping System) soils, landuse
developed through the TSSWCB CWA §319(h) project 06-10 Arroyo
Colorado Agricultural NPS Assessment, and the USGS 30-meter
resolution digital elevation model (DEM). Measured precipitation
and temperature will be collected from National Weather Service
climate stations for input to SWAT. Measured stream flow will be
collected from USGS, IBWC and other stream gage stations. Water
quality data will be compiled from USGS, IBWC, NRA, TCEQ and Clean
Rivers Program sources.
Information on typical crops and management practices (e.g.
tillage practices, irrigation management, and nutrient application
rate and timing) will be obtained from TAES, TCE, TSSWCB, and local
NRCS and SWCD field offices. Existing BMPs (e.g. land leveling,
irrigation management, nutrient management methods) will be
obtained through the TSSWCB CWA §319(h) project 06-10 Arroyo
Colorado Agricultural NPS Assessment for the period of
1999-2006. Non-agricultural input data will be obtained from TCEQ,
cities, counties, and other entities with jurisdiction over these
issues. SWAT inputs will be prepared to accurately represent
existing conditions and management.
After compiling all available data for the watershed, the SWAT
model will be set up and calibrated using measured flow and
in-stream measurements of sediment, BOD, and nutrient
concentrations for the period of 1999-2003 with 1999 as warm-up
period. If measured data is not available for a particular
sub-watershed, SWAT inputs will be selected and adjusted based on
recent research and calibration in other watersheds. After
calibration, the model will be validated using measured flow and
in-stream measurements of sediment, BOD, and nutrient
concentrations for the period of 2004-2006. Existing conditions and
load reduction scenarios specified by TSSWCB will be simulated to
determine nutrient, BOD, and sediment loadings.
SWAT output will be provided to the TSSWCB formatted for input
into the model by TCEQ. A final report for the project will also be
prepared.
Task 1: Coordinate and Administer Project
Objective: TWRI will effectively coordinate and
monitor all work performed under this project, including technical
and financial supervision, preparation of status reports, and
maintenance of project files and data. This project will be
coordinated with ongoing efforts in the Arroyo Colorado, especially
the TSSWCB CWA §319(h) project 06-10, but also the TSSWCB CWA
§319(h) projects 05-10 and 05-12, and other projects with which
TWRI has involvement. In addition, TWRI will perform quality
assurance functions.
Task 2: Watershed data compilation, analysis, and simulation
using SWAT
Objective: TAES, with technical oversight and
assistance from SSL, will compile data needed for the SWAT model
and characterize the flow and watershed loadings to the Arroyo
Colorado for both agricultural and urban areas.