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Evaluation of the Effects of Ashe Juniper (Cedar) Control on Water Quantity, Water Quality, and Suspended-Sediment Loads in three watersheds on the Edwards Aquifer Recharge and Catchment Area, South-Central Texas
Honey Creek in the Honey Creek StateNatural Area, Comal County, Texas.
The U.S, Geological Survey (USGS), in cooperation with the U.S. Department of Agriculture, Natural Resources Conservation Service, and in partnership with several state and local agencies, began a study in 1999 to evaluate the effects of ashe juniper (Juniperus ashei) control (removal) as a best-management practice (BMP) for increasing water quantity (aquifer recharge and streamflow) and protecting water quality in two watersheds located in the Honey Creek State Natural Area (HCSNA) in Comal County, Tex. (fig. 1 and 2). In 2002, the study was expanded to include an additional watershed, Laurel Canyon Creek, located in the Government Canyon State Natural Area (GCSNA) in Bexar County, Tex. (fig. 1 and 3).
This study is a follow-up to the Seco Creek Water Quality Demonstration project (Seco Creek project). Results from the Seco Creek project indicate that removal of ashe juniper trees might reduce evapotranspiration (ET) and runoff and increase water infiltration, aquifer recharge, and springflow. The Seco creek study was conducted on two, 40 acre sites; the current study applies the Seco Creek project site-specific study design to a larger watershed scale. The objective of this new study is to measure the effect of ashe juniper removal on streamflow, ET, and water-quality. This will be accomplished by measuring changes and differences in streamflow, ET, and water-quality of runoff of two watershed in the HCSNA, and of one watershed in the GCSNA; comparing the pre-treatment and post-treatment periods, and the treated and untreated watersheds.
In the HCSNA, using a paired watershed study design, two adjacent watersheds will be studied. One of the two watersheds, referred to as the “reference watershed”, the BMP will not be applied, and no ashe juniper trees removed. In the adjacent watershed, referred to as the “treatment watershed”, the BMP was applied in 2004, and a selected portion of ashe juniper trees removed. In the GCSNA, in the Laurel Canyon watershed, the BMP is currently being applied. Prior to juniper removal, baseline rainfall, streamflow, ET, and water-quality have been monitored at selected sites in each of the watersheds.
Field of Little Bluestem in the Honey Creek State Natural Area, Comal County, Texas.
Description of Watersheds and Sites
The two watersheds in the HCSNA are in the catchment area of the Edwards aquifer about 25 miles (40 kilometers) north of San Antonio (fig. 1 and 2, table 1). The catchement area, adjacent to the outcrop (recharge zone) of the Edwards aquifer to the north, comprises the upper parts of drainage basins of streams that lose water to the Edwards Aquifer as the streams cross the recharge zone. The watersheds are drained by ephemeral, first order streams that are tributaries to Honey Creek, a tributary to the Guadalupe River. The reference watershed has a drainage area of about 0.36 square mile (0.93 square kilometer). The treatment watershed has a drainage area of about 0.56 square mile (1.5 square kilometers). The upper 0.16 square mile (0.41 square kilometer) of the treatment watershed is on private land outside the HCSNA. For more information about the HCSNA, please see the U.S.D.A. Natural Resources Conservation Service website.
During the first three years of the study, the reference and treatment watersheds remained untreated while baseline rainfall, streamflow, ET, and water-quality data were collected. In 2004, the cedar removal BMP was implemented in the treatment watershed while the adjacent reference watershed remained untreated. The baseline, pretreatment data is documented in the report "Hydrologic and Water-Quality Data, Honey Creek State Natural Area, Comal County, Texas, August 2001-September 2003".
Cedar growth in the Reference watershed in the Honey Creek State Natural Area, Comal County, Texas.
Ashe juniper, commonly referred to as "mountain cedar" or "cedar" is a pervasive phreatophyte covering large areas of central Texas, including large portions of the Edwards Aquifer recharge zone and recharge catchment area. Groundwater consumption by phreatophytes is thought to be an important component of the hydrologic cycle in riparian corridors.
The Laurel Canyon Creek watershed is in the Edwards recharge zone, on the outcrop of the Edwards aquifer immediately outside the north-west city limits of San Antonio (fig. 1 and 3, table 1). Laurel Canyon Creek is also an ephemeral, first order stream and tributary of Leon Creek, a tributary of the San Antonio River. Most of the runoff from streams crossing the recharge zone infiltrates unconfined parts of the Edwards aquifer through faults, fractures, and sink holes that characterize the recharge zone. The Laurel Canyon Creek watershed has a drainage area of about 0.61 square miles (1.58 square kilometers). Baseline rainfall, streamflow, ET, and water-quality data were collected from 2002 to 2006. At the end of 2006, the cedar removal BMP began and should be completed in 2007.
Hydrologic and Water-Quality Data
The data collection phase of the study is ongoing, and throughout this period the USGS will gage as completely as possible the hydrologic characteristics of each watershed, measuring continuous rainfall, stream discharge, and evapotranspiration (ET). Following this period, these data will be used to quantify the effectiveness of the applied BMP’s by evaluating statistically significant changes in the hydrologic characteristics of both watersheds – comparing the pretreatment and post-treatment periods and the treated and untreated watersheds.
During 2000 and 2001, the USGS installed continuous data-collection stations at the nine sites in the HCSNA (fig. 2; table 1). And in 2002, two sites were installed in the GCSNA (fig. 3; table1). Rainfall data at sites 1-4 and 10, streamflow data at sites 2-4 and 10, and groundwater-level data at sites 8 and 9, are transmitted every 4 hours by way of Geostationary Operational Environmental Satellite (GOES) to the USGS National Water Information System database and are available on the internet in near real-time. ET data are retrieved from sites 6, 7, and 11 during periodic station service visits. Rainfall and storm-flow water-quality samples are collected as composite samples over the duration of the runoff events. Data-collection site number, station number, drainage area, type of data, and hyper links to real time data are listed in table 1. The site number in table 1 corresponds with the site number and location of each site provided in figures 1, 2, and 3.
Evapotranspiration system in the Treatment watershed (site 7) in the Honey Creek State Natural Area, Comal County, Texas.
Evapotranspiration system in the Laurel Canyon watershed (site 11) in the Government Canyon State Natural Area, Bexar County, Texas.
Meteorological data collection
Five tipping-bucket rain gages located at each of the four streamflow gages (sites 2-4, and 10), and one at a stand-alone site (site 1), continuously monitor rainfall amounts. The rainfall data will be applied to the analysis of rainfall-runoff relations, suspended-sediment and nutrient loads, soil-infiltration rates, and will be a component of the hydrologic budget analysis of each watershed.
The three Bowen Ratio evapotranspiration systems (sites 6, 7, and 11) collect continuous meteorological data, these measured properties include: net radiation, soil heat flux, soil temperature, soil moisture, differences in air temperature at two heights, and differences in vapor pressure at the same two heights. From these data, the energy-balance Bowen ratio method is used to calculate sensible- and latent-heat fluxes, which derive a value for ET. The ET data will be used to measure the effect of cedar removal on ET rates and will be a component of the hydrologic budget analysis of each watershed.
Storm-flow over the weir in the Reference watershed (site 2) in the Honey Creek State Natural Area, Comal County, Texas.
Storm-flow over the weir in the Treatment watershed (site 3) in the Honey Creek State Natural Area, Comal County, Texas.
Surface water data collection
Four streamflow-gaging stations continuously monitor ephemeral-streamflow discharges in the three watersheds (sites 2-4, and 10). Stage data will be measured and recorded continuously by pressure transducer gages. Weirs constructed at the sites are used as streamflow-discharge measuring devices applying theoretical stage-discharge relations developed for each of the weirs, and crest-stage gages will be used to verify recorded peak stages. Daily values of streamflow will be applied to the hydrologic budget analysis of each watershed. Unit values of streamflow will be used to analyze for differences in the streamflow characteristics of each site; analyzing peak-flows, flow durations, and rainfall-runoff relations.
Water-quality sampling by the USGS and video and logging by the Edwards Aquifer Authority of the Deep Well (site 8), in the Honey Creek State Natural Area, Comal County, Texas.
Groundwater data collection
Water-levels are monitored continuously in the HCSNA at two wells (sites 8 and 9) at different depths; one shallow, approximately 15 feet below the land surface datum, and the other deep, approximately 200 feet below the land surface datum. The Water-level data is measured by a pressure transducer gage and recorded at 1-hour intervals. The record will be used to evaluate infiltration rates and to attempt to monitor any changes in storage in the local Trinity aquifer.
Water-quality data collection
Automatic samplers installed at each streamflow gaging station collect water-quality samples during runoff events at sites 2-4, and 10. Analysis of the samples include pH, specific conductance, organic carbon, major ions, nutrients, the isotope ratios for oxygen (oxygen-18/ oxygen- 16) and for hydrogen (deuterium/protium), and suspended sediment (concentration and sand/silt composition). The automatic samplers are programmed to begin sampling upon the occurrence of streamflow, and will collect as many as 24 discrete samples at variable time intervals for the duration of the runoff event. Selected discrete samples are flow weighted and combined into a single, composite sample for all analyses except sediment. Suspended sediment is analyzed from up to five of the discrete samples that coincide with different phases of the rainfall-runoff hydrograph.
A rainfall sampler is used to collect rainfall water-quality samples which are analyzed for pH, specific conductance, major ions, nutrients, and isotopes (site 5).
The runoff water-quality data will be used to estimate event loads and will provide a water-quality comparison between pre-treatment and post-treatment periods and between treated and untreated watersheds. The rainfall water-quality analysis will be used to account for atmospheric inputs of these constituents to the watersheds and to determine/and or verify the meteoric isotope composition.
A storm-flow discharge hydrograph with corresponding time of water-quality and suspended sediment samples collected by an Isco Auto-Sampler.