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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