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.
Funding Source
Funding provided by the Texas State Soil and Water Conservation Board as part of a Clean Water Act Section 319(h) Grant.
