Randy Mayer and Bradford Wilcox TX A&M University Department of Rangeland Ecology and Management

Streamflow Characteristics of Individual Watersheds on the Edwards Plateau

Eco-hydrology of Karst Shrublands

Randy Mayer and Bradford WilcoxTX A&M University

Department of Rangeland Ecology and Management

Randy Mayer

You wouldn't think that a semi-arid system like the one pictured here had the capacity to yield much water. The fact is that this type system does produce vast sums of water for the Austin-San Antonio area of Central Texas. It is important to remember that there are two components to this system, surface flow and subsurface flow.

• Climate: precipitation, 375-800mm temperature, 18-20 degrees C

• Geology: Edwards & Glen Rose limestones with large areas exposed

• Soils: Shallow calcareous, rocky, underlain by fractured limestone

Randy Mayer

The Edwards Plateau is located in this region of Texas. Precipitation varies from 35 in/yr in the east to 15 in/yr in the west. The south and east comprise the "Texas Hill Country", and as can be seen in this slide, consists of canyons, ravines, and steep hillslopes. This is the Balcones Escarpement. The west is marked by gently sloping hills and upland plateaus. The soils are rocky and extremely shallow, which makes for some unique and intersesting hydrology.

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Randy Mayer

Top Graph - The population of Texas is expected to almost double by 2050 (blue). Water demand is expected to increase by almost 20% (red), while water supply from existing sources is expected to decrease by almost 20% (green). This means that the State of Texas must locate new sources of water equivilent to 40% above what is currently available.Bottom Left - $30 billion in proposed water supply projects to meet this need. Notice the Austin-San Antonio area.Bottom Right - Rivers in Texas with red delineating those streams that are already 100% appropriated. Notice the Colorado River and its tributaries that flow off the Edwards Plateau.

Randy Mayer

Brush control is one method being implemented to increase regional water yield. The State of Texas has already spent over $25 million and is considering a more extensive program.

North Llano Watershed

• Semi-arid environment

• Perennial baseflow

Randy Mayer

Before looking at the role vegetation may play in increasing regional water yield, we first need to get an understanding of the characteristics of watersheds on the Edwards Plateau. The watershed of interest is the North llano watershed in Kimble and Sutton Counties. The difference between this photo and the one on the first slide is that the North Llano has perrennial baseflow supplied by springs.

Daily Streamflow

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Randy Mayer

Large Graph - Daily streamflow for the period of record with year on x axis and streamflow in cf/s on y axis. Notice the spikes, of which about 45 are over 10,000 cf/s. To put this in perspective let's look at the inset graph.Small Graph - Daily streamflow (log scale) shows the average streamflow is about 150 cf/s. It is interesting to note that even during the drought of record in the 1950s, these springs didn't dry up.

Randy Mayer

These photos contrast baseflow with streamflow generated by a large precipitation event. On Nov. 3rd, 2000, the North llano watershed received enough preipitation to raise the guage height on the North Llano to 32 ft (which is still far below the 45 ft record). A few days later the first dam in Austin's Highland Lakes System had all flood gates open. Water continued to rise and poured over the top. What is interesting is that 3 days earlier this reservoir was at 50% capacity. This may indicate that these reservoirs aren't filled by baseflow or small rain events, but rather by large events such as this one.

What is the Interaction of Shrubs and Hydrology Given this Type of Flow Regime?

• Widespread conversion of grasslands and savannas to shrublands in the last 100 years.– Mesquite, ashe and redberry juniper– Dramatic shift of herbivory and fire regime

Preliminary Analysis of Vegetation, Patch Analyst

1966 1995

2% increase in shrub density

Randy Mayer

Now that we know a little about the characteristics of the watershed, what role might vegetaion play with respect to streamflow? There is a widespread belief that an increase in shrubs over the past 100 yrs has decreased streamflow on the Edwards Plateau. Preliminary analysis of aerial photos from 1955 and 1966 compared with DOQQs from 1995 has shown only a slight increase in shrubs, but this work is ongoing. Bottom right is an example of the analysis using Patch Analyst and FragStats. The dominant shrub is ashe juniper.

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* Keith Owens, TX A&M

Research Center (Uvalde)

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* Juniper interception is significant for small events, but has less influence on larger events

Randy Mayer

What influence does ashe juniper have on the hydrologic cycle? Studies have shown that ashe juniper has greatest influence in its ability to intercept large amount of rainfall.Top Graph - Precipitation events for period of record. Vast majority are small events.Bottom Graph - One of ten sites set up by Keith Owens to record Juniper interception. All ten sites have similar findings. Ashe juniper has the capacity to intercept a large percentage of rainfall coming from small events, but has decreasing influence as the rain event increases in size.

Annual Precip & Resulting Streamflow

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Randy Mayer

Top Graph - X axis is year, y axis (left) is mm of precipitation for top line. While the average falls around 600 mm, it is highly variable. Secondary y axis (right) is percent of rainfall that becomes streamflow and corresponds with bottom line. While the average is just under 6%, this increases considerably with increasing rainfall. So the more rain this watershed receives, the resulting streamflow increases at a rate greater than linear.Bottom Graph - x axis is year, y axis is precipitation in mm for the top line and runoff in mm for the bottom line. Runoff is highly dependent on rainfall. For the period of record, there is no significant trend in runoff. So if there has been any change in surface cover, it is not reflected in streamflow.

Runoff

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Randy Mayer

This contrasts baseflow with stormflow.

Preliminary Conclusions

• It appears that the runoff necessary to fill reservoirs on the Edwards Plateau is generated by large precipitation events.

• Surface cover has less influence on these large events than the small events.

• While there are many benefits to shrub control in the North Llano basin, increasing regional water yield does not appear to be one of them.

Randy Mayer and Bradford Wilcox TX A&M University Department of Rangeland Ecology and Management

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