Performance of water catchments and the land-based water cycle is heavily influenced by vegetation and the management of that vegetation. Vegetation in south central Texas has undergone significant change over time, shifting from a grassland dominated savanna to a heavily wooded landscape. This is due to the invasion of Ashe juniper (sometimes called “cedar”) in the Edwards Plateau and mesquite in the South Texas Plains. This transformation to woodland may have reduced water available for recharge and streamflow.  Woody plant invasion can be reversed through rangeland restoration. Initial costs, however, are generally more than a landowner can justify when considering livestock production alone. 

Under certain  circumstances, additional water yield results from rangeland restoration. As is suggested by several studies, there may be opportunities for creating incentive-based programs that lead to additional water yield through rangeland restoration.

With sponsorship from the Texas Wildlife Association Foundation, a team of Texas A&M University scientists have conducted a technical evaluation of land-based water conservation practices and their potential for water yield in south central Texas. The purpose of this evaluation was to determine the feasibility of rangeland restoration for increasing water yield. This team identified areas in south central Texas suitable for rangeland restoration programs. This report is a summary of their findings.

Background

The South Central Texas Regional Water Planning Group, also known as Region L, is one of 16 regional water planning groups across the state. These groups identify water needs, assess potential water supplies, and recommend strategies for meeting those needs for the Texas Water Development Board. It includes over 20 counties and the seventh largest city in the United States—San Antonio. The population of this area was about 2.0 million in 2000 and is projected to grow to about 4.3 million by 2060. Obviously, securing adequate supplies of fresh water for Region L is a mounting concern. Total water use (municipal, industrial, and agricultural uses) in the region was approximately 896,000-acre feet/year (an acre-foot is about 325,000 gallons) in 2000 and is projected to increase by 43 percent to 1.28 million acre-feet/year by 2060 (SCTRWPG 2006). Nearly 80 percent of the region’s fresh water is obtained from underground aquifers— primarily the Edwards and the Carrizo-Wilcox. Due to the high demand placed on these aquifers, the amount of water withdrawn exceeds recharge. Thus the need to increase water available for recharge and streamflow is essential.

LAND STEWARDSHIP LANGUAGE IN THE TEXAS CODE

Anyone seeking to officially fund and implement land stewardship programs can now find authorization in our state law. Senate Bill 3 of the 2007 Session of the Texas Legislature amended Article 2, Section 2.02, Subchapter A, Chapter 1 of the Texas Water Code by adding Section 1.004. In this section, our leaders note the benefits of voluntary land stewardship and define the term. It reads:

Sec. 1.004. FINDINGS AND POLICY REGARDING LAND STEWARDSHIP.
(a) The legislature finds that voluntary land stewardship enhances the efficiency and effectiveness of this state’s watersheds by helping to increase surface water and groundwater supplies, resulting in a benefit to the natural resources of this state and to the general public. It is therefore the policy of this state to encourage voluntary land stewardship as a significant water management tool.

(b) “Land stewardship,” as used in this code, is the voluntary practice of managing land to conserve or enhance suitable landscapes and the ecosystem values of the land. Land stewardship includes land and habitat management, wildlife conservation, and watershed protection. Land stewardship practices include runoff reduction, prescribed burning, managed grazing, brush management, erosion management, reseeding with native plant species, riparian management and restoration, and spring and creek-bank protection, all of which benefit the water resources of this state.

With this language, the Texas Legislature officially recognized voluntary land stewardship practices as a means to improve water quality and quantity, opening the door for the development of effective land conservation/management programs designed to benefit water resources.

Influence of Woody Species on the Water Cycle

The two major natural areas of Region L are the Edwards Plateau and the South Texas Plains.  Prior to European settlement in the 1800s, the Edwards Plateau and South Texas Plains were dominated by grassland. In the absence of fire and the presence of overgrazing, Ashe juniper has invaded large areas of the Edwards Plateau, and mesquite and mixed brush have overtaken much of the South Texas Plains. There is some evidence that rangelands dominated by these woody shrubs and trees may not yield as much water as rangelands dominated by grasses and herbaceous vegetation.  A significant amount of research has been performed that addresses this issue at several scales.

Edwards Plateau—Research within the Edwards Plateau has focused on the water use of Ashe juniper at the individual tree level.  A single, mature juniper tree can transpire approximately 550 gallons per day (Owens and Ansley 1997).  In addition, as much as 40% of the precipitation that falls on a juniper tree is intercepted by the canopy and lost to evaporation (Owens et al. 2006). As a consequence, in an area that receives approximately 29.5 inches of annual rainfall, a mature juniper tree will intercept and transpire nearly all of the available water falling within its canopy. 

How much water yield?  An additional acre-foot can be gained for every 5 to 8 acres of brush converted to native rangeland in the Edwards
Plateau and for every 15 to 30 acres converted in the South Texas Plains.

Based on this, available water can be negatively impacted in regions where extensive juniper cover exists.  However, this does not take into account the difference in water usage between juniper and the grasses and other vegetation that would replace it when restored.

Studies conducted at the stand scale have compared the differences in water usage between rangeland grasses and Ashe juniper. These studies show that in areas cleared of juniper, evapotranspiration rates (water lost to the atmosphere through evaporation and use by plants) were 1.6 inches/acre/year less than areas with intact juniper (Dugas et al. 1998).

There are several anecdotal accounts of springs drying following encroachment by juniper.  Studies at the small catchment scale that focus on catchments with springs have shown increases in spring discharge following the removal of juniper. In some instances this occurred even when precipitation was below average following juniper removal. These studies estimate that 1.6 to 1.8 inches/acre/year of additional water were made available after restoration of rangelands (Wright 1996, Huang et al. 2006).

Although there have been no large-scale experiments conducted that look at the landscape-scale impacts of juniper encroachment on streamflow, confidence is increasing that restoring rangelands on the Edwards Plateau will increase streamflow and recharge at the small catchment scale.

Based on current research, the best estimate is that converting 5 to 8
acres of juniper to rangeland would result in an approximate increase in recharge and streamflow of 1 acre-foot of water per year.

SOUTH TEXAS PLAINS—Compared to the Edwards Plateau, there has been little effort to study the influence of woody plants on recharge in the South Texas Plains. In studies that have been conducted, there has been evidence that rangeland restoration increases recharge. One field study indicated that there was no recharge on plots with dense shrub cover and 0.9 inches/acre/year of recharge on plots with grass cover (Weltz and Blackburn 1995). On the basis of this study, it can be concluded that recharge is limited on sites dominated by brush and that recharge can be increased if dense shrub cover is converted to grassland. Studies in other regions of Texas dominated by brush have yielded similar results.

Recent work using hydrologic modeling focuses on the strong influence of climate, soils, and vegetation on recharge in Texas.  Based on these simulations, it is estimated that recharge for the South Texas Plains is less than 0.2 to 0.4 inches/acre/year (Keese et al. 2005). Further investigation shows that the recharge rate declined by a factor of 2 to 30 times when vegetative cover was included in the model rather than focusing solely on climate and soil texture (Keese et al. 2005). This suggests that rangeland restoration on sandy soils where water has a chance to make it past the root zone may have a strong effect on recharge in the South Texas Plains. 

All of the available research in the South Texas Plains suggests that little, if any, recharge occurs in the presence of dense shrub cover. However, both hydrologic modeling and fieldwork suggest that when rangeland grasses are restored, recharge will be higher—especially on sandy soils. On the basis of current research, best estimates are that 15 to 30 acres of south central Texas brushland restored to rangeland would yield 1 acre-foot of water per year.

RANGELAND RESTORATION

Rangelands comprise 60 percent (90 million acres) of Texas land. In addition to supporting livestock production and providing habitat for native wildlife, they serve as the state’s largest watershed.  The precipitation that falls on Texas rangeland is a major source of surface flow and aquifer recharge. The management of rangelands can have major impacts on the water available to Texas. Healthy rangelands provide high-quality drinking water, promote recharge, conserve soil, filter overland flow of water, provide forage for livestock, and provide wildlife habitat (Hays et al. 1998). Over the last century, encroachment of woody species across much of Texas’ rangelands has degraded many of these services.

Rangeland restoration programs strive to reverse this trend. Through the use of sound management practices, woody species on Texas rangelands can be controlled, and rangelands can be restored.

There are many benefits to rangeland restoration including increased forage for livestock production. This allows the opportunity to increase stocking rates and ultimately increase revenue for landowners. Rangeland restoration can enhance wildlife habitat, thus enhancing hunting opportunities—a major source of income for Texas landowners. 

Healthy rangelands provide a tremendous public benefit to Texans.  Wise stewardship of this resource will have positive impacts on Texas for many generations—the greatest of which is a plentiful and clean supply of fresh water.

Saving the water and the soil must start where the first raindrop falls.
—Lyndon B. Johnson, 1947

Rangeland Restoration Techniques and Brush Management

The vast differences in terrain and vegetation between the Edwards Plateau and South Texas Plains require different approaches to rangeland restoration and brush management.  Costs for both regions are highly variable based on a number of factors including size and density of the target brush species; the type, rock content, and slope of soil in which the target species is growing; whether the target species sprouts re-growth from root buds; and whether cost-effective herbicides are available for controlling the target species.

Edwards Plateau—Within the Edwards Plateau, Ashe juniper is the primary species targeted for brush management and subsequent restoration of rangelands. Ashe juniper is a non-sprouting species; i.e., juniper will die when all of the aboveground green material is removed. There are three primary methods used for controlling Ashe juniper— mechanical, prescribed fire, and biological. There are limited chemical treatments for the control of Ashe juniper.

How much does it cost? Assuming rangeland restoration practices are effective for at least 10 years, the cost to produce an additional acre-foot of water in the Edwards Plateau would be $40 to $180 depending on the method. Likewise, over the Carrizo-Wilcox Aquifer in south central Texas, the range is $100 to $300 per additional acre-foot.

Mechanical treatments involve the use of large equipment such as a bulldozer or skid steer loader that physically removes the aboveground portion of the juniper. Costs are variable, but based on current market rates, they range from $75 to $400/acre (Pestman 2007). Perhaps the most economically effective treatment for juniper control is prescribed fire.  Prescribed fire can be combined with other high-cost initial practices such as mechanical brush control to enhance or maintain brush control benefits for many years. Costs are variable, but based on current market rates, they can range from $3 to $8/acre.  Biological control of Ashe juniper can also be achieved through the use of goats. Goats will browse on the young saplings of both juniper and hardwoods. When concentrated in high densities and rotated through pastures, they represent an effective means of controlling woody brush.

SOUTH TEXAS PLAINS—The South Texas Plains are the heart of the Texas “Brush Country,” and no other region in Texas has seen more widespread implementation of brush management practices. Brush stands in this region are often mixtures of more than 15 species such as mesquite, acacia, and prickly pear.  Most brush species in this region will re-sprout after treatment, which causes significant management challenges.  The primary methods of controlling brush in the South Texas Plains are mechanical, chemical, and prescribed fire (Hamilton et al. 2004). 

Mechanical methods that involve the use of a root plow, bull dozer, or chaining are highly variable depending on density and target species. These methods range from $30 to $250/acre based on current market rates (Pestman 2007). Due to the re-sprouting nature of most target species in the South Texas Plains, chemical methods represent a cost- effective method for controlling brush and maintaining range conditions. Based on current market rates, costs for chemical control methods range from $20 to $120/acre. Prescribed burning can be combined with other initial practices such as mechanical brush control to enhance or maintain brush control benefits for many years. Costs are variable, but based on current market rates, they range from $3 to $8/acre.

Where does rangeland restoration work?  Water yield resulting from rangeland restoration is likely to be effective only on sites receiving at least 18 inches of annual rainfall and having geology and soil characteristics leading to rapid runoff and infiltration (Hibbert 1983, Seyfreid et al. 2005, Wilcox et al. 2006).

Aquifer recharge zones and river basins within Region L suitable for rangeland restoration efforts to increase water yield based on vegetative cover, geology, and soil characteristics.

Cost of other methods?  The cost of additional water yield through rangeland restoration compares favorably to other methods of acquiring additional available water. For example, municipal conservation efforts cost $50 to $200/acre-foot, and seawater desalination costs $619/acrefoot (SCTRWPG 2006).

Rangeland Restoration and Water

Areas within Region L with the greatest potential for increasing recharge through brush management and rangeland restoration are those areas where deep drainage (water movement beyond the root zone) can occur. This characteristic is found where soils are shallow and overlie relatively permeable bedrock. The Edwards Plateau region is a prime example of this type of situation. It has considerably more “flowing water” than would be expected for a semiarid climate (about 27.5 inches of precipitation per year). The explanation lies in the karst geology—a substrate of fractured limestone that allows rapid flow of water to the subsurface.  Other soil types that may enable deep drainage are sandy soils—like those found in the Carrizo-Wilcox Aquifer recharge zone. Large areas of Region L exhibit these characteristics and support vast expanses of brush that provide opportunities for increasing water yield through rangeland restoration.

Based on current research within the Edwards Plateau, the best estimate is that the conversion of Ashe juniper woodlands into grassland-dominated savannas (woody cover < 10%) would result in an average increase in water yield (streamflow and recharge) of approximately 1.5 to 2.4 inches/acre/year. Thus, for every 5 to 8 acres of Ashe juniper converted to open savannas, an additional acre-foot of water would become available for recharge and streamflow. Current research within the south central Texas shrublands indicates that average recharge on sandy soils could be increased by shrub control from 0.4 to 0.8 inches/acre/year. This translates to an additional acre- foot of water for every 15 to 30 acres of brush cleared. For example, if recharge rates were at the lowest estimated level (0.4 inches/acre/year), restoring 200,000 acres of rangeland over the Carrizzo-Wilcox recharge zone would increase recharge by about 5,000 acrefeet/year. Based on average annual water consumption, an acre-foot is enough water to satisfy the needs of five Texans for an entire year.

The cost-effectiveness of implementing a brush management program for increasing recharge and streamflow cannot be assessed strictly on the basis of the initial cost of rangeland restoration. For example, if a rangeland restoration program is limited to the initial practice, regrowth of brush will eventually occur to the point that there will no longer be any increase in groundwater recharge. Alternatively, by using follow-up brush control practices after the initial treatment to control brush re-growth, the increased groundwater recharge gained from the initial brush control practice can be maintained for many more years into the future. Fortunately, followup range management practices, like prescribed fire, are relatively inexpensive. Therefore, rangeland restoration programs consisting of an initial treatment plus appropriate maintenance practices at 3- to 6-year intervals after the initial practice can result in maintaining range condition and the resulting increase in ground water recharge for many years into the future.

For example, clearing juniper on the Edwards Plateau ranges from $100 to $400 per acre. On average, this yields an increase in recharge
of approximately 0.167 acre-feet per year. The results of extending the years of reduced brush cover, and the subsequent increased groundwater recharge, on the cost per acre-foot of added groundwater recharge are illustrated in Table 1. The cost estimates are obtained by taking the per-acre cost of the restoration practice, or cost of a program consisting of an initial restoration plus follow-up practices, and dividing it by 0.167. This yields the estimated cost per acre-foot of added groundwater recharge resulting from brush control if the program is effective for only one year. Results for several alternative levels of brush control costs are detailed in Table 1. The third and fourth columns of Table 1 illustrate the per-acre-foot costs of added groundwater recharge resulting from brush control if the brush control practice, or program, is effective for a period of five and ten years, respectively.

If rangeland restoration programs were implemented that require participating landowners to maintain range conditions with 5 percent woody cover for a period of 10 years, then the costs per acre-foot of added groundwater recharge would be expected to range between $40 and $180 per acre-foot in the Edwards Plateau and between $100 and $300 per acre-foot in the Carrizo-Wilcox Aquifer recharge zone. It should be noted that the estimates of added groundwater recharge costs are based on the highly variable costs of the brush control practices and/or programs. Additionally, there are many other factors that would impact the ultimate costs, including landowner participation
rates, proper implementation and maintenance of brush management, and availability of cost-share funding.

Incentive-Based Programs

Rangeland restoration is generally recognized to be a long-term investment that often requires a large monetary investment with benefits extending several years into the future. Landowner benefits are based on expected increases in returns from livestock and wildlife enterprises (Conner and Bach 2000).

Even after benefits are realized, they may not be enough to cover the costs of clearing and maintenance activities. Landowners who receive cost-share funding will realize a profit from their investment more quickly regardless of the cost of clearing if they are reimbursed for a portion of restoration expenses.  In addition, studies have shown that when cost-share funds are available, landowner participation rates increase (Olenick et al. 2004). An incentive-based program that shares the cost of rangeland restoration with the landowner could provide opportunities for increasing water yield.

Endangered Species Management

A rangeland restoration program, if implemented, would need to  account for endangered species habitat. Within Region L, the golden-cheeked warbler, a federally listed endangered bird, inhabits mature oak-juniper woodlands. This habitat would not be a candidate for rangeland restoration efforts. Impacts to the Black-capped vireo, another federally listed endangered bird, would also have to be considered. Any juniper removal performed in areas that have the potential for becoming black-capped vireo habitat would have to be done selectively and followed with prescribed fire to enhance that potential. Any brush clearing should be conducted by a certified brush management contractor who has received training on how to recognize and work around endangered species habitat.

Conclusion

Over the next several decades, Region L will continue to face an ever-increasing demand for a limited resource—fresh water. Based on the research available, there is a better understanding of the influence of vegetation and vegetation management on the performance of water catchments and the land-based water cycle. Land-based water conservation practices, specifically rangeland restoration, offer great promise for augmenting fresh water supplies in the Edwards Plateau and South Texas Plains of Texas. With long-term costs per acre-foot of added water ranging from $40 to $180 for the Edwards Plateau and $100 to $300 for the South Texas Plains, brush management represents a cost-effective alternative for increasing available water when compared to other water management strategies.  In order for a rangeland restoration program to be successful, an incentive-based program that helps defray the high initial landowner costs of brush clearing must be developed. Additional programs should be developed that provide incentives for landowners to maintain rangeland in a manner producing the greatest public benefit, i.e., increased water yield and the other benefits healthy rangelands provide.

Where do we go from here?  Additional work should be performed to determine the number of acres suitable for rangeland restoration efforts to increase water yield. In addition, studies should be conducted that assess the impact of incentivebased programs on landowner participation. When combined, these two efforts will give a better estimate of expected water yield from rangeland restoration.

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A summary of a technical report by:  Richard Conner, Wayne Hamilton, Brad Wilcox

With input from: Neal Wilkins, David K. Langford, Todd Snelgrove

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