3.1. Study Site

The case study site is the The 471 km² Dolan Creek watershed in Val Verde County, TX, USA. USGS Gage 08449100 is located on Dolan Creek near the watershed outlet and the confluence of Dolan Creek and Devils River.

Dolan Creek watershed

This stream gaging station has been in operation since November 2011. As shown on Figure Dolan Creek watershed, most of Dolan Creek is ephemeral with the exception of the downstream-most reach where a number of springs, including Dolan Springs, provide flow to the stream throughout the year.

Figure Dolan Creek watershed shows the mapped Hydrologic Soil Group from SSURGO for the site watershed. Table SSURGO soil properties provides a listing of the pertinent hydrologic soil properties for HSPF simulation considerations, also from SSURGO. The ridgetops and valley/mesa sides (92-93% of total watershed area) have low water storage and high runoff potential while the dry stream beds and valley bottoms (7-8% of total watershed area) provide high infiltration rates and near surface water storage.

SSURGO soil properties

Descriptor	Value	% of Watershed	Notes
Hydrologic Soil Group	D	92%	Everything except for dry stream beds
	C	1%
	A	7%	Dry stream beds and valley bottoms
Available Water Supply	1 - 2 cm	88.5%	Uplands away from streambeds
	3 - 4 cm	4 %	Valley bottoms away from stream beds
	10 - 23 cm	7.5%	Dry stream beds
Depth to restrictive layer	> 201 cm	8.2%	Dry stream beds
	30 - 50 cm	53.7%	Hillsides
	20 - 25 cm	38.1 %	Ridgetops

Hydrologic Soil Types:

D = high runoff and slow infiltration
C = moderate runoff and slow infiltration
A = little runoff and high infiltration rate

Land cover across the watershed is relatively uniform shrub/scrub. Development-related impervious surfaces within the watershed are limited as there are no paved roads and only a few scattered cabins. Most of the watershed, 92% in Table SSURGO soil properties, is naturally impervious due to the rocky nature of the mesa-dominated landscape. The site watershed is in karst terrain and the high infiltration rate, valley bottom-areas exhibit enhanced secondary porosity from limestone dissolution.

In terms of surficial geology, the Dolan Creek watershed is at the southwestern margin of the Edwards Plateau, a resistant carbonate upland of nearly flat-lying limestone and dolostone with thin soils, caprock mesas and dry arroyos. Dolan Springs and three other mapped springs, in or near the site watershed, are located on an outcrop of the Fort Terrett limestone of the Edwards Formation. Conduits and caves are present in the overlying Segovia limestone above the contact with the Fort Terrett limestone. The majority of the watershed has Segovia limestone of the Edwards Formation mapped at the surface with the exception of a few ridgetops mapped as Buda limestone of the Edwards Formation.

The hypothesis for integrated hydrologic model development is that the Fort Terrett is relatively impermeable and spring discharge occurs at or associated with the contact of the Fort Terrett and overlying material. Additionally, the perennial portions of Dolan Creek coincide with locations where the creek bed crosses Fort Terrett outcrop.

As shown on Figure Dolan Creek watershed most of Dolan Creek is ephemeral with the exception of the downstream-most reach where a number of springs, including Dolan Springs, provide flow to the stream throughout the year. A probabilistic, day of the year discharge plot is provided for USGS Gage 08449100 Dolan Creek on Figure Dolan Creek probabilistic discharge. In this figure, the interquartile (25th percentile to 75th percentile) and lower quartile (0th percentile to 25th percentile) ranges suggest relatively consistent discharge across the year. The consistent flow pattern is attributed to the dominance of spring discharge from Dolan Springs and YR-70-01-701 (see Figure Dolan Creek watershed) on the Dolan Creek discharge hydrograph.

Dolan Creek probabilistic discharge