Effects of Brush Management on Water Budget and Water Quantity, Honey Creek State Natural Area
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Woody vegetation, including ashe juniper (Juniperus ashei), has encroached on areas in central Texas that were historically oak grassland savannah. Encroachment of woody vegetation is generally attributed to overgrazing and fire suppression. Removing ashe juniper and allowing native grasses to reestablish in the area as a brush management conservation practice might change the hydrology in the watershed. These hydrologic changes might include changes to surface-water runoff, evapotranspiration, or groundwater recharge.
The USGS Texas Water Science Center conducted a study during 1999 to 2010 to evaluate the effects of ashe juniper (Juniperus ashei) removal as a best-management practice (BMP) for changes in water quantity and water quality in two watersheds located in the Honey Creek State Natural Area in Comal County, Texas. In 2002, the study was expanded to include an additional watershed, Laurel Canyon Creek, located in the Government Canyon State Natural Area in Bexar County, Texas.
The idea of hydrologic changes resulting from brush management generally is based on a simplified mass balance approach to the hydrologic cycle in which rainfall accounts for the water coming into the system, and rainfall is distributed to:
- surface-water runoff (streamflow),
- evapotranspiration (combination of evaporation and transpiration), or
- groundwater recharge (subsurface flow that contributes to the groundwater table or contributes to spring discharge downstream from the study area).
If the amount of rainfall remains constant and the evapotranspiration rates change because of a change in vegetation cover, then the surface-water or groundwater components of the hydrologic budget will change.
The U.S. Geological Survey, in cooperation with Federal, State, and local partners, examined the hydrologic effects of brush management in two adjacent watersheds.
Hydrologic data (rainfall, streamflow, evapotranspiration, and water quality sampleese) were collected in the watersheds for 3–4 years, after which brush management occurred on one watershed and the other was left in its original condition. The same hydrologic data were then collected in the study area for another 6 years. Groundwater recharge was not directly measured, but potential groundwater recharge was calculated by using a simplified mass balance approach.
The following highlights of the study are summarized from Effects of Brush Management on the Hydrologic Budget and Water Quality In and Adjacent to Honey Creek State Natural Area, Comal County, Texas, 2001–10
- The streamflow to rainfall relation did not change between the watersheds during pre- and post-treatment periods.
- Analyses indicate that the difference in evapotranspiration rates between the two watersheds is greater during the post-treatment period than it was during the pre-treatment period; an approximate 8 percent reduction in evapotranspiration in the treated watershed is likely.
- Average annual rainfall, streamflow, evapotranspiration, and potential groundwater-recharge were incorporated into a hydrologic budget (expressed as a percentage of the average annual rainfall) for each watershed pre- and post-treatment to evaluate the effects of brush management. Differences in percentages of average annual evapotranspiration and potential groundwater recharge during the post-treatment period were more appreciable between the watersheds than during the pre-treatment period.
- There were no notable differences in major ion or nutrient concentrations between water samples collected at the reference watershed and the treatment watershed during pre- and post-treatment periods.
- Suspended-sediment loads calculated from water samples collected from the reference watershed and the treatment watershed did not exhibit a significant difference during the pre-treatment period, whereas during the post-treatment period, suspended-sediment loads calculated from the treatment watershed were generally less than those from the reference watershed.
The results of the Honey Creek study are published in Effects of Brush Management on the Hydrologic Budget and Water Quality In and Adjacent to Honey Creek State Natural Area, Comal County, Texas, 2001–10
The results of the Government Canyon study are published in Hydrologic and water-quality data at Government Canyon State Natural Area, Bexar County, Texas, 2002-10