zopff - thesis proposal - a change detectioin study of the edwards and trinity aquifers - eighth...
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A CHANGE DETECTION STUDY OF THEEDWARDS AND TRINITY AQUIFERS: A GEOSPATIAL SURVEY
OF THE EFFECTS OF HUMAN DEVELOPMENT AND ASHE JUNIPER ON THE AQUIFER RECHARGE ZONES.
A THESIS PROPOSAL PRESENTED TOTHE DEPARTMENT OF HUMANITIES AND SOCIAL SCIENCES
IN CANDIDACY FOR THE DEGREE OFMASTER OF SCIENCE
By
WILLIAM M. ZOPFF III
NORTHWEST MISSOURI STATE UNIVERSITYMARYVILLE, MISSOURI
February 2016
Table of Content
1. Introduction……………………………………………………………………………. 3
2. Research Objectives……………………………………………………………………. 10
3. Justification……………………………………………………………………………..11
4. Description of Study Area……………………………………………………………....12
5. Description of Data Sources…………………………………………………………….14
6. Proposed Methodology……………………………………………………………….....15
7. Proposed Timeline………………………………………………………………………17
8. Expectations……………………………………………………………………………..17
9. References……………………………………………………………………………….18
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1. Introduction
The water that makes life possible on our planet has been a requirement for the
development of any civilization. The westward expansion and settlement of the western United
States hinged upon abundant safe dependable sources of water. While the settlement of Texas
was also dependent upon sources of water; many Texas settlements like San Antonio depended
upon the waters that issued forth, from artesian wells and springs, directly from aquifers (Porter,
2009). This thesis will explore a couple of these sources of water that have become threatened by
manmade and natural causes. This study will discuss these manmade and natural activities,
looking at their relationships. Attempting to describe the links between the acts of men and their
environment; examining some of their various endeavors across a portion of the State of Texas.
There are many ecosystems in Texas, with a diversity that ranges from the coastal plains
to the desert chaparral, and from the East Texas pine forests (Texas Almanac, 2013) to the sparse
piñon pines. The northern central portion of Texas is located at the foot of the southern Great
Plains and contains as many as five distinct regions. Texas being at the southern end of the Great
Plains (Antle et al., 2014), is covered with a variety of terrain types, with broad grasslands being
the dominant features of the state.
The geography of Texas rises from the Gulf of Mexico in the south, with the coastal plain
extending from the border of Mexico to Louisiana, to the High Plains of the north. The pine
forests and ranch lands of East Texas to the deserts and scrublands in the panhandle of West
Texas. One factor that ties the regions of Texas together is the need for water (Chaudhuri and
Ale, 2013) and a network of underground aquifers. The western two-thirds of Texas are a more
arid environment with desert like conditions. During rainy seasons there is often more abundant
surface water, especially in the eastern one-third of the state. The central portion of Texas
includes the Edwards Plateau which is the southernmost portion of the Great Plains. One 3
interesting fact that details the enormous dimensions of Texas is that the Texas City of El Paso at
its western tip is closer to San Diego, California, than it is to the City of Beaumont, Texas.
Due to the complex nature of the aquifers in Texas, a smaller study area was chosen in
Central Texas close in proximity to two major population centers. Both cities (Austin and San
Antonio) rely primarily on the aquifers (George et al., 2011) that lie beneath them for their water
supply. Additional water supplies come from the state’s hydrology, a system of U.S. Army Corps
of Engineers lakes and reservoirs along the state’s natural rivers; which has made Texas’ modern
growth possible. Almost any area within Texas could have been chosen for this study due to the
extensive natural formation of at least 30 known aquifers.
Most of the water in Texas lies underground and for centuries there has been plenty for
citizens, commerce, industry, and agriculture. The source of this abundant water that is contained
in Texas’s many aquifers comes from rainfall (Ferrill et al., 2004) and the transport of water
through its many rock formations. Due to the immense size of Texas (Scanlon et al., 2002), a
smaller region was chosen as the study area extending along the southern portions of the
Edwards and Trinity (Echardt, 2013) Aquifers, shown in Figures 1. The nine counties chosen are
located centrally and provided a decent cross section of ecosystems and environments for the
Edwards and Trinity Aquifers in Texas. In this region the Edwards and Trinity Aquifers have
adjacent outcroppings (OC) acting as recharge zones and overlapping sub-crops (SC). These
recharge zones acting as refilling points and storage zones that store the captured rain water, and
through fractures (Smith and Hunt, 2013; Ferrill et al., 2004) allow for some water transport
between aquifers.
From the early 1900s to the present, population increases and the accompanying
economic development (Mace et al., 2000) in Texas have the potential of having a serious impact
on the aquifers that provide this much needed water resource. There are forests, grasslands, and
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Figure 1: Major Aquifers of Texas and their proximity to the selected Study Area.
hills in central Texas and specifically within the study area that lay over portions of the Edwards
and Trinity Aquifers. The cities, communities, and ranches within the study area have proven to
support the presence of grazing domestic cattle and wildlife; and have endured the competition
imposed by an invasive plant species namely the Ashe (Juniperus ashei) juniper (Heilman et al.,
2009).
The cattle grazing and invasive juniper have a direct impact on the grasslands. When
practiced improperly, ranching with beef cattle, dairy cows, goats, horses, and sheep can lead to
overgrazing of the native prairie grasslands; consequently, the removal of these competitive
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natural grasslands provides an opportunity for the juniper species to flourish. The invasive Ashe
juniper or cedar trees can encroach upon the grasslands when they are overgrazed. The weather
patterns visited upon Texas often contribute; the cycles of high rainfall in spring followed by
summer droughts (Smith, 2013) are strongly affected by the Pacific Ocean weather, tropical
storms from the Gulf of Mexico, and the ever changing jet stream. The drought patterns can also
lead to overgrazing when ranchers don’t rotate their grazing pastures frequently enough to
prevent over grazing, also due to the slower growing pasture grasses during drought conditions.
The cycles of rain and drought (Smith, 2013) mentioned in the preceding paragraph
directly affect the aquifers in Texas. The drought conditions only exacerbate the water shortages
and increase the strain on the natural recharging of the aquifer system. The reduction of natural
refilling places increases pressure on the aquifers along with the increased demand from the
human population. Drought conditions also place additional pressure on farmers and cattle
ranchers due to the low levels of their traditional water sources such as lakes, rivers, and streams.
These farmers and ranchers have requested more access to water. Since most farms and ranches
have wells that access water from the aquifers directly, during the droughts the existing wells can
be inadequate, so additional well permits are submitted to the government. Often existing wells
are of a certain depth, and as aquifer levels drop, new wells are needed and can only be
developed with government permits, thus placing further demand upon the aquifer resources.
The studied portion of the Edwards and Trinity Aquifers (shown in Figure 1) lie along the
I-35 corridor shown in Figure 2, 3, 4, and 5, which is a compilation of maps that will
demonstrate a time lapse (1900, 1990, 2000, & 2010) view of the population centers and
transportation established throughout Central Texas along these major aquifers. The Edwards
and Trinity Aquifers supply water to two of Texas’s densely populated cities: Austin and San
Antonio clearly shown in Figure 5 (George et al., 2011). The population growth in the west and
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south matched the growth seen elsewhere in the westward expansion of the United States from
1840 to 1990; that growth took place along established transportation (Beeson et al., 2001)
routes. Shown on Figures 2 thru 5, the population in Texas (almost doubled) became more urban,
and the cities became more densely populated, and the rural areas in between became less
populated. The migration of the population to the more industrialized and service oriented cities
during the 1900s led to larger land holdings that were ideal for cattle ranching.
Figure 2: The map demonstrates the population density of farms and ranches along the railroads in Texas; the railroads gave rancher’s convenient access to growing markets for their crops and livestock in the East. The extensive railroad development in East Texas is a result of a booming lumber industry during the period 1880s to 1920s called the “The Bonanza Period”. (Texas Beyond History, 2004).
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Figure 3: This map shows 90 years of progress as numerous small ranches along the railroads dissipated into larger ranches and regional populations moved from rural to urban. More trucks transported livestock on highways, cheaper and more efficient than the railroad.
The continued growth of Texas’s population centers are placing ever increasing demands
upon these crucial water sources. Since dependable water supplies are critical and these aquifers
are apparently threatened, the lawmakers and the people of Texas charged the Texas
Commission on Environmental Quality (TCEQ) (TCEQ, 2015) with the responsibility of
maintaining this reliable supply of water. One of the many missions for the TCEQ is that of
ensuring this water resource will last for the future generations of Texans. The TCEQ also is
responsible for developing plans and ensuring they are followed to protect this essential resource.
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Figure 4: Displays the same information, but during 2000, more recent time. The interstate highways allow commuters to live outside cities increasing population along the interstates, but still near population centers.
The many efforts of humans have an impact on their local environments, whether its
residential , commercial, or agricultural development. Each of these endeavors leave their mark
on the land. It is these marks that this research will attemtp to demonstrate that they are having
an effect on the refilling of the Edwards and Trinity Aquifers. This study will attempt to
demonstrate that human development either residential or commerical is reducing available
grasslands. And this study will also attempt to demonstrate how agricultural development or
ranching is having an impact on the avialable grasslands and encouraging the growth of an
invasive plant species.
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Figure 5: This map demonstrates the Major Aquifers in Texas and the highlighted Study Area. The City of San Antonio has swelled to over 2 million persons, placing more demand for more water on the regions system of aquifers and reservoirs. For a comparison see Figure 5 and 6, to visualize how the highways and population centers from Dallas to San Antonio almost mirror the locations of the Edwards and Trinity Aquifer.
2. Research Objectives
The main objective of this research is to detect landscape change within the study area
(see Figure 6) in the Edwards and Trinity Aquifer recharge area. This project will identify the
landscape changes caused by human economic development and invasive Ashe juniper growth
that may correlate with a reduction of the aquifers levels over time. It is an objective of this study
to prove that these two factors reduce the acreage (grasslands) available for soakage of rainwater
near the aquifer recharge zones, which is responsible for the normal refilling of the aquifers.
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Figure 6: This map is a reminder of the study area in the State of Texas, demonstrating how the aquifers are positioned throughout the state. See Figure 3 for a focused view of the study area.
3. Justification
As the human and economic development mentioned above increases its coverage of the
natural soakage areas, it interferes with the slow percolating seepage of water into the aquifers.
These impervious coverings consist of concrete, black top, and buildings, (Sung, 2013) which
provide flat surfaces that speed the transport of water away from its traditional soakage areas and
into the creeks, streams, lakes, and rivers. This transportation of water away from the recharge
zones has the effect of reducing aquifer recharge.
The Ashe juniper (juniperus ashei), or commonly called cedar trees, are a hydrophilic
species, absorbing over 60% of rainfall through their foliage and dense root systems. The Ashe
junipers are a native species to Texas and the South Central United States. Cattle overgrazing of
grasslands removes the natural competition provided by the dense nature of the grasslands.
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Drought conditions retard the natural growth of the Texas grasslands reducing available grazing
land for cattle. This reduction has left an opening for a species of juniper to flourish. Sparse
grasslands also lead to the reduced soakage of rainwater during drought; this in turn leads to
erosion and fast runoff of rainwater into the streams and rivers (flashfloods) and reduces the time
any rainwater spends in the aquifer recharge zones.
Agriculture in Texas and its demands for more access to water have compounded Texas’s
water issues. Commercial development and the demands for water have a positive and negative
impact on the population growth; the more positive aspects that have developed are innovative
water conservation methods. Modern industry (high tech and oil well development) (Ramos,
2001) provides important jobs, materials, and resources for local and national economies, and it
takes all levels of education to prepare a society to work in technology and industry occupations.
Texas falls within the global and national trends in population growth (Perryman, 2013).
These growth trends are placing increased demands upon all resources as a result of various
aspects of human endeavors including agricultural, commercial, education, environment,
industrial, and transportation networks. All of these demands directly affect the amount of
available water, whether it’s due to consumption, contamination, or diversion (Heilman et al.,
2009) of the natural water supply. So, determining some of the environmental causes of this
consumption, contamination, and diversion can be important to planners and consumers.
4. Description of Study Area
The state of Texas is vast; choosing which area to study wasn’t difficult. The region
chosen was close to one of the major population centers (San Antonio), which has experienced
tremendous growth. This study will attempt to demonstrate the impact of development,
agriculture, and the effects of an invasive species to the aquifers in this region. The Texas
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counties involved within the area of research include: Bexar, Blanco, Bandera, Comal, Gillespie,
Hayes, Kendall, Kerr, and Medina (as shown in Figure 7). These counties cover most of the
southern reaches of the Edwards and Trinity Aquifers recharge zones and the outcrop. Figure 7
includes an inset map of Texas that localizes the study area. Bexar County contains one of
Texas’s larger cities (San Antonio). San Antonio is directly affected by the state’s main water
issues and is dependent upon the Edwards and Trinity Aquifers for its water supplies. San
Antonio is not supplied by water from any nearby reservoirs (San Antonio Water System,
SAWS, 2015). The subterranean aquifer structures within the study area are stratified by
amazing geological structures (Ferrill et al., 2004; George et al., 2011, p.40) that make up
Figure 7: Overview Map of Study Area. This Map of Study Area demonstrates the Study Areas location within Texas.
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the region. It is these geological structures that contain the waters of the Edwards and Trinity
Aquifers. Fractures within these two aquifers are believed to allow the transport water from
above and below ground, filling numerous aquifers. Some of these underground reservoirs are
tapped by wells, and a few even emit water through several artesian wells.
Texas is an agricultural state, producing cattle and crops of corn and soybean, cotton, and
hay for cattle ranches. The Hill Country region of South Central Texas is better suited to cattle
ranching due to the rugged terrain, exposed rock bluffs and hills and is less suitable for farming.
The reported historical drought from the 1950’s, 1961, 1990, 2002, 2012 to present (Texas Water
Development Board, TWDB, 2014) forced cattle ranchers to reduce their herds to save on
feeding and watering costs during the drought; some reduced to breed stock only. The State of
Texas has worked since the severe drought of the 1950’s to address their water issues, and
created the Texas Water Development Board (TWDB) (TWDB, 2014) to oversee Texas’s plan
for its future water needs as Texas grows.
5. Description of Data Sources
Data for this thesis is acquired from Texas State Government Websites with Hydrology
data (see Table 1); United States Geological Survey (USGS) (USGS, 2014) Data on Hydrology
and Remotes Sensing (MODIS, Landsat, and NDVI). Additional websites are included in the
references and referenced within this text, due to information about Texas history, culture,
weather, economics, and other topics.
Over the period of study 1983 to 2013, the areas of coverage vary along with the quality
of the imagery. Change detection will be conducted using NLCD land cover images from 1992,
2001, and 2006 from EarthExplorer. Change detection will also be applied to LandSatLook’s
TM imagery from 1982, 1992, 2000, 2011, and 2013. Within the nine counties of the study area
Table 1: Data Table14
Name Date Description Raster/ Vector Source
USGS 1982 to 2013
Edwards and Trinity Aquifers Raster Tiff image files from USGS’s LandsatLook Viewer, website:
http://landsatlook.usgs.gov/ 17 images
USGS 1982 to 2013
Study Area region Raster NDVI and NLCD data from USGS EarthExplorer, website:
http://earthexplorer.usgs.gov/ 3 images
TCEQ 1995 to 1996
Edwards and Trinity Aquifers
Land UseVector
Shape file data from Texas Commission on Environmental Quality, website: http://www.tceq.state.tx.us/gis/download-tceq-gis-data/#water
a number of locations were selected as sampling points, using five points within each county.
Cluster sampling will be conducted from these points randomly to evaluate the land cover
changes. These samples were initially selected using Google Earth, and then verified by
physically visiting each points. The physical verification was used to validate the imagery (1983
to 2013, specifically 2013 as this imagery is closest to the period of this study) used when
comparing existing foliage and historical imagery within the study area. These points also
demonstrated regions of visible change, which were either residential or commercial
development or areas of identified overgrazing or cedar encroachment.
6. Proposed Methodology
ERDAS Imagine and Esri ArcMap software will be utilized in this study to create
working maps, image evaluation, digital image processing, and statistical analysis. ERDAS
Imagine’s change detection methods will enable the process of looking for patterns of change.
Esri ArcGIS will also be used with Landsat (USGS, 2014) national land cover data (NLCD)
imagery (1982 to 2013) and normalized difference vegetation index (NDVI). Major steps in this
study include: image pre-processing, land cover evaluation and change detection, and statistical
analysis.
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Image pre-processing will be performed to ensure a proper spatial extent for any and all
of the tasks to follow. ArcGIS is used to generate layers that include the State of Texas, its
counties, and the aquifers within the state. The generated map of aquifers in this region will have
the same geographic reference as the overlays in the county and state maps. The aquifer map of
Texas is utilized for description and location of study area for this study.
This study utilizes NLCD data to evaluate the land cover features in the study area.
Images for the study area will be analyzed using the existing classification and evaluated for land
use changes. The land cover classifications used will be grasslands, juniper forests, native
forests, open ground, and impervious surfaces. These classifications will be evaluated by image
algebra and change detection processes. The NLCD and NDVI imagery will be analyzed from
the four available study periods (1992, 2001, 2006, and 2011), comparing classification types
and collecting statistical data from the comparisons. An accuracy assessment will compare the
NLCD imagery data to the cluster samples collected within the study area, and an error matrix
will test all of these data points. During the land-cover analysis the use of transformed
divergence may be used to determine the greatest separation from land cover classes.
Data will be collected from the counties within the study area for historical aquifer levels,
rainfall, and hydrology to compare and correlate findings to support the research questions and
examined using statistical analysis. An inspection of the historical aquifer levels, average yearly
rainfall, and land cover changes could reveal some correlation between these data. An
examination of the data will involve applying the basic statistical tests to look for trends.
Applying variance-covariance matrix, and the correlation matrix (if applicable) for all the values
found for the historical data for rainfall, and the aquifer levels during the study period.
Hypothesis testing will also be applied, testing the Null and Alternate Hypotheses for the
historical data as they relate to this proposals research objectives, making inferences about the 16
magnitude of the data over the study period. The matched-pairs tests will be applied to historical
rainfall and the aquifer levels as well. The probability distribution of the periods of drought will
be examined to determine whether it’s a Normal or a Poisson distribution. ArcGIS will be used
to create maps displaying the data (correlations and trends) over the study area. Looking for
mapped and statistical patterns offer the probability of finding a correlation between the human
development (agricultural, commercial, and residential) and its possible effects on the normal
aquifer recharging processes.
7. Proposed Timeline
The first three phases of this thesis have been accomplished, and the fourth phase is in
progress (see Table 2). The fourth phase involves submitting additional revisions of my thesis
proposal until approved. After proposal approval, the thesis process will continue with writing
and further research, until the thesis is ready for submission in the spring semester 2016.
Table 2: Proposed Time Line
Phase I Phase II Phase III Phase IV Phase VJanuary to May 2014 Summer 2014 Fall 2014 and Spring 2015 Fall 2015 Spring 2016Submitted thesis pitch Resubmit thesis proposal Resubmit thesis proposal to
committeeReview committee suggestions
Implement committee suggestions and resubmit
Submitted thesis proposal
Collect data samples Review committee suggestions, and implement
Implement committee suggestions and resubmit
Argue thesis as necessary
Submit thesis
8. Expectations
The expected outcomes from this study should demonstrate whether or not human
development and/or the invasive Ashe juniper are impacting the Trinity and Edwards Aquifers.
The Trinity and Edwards Aquifers have long been a primary source of water for human
population in Texas. This study will demonstrate the percentage of change in landuse and
include the comparative amounts of growth of the invasive juniper within the timeline.
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One possible solution for both the juniper problem and impervious surfaces is to build
small reservoirs along the water transportation routes, strategically placed to slow down rain
water runoff. Reservoirs can provide numerous locations for soakage and possible locations for
direct aquifer injections (with all water purity considerations in place). The consequences of
flash flooding in Texas created the necessity to control flash flooding (Texas State Soil and
Water Conservation Board (TSSWCB), 2009), which led to a system of flash flood control dams
developed along flash flood zones throughout Texas. Around and in the areas of flood control
zones, native grasses should be sown to increase soakage acreage. As an incentive to participate
the government could offer the native grass seed for free to ranchers.
Another solution to address the consequences of overgrazing of land is the removal of
more juniper stands (Hill Country Alliance, 2011) and planting more native grasses. Like every
topic of science there are those who believe and those who don’t believe that eradicating the
junipers will present the desired benefits. However, having more land available for grazing
should make it possible for ranchers to rotate pastures and prevent overgrazing. Selectively
removing or controlling the invasive junipers could better address their effect on the grasslands.
The government could offer incentives to keep grasslands from being overgrazed or even a
penalty for ranchers that allow their fields to be overgrazed. Restoring the grasslands and
reducing erosion will slow down the water transport away from recharge systems and keep the
water in the aquifer regions longer.
9. References
Antle, J.M., Kluck, D., McPherson, R. A., Scanlon, B., and Sherman, K., 2014, “Great Plains”, [online]. Available from: http://nca2014.globalchange.gov/highlights/regions/great-plains. [Accessed 1 July 2014].
Beeson, P., Dejong, D., and Troeskon, W., 2001. Population growth in U.S. Counties, 1840-1990, Regional Science and Urban Economics, 31, 669-699.
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Chaudhuri, S. and Ale, S., 2013. Characterization of groundwater resources in the Trinity and Woodbine aquifers in Texas, Science of the Total Environment, 452-453, 333-348.
Echardt, G., 2013. The Edwards Aquifer Website, [online]. Available from: http://www.edwardsaquifer.net/trinity.html. [Accessed 25 February 2014].
Ferrill, D., Sims, D., Waiting, D., Morris, A., Franklin, N., and Schultz, A., 2004. Structural framework of the Edwards Aquifer recharge zone in south-central Texas. Geological Society of America Bulletin, 116(3-4), 407-418.
George, P., Mace, R., and Petrossian, R., 2011. Aquifers of Texas, [online]. Texas Water Development Board, Available from: http://www.twdb.state.tx.us/publications /reports/numbered_reports/doc/R380_AquifersofTexas.pdf. [Accessed 6 September 2013].
Heilman, J., McInnes, K., Kjelgaard, J., Owens, M., Schwinning, S., 2009. Energy balance and water use in the subtropical karst woodland on the Edwards Plateau, Texas. Journal of Hydrology, 373 (3-4) 426-435.
Hill Country Alliance, 2011. Cedar/Brush Management, [online]. Available from: http:// www.hillcountryalliance.org/HCA/ juniper , [Accessed 28 June 2013].
Mace, R.E., Chowdhury, A., Anaya, R., Way, S., 2000. Groundwater Availability of the Trinity Aquifer, Hill Country Area, Texas: Numerical Simulations through 2050 [online]. Texas Water Development Board. Available from: http://www.edwardsaquifer.net/pdf/Report_353.pdf [Accessed 6 March 2014].
Perryman, M.R., 2013. Population of Texas could Double by 2050, [online]. San Antonio Business Journal. Available from: http://www.bizjournals.com/sanantonio/print-edition/2013/05/10/population-of-texas-could-double-by-2050.html?page=all. [Accessed 20 September 2013].
Porter Jr., C.R., 2009. Spanish Water, Anglo Water - Early Development in San Antonio. College Station: Texas A&M University Press.
Ramos, M., 2001. Oil and Texas: A Cultural History, [online]. Texas Almanac. Available from: http://www.texasalmanac.com/topics/business/oil-and-texas-cultural-history. [Accessed 29 November 2013].
San Antonio Water System (SAWS), 2015. Your Water. Edwards Aquifer. [online] Available from: http://www.saws.org/Your_Water/index.cfm . [Accessed 30 August 2015].
Scanlon, B. R., and Dutton, M., 2002. Groundwater Recharge in Texas, [online]. Texas Water Development Board. Available from: http://www.beg.utexas.edu/environqlty/vadose/pdfs/webbio_pdfs/TWDBRechRept.pdf [Accessed 21 February 2014].
Smith, B.A., and Hunt, B.B., 2013. Enhanced recharge to the Barton Spring segment of the Edwards Aquifer, Central Texas. Carbonates and Evaporites, 28(67-73).
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Smith, R., 2013. Current Texas Drought on Track to be the Second Longest, [online]. Southwest Farm Press, Available from: http://southwestfarmpress.com/irrigation/current-texas-drought-track-be-second-longest / [Accessed 25 January 2014].
Texas Almanac, 2013, Physical Regions of Texas, Environment, [online]. Available from: http://www.texasalmanac.com/topics/environment/physical-regions-texas . [Accessed 10 February 2014].
Texas Beyond History, 2004. Aldridge Sawmill and the East Texas Logging Bonanza, [online]. Available from: http://www.texasbeyondhistory.net/aldridge/ [Accessed 1 February 2014].
Texas State Soil and Water Conservation Board (TSSWCB), 2009. Flood Control Program, [online]. Available from: http://www.tsswcb.texas.gov/floodcontrol . [Accessed 10 December 2012].
The Texas Water Development Board’s (TWDB), 2014. Water Data for Texas, Drought in Texas, [Online]. Available on-line at URL: http://waterdatafortexas.org/drought/ [Accessed on March 10, 2014].
United States Geological Survey (USGS), 2014. LandsatLook Viewer, [online]. Available from: http://landsatlook.usgs.gov/. [Accessed 29 March 2014].
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