1.  A LINEAR OASIS

The dictionary has several definitions of oasis. One: A fertile spot in the desert, where water is found. Two: A small area in a desert that has a supply of water and is able to support vegetation. Three: An area where groundwater lies close enough to the surface to form a spring. These definitions point to the four features defining an oasis: A relatively confined spot, an arid climate, the presence of water, and the presence of vegetation.

The community of Tierra del Sol, in southeast San Diego County, in California, has an arid climate, with mean annual precipitation averaging 15 in. There is, however, high variability in annual rainfall and associated environmental moisture. The record shows that annual rainfall has been as low as 4 inches in 2001-02 and as high as 31 inches in 1992-93 (Ponce et al., 2000; Ponce, 2006). Significantly, the region is underlain by a shallow fractured rock aquifer featuring an extensive system of fractures and dikes. The prevailing ecosystem is the chaparral, which is present in Southern California, extending into Baja California, Mexico. The region contains the ideal conditions for an oasis: a limited area, a desert climate, a shallow aquifer, and upland vegetation.

Figure 1 shows a color-enhanced aerial view of a section of Tierra del Sol, near the Morning Star Ranch, a 358-acre rural property owned by Ed and Donna Tisdale. Vegetation is shown in bright red color. Two distinct features are noted:

1. A lowland riparian forest of coast live oak (Quercus agrifolia) along the geomorphological lowlands, where surface and near-surface drainage collects. This forest aligns itself along ephemeral watercourses and is characteristically sparse.

2. An upland linear forest of red shank (Adenostoma sparsifolium) located on the lower right hand corner of the image. This forest is quite dense, aligning itself with what appears to be a significant source of moisture. Herein we refer to this red shank forest as a linear oasis.

Geography Department, San Diego State University (2006) Fig. 1  Color-enhanced image clearly showing the upland linear forest of red shank.

Figure 2 shows a recent (2015) satellite image showcasing the linear forest. It is clearly seen that this forest would not be there if it were not for the presence of sufficient moisture to sustain it. In all probability, the linear forest overlies an unusually large fracture through which water and moisture are convected from the underlying aquifer to the surface. The objective of this study is to substantiate the relation between aquifer fractures and linear vegetative features, with particular focus on the Tierra del Sol watershed.

Google Earth ® Fig. 2  Aerial view of the linear forest of red shank.

2.  THE TIERRA DEL SOL WATERSHED

The Tierra del Sol watershed comprises 1,760 acres near the southeast corner of San Diego County (Fig. 3). It drains gentle slopes and foothills of the California Peninsular Range in a predominantly southern direction, with numerous rock outcrops, dikes, and springs. The land use is a mixture of the native chaparral ecosystem, albeit with a substantial presence of phreatophytes along well defined moisture paths and, notably, a few mesophytes, such as the blue elderberry (Sambucus mexicana).

The anthropogenic land uses include ranch clearings, rural housing, and other dwellings. The main watercourse is an ephemeral stream which flows predominantly in a southern direction, crossing into Mexico at 32° 35" 56.89' N and 116° 20" 55.62' W. The hydrology and hydrogeology of the Tierra del Sol watershed have been described by Ponce (2006).

Fig. 3  The Tierra del Sol watershed, located in the community of Boulevard, southeast San Diego County, California.

Existing studies have revealed the extensive fracture system in the underlying rock aquifer. Figure 4, prepared for the proposed Campo landfill project (1991), documents the presence of many photogeologic lineaments along the watershed's western boundary (Ponce, 2006). The lineaments, highlighted in green, depict changes in type and density of vegetation, reflecting the presence of fractures and other discontinuities in the underlying rock. Unlike the flow of water through an alluvial aquifer, where diffusion is the predominant process, transport through a fractured rock aquifer is dominated by advection.

Fig. 4  Geologic fractures identified in the Tierra del Sol watershed and vicinity.

A striking feature of the Tierra del Sol watershed, and one possibly related to the fractures, is the striking number of white, mostly pegmatitic, seemingly vertical dikes that cut through the underlying aquifer. These dikes are formed by quartz and feldspar-rich magmas left over in the final stages of cristallization of the granitic rocks (Ponce, 2006). One significantly major dike (Fig. 5) has been documented to be 1,465-ft long and about 6-ft wide (Fig. 6).

Google Earth ® Fig. 5  A large pegmatitic dike, upstream of an upland specimen of coast live oak (Quercus agrifolia), shown towards the left.

Fig. 6  Closeup of the large pegmatitic dike.

The Tierra del Sol watershed has its highest point on its northern upland ridges, at elevation 3,880 ft. Its lowest point is at its mouth on the U.S.-Mexico border, at elevation 3,440 ft. Thus, the total watershed relief is 440 ft. The numerous springs located in upland areas are a good indication of the hydraulic connection between groundwater and surface water.

Figure 7 shows the location of eleven (11) springs which drain mostly headwater slopes at elevations above 3,700 ft. The southernmost spring shown in Fig. 7 is located precisely at the western extremity of the large dike (portrayed with a red line) shown in Fig. 5. This spring sustains a rather large specimen of coast live oak, shown in Fig. 8. Notable in this figure is the pool of water (spring) near the base of the tree. Curiously, the figure shows a conveniently placed bathtub which functions as a drinking trough for animals. These facts clearly point to the association of groundwater exfiltration through springs with the presence of vegetation in the immediate vicinity.

Fig. 7  Location of springs within the Tierra del Sol watershed.

Fig. 8  A large specimen of coast live oak (Quercus agrifolia) located at the downtream end of the large pegmatitic dike.

3.  RED SHANK vs CHAMISE

Two closely related chaparral species, chamise (Adenostoma fasciculatum) and red shank (Adenostoma sparsifolium) are significantly represented in the Tierra del Sol watershed. Notably, these two shrubs are the only two species in their genus. Chamise is widely distributed throughout California and Baja California, with a range of nearly 800 miles. In contrast, red shank occurs only in four isolated places in Central California, Southern California, and Baja California as co-dominant with chamise, with a range of only 300 miles (Marion, 1943; Sampson, 1944; Hanes, 1965).

Although chamise and red shank have overlapping geographical distributions, Beatty (1984) has noted that they do not grow in close association within a given stand. Red shank is commonly found on all aspects of gentle slopes and foothills of inner mountain ranges. Pure stands of red shank can be found in only 3% of the area in which red shank is the dominant species (Marion, 1943).

Fig. 9  The stand of red shank in the vegetative lineament of Tierra del Sol.

Chamise and red shank, although congeners, are not at all alike in appearance. Stands of chamise are dull, of dark-green color and uniform in appearance. The mature chamise plant is a medium-sized shrub 2 to 8 ft tall, with sparse leaf litter. In contrast, stands of red shank are distinctive, spreading above the general level of the chaparral. Red shank is a tall, round-topped arborescent plant 6 to 20 ft tall, with thick naked reddish stems and considerable leaf litter, 0.5 to 2 inches in depth (Hanes, 1965). The difference in amount of leaf litter between chamise and red shank may be attributed to their different water affinities (moisture availability and, consequently, actual evapotranspiration).

Fig. 10  Closeup of red shank.

There is a substantial difference in the rooting habits of the two Adenostoma species. Chamise, even though less than half the average size of red shank shrubs, has a more agressive root system, possessing stronger roots with greater length and vertical penetration, appearing to be well adapted to moisture deficiency. On the other hand, the roots of red shank are massive, smaller, weaker, and are more likely to spread laterally rather than vertically. Chamise blooms and sets seed best, with low seed viability, following winters of ample rainfall. In contrast, red shank blooms well, with low seed set and viability, regardless of the amount of rainfall the previous winter (Hanes, 1965). Red shank appears to dominate over chamise on sites with higher moisture content and nutrient availability (Beatty, 1984).

Hanes (1965) noted that although both Adenostoma species resumed growth in early winter, chamise displayed a sudden flush of growth in the spring, whereas red shank grew more steadily, with continued growth throughout the summer and fall. Chamise flowered in April and May, whereas red shank flowered abundantly in August. While chamise has an apparent enforced dormancy in the fall, red shank has been known to experience substantial autumnal growth, suggesting a sustained moisture source.

Red shank appears to violate several definitions of sclerophyllous plants. First, it remains physiologically active during summer drought; thus, it is drought tolerant without being drought dormant (Hanes, 1965). Secondly, its shallow root system suggests that its moisture for summer growth must come from the top layers of the substrate. Therefore, red shank seems to be a type of shrub well adapted to drought conditions, but lacking the obvious morphological characteristics suggesting such adaptability (Shreve, 1934). The water affinities of red shank appear to lie in between those of the xerophytes, which are well adapted to drought, and those of the mesophytes, which habitually require a more sustained moisture source (Ponce, 2006). Thus, the link between red shank and groundwater exfiltration in an arid environment may be envisioned.

4.  STUDY METHODOLOGY

Soil moisture measurements were performed in two sets of paired locations: (a) the target point, located close to the center of the range delimited by the vegetative lineament, and (b) the control point, located outside of this range. To be meaningful, the measurements are performed periodically, at regular intervals, for a period of two years. The objective is to show that there is a moisture gradient between the target point and the control point, wherein the soil moisture in the target point is significantly greater than that of the control point. This explains the presence of the vegetative lineament, closely tied to a sustained moisture source originating in groundwater exfiltration through the linear rock fractures.

The following field strategy was adopted:

1. Number of points. Two pairs of points were selected, for a total of four points: 1A, 1B, 2A and 2B. The A points are the target points and the B points are the control points.

2. Location of the boreholes. The location of the boreholes was selected to lie within the confines of the Morning Star Ranch (Fig. 11). This decision assures unrestricted access and enhanced protection against possible eventual risks. A cleared area was chosen for this purpose (Figs. 11 and 12).

   Google Earth ® Fig. 11  General view of the study area, with the clearance near the middle.

   Google Earth ® Fig. 12  Detail of the borehole locations.

3. Borehole coordinates. The actual physical location of the boreholes was determined by water witching, performed by Ed Tisdale, owner of the Morning Star Ranch (Fig. 13). Mr. Tisdale has about 50 years of experience with water witching in Tierra del Sol and vicinity.

   Fig. 13  Well witching by Ed Tisdale in Tierra del Sol.

   Table 1 shows the coordinates of the borehole locations and the distance between target (green) and control (yellow) points, which are shown in Fig. 12. Points 1A (green) and 1B (yellow) lie north of points 2A (green) and 2B (yellow).

   Table 1.  Geographical coordinates of the boreholes. Borehole Latitude N Longitude W Distance (ft) 1A 32° 37' 37.8" 116° 20' 45.2" 30.384 1B 32° 37' 37.5" 116° 20' 45.1" 2A 32° 37' 36.6" 116° 20' 44.9" 20.256 2B 32° 37' 36.8" 116° 20' 45.0"

4. Moisture sensor. Soil moisture was measured with an Aqua-Pro capacitance moisture probe. This instrument transmits a very low powered radio frequency through the soil to measure moisture. Two copper bands (radio antennas) are located on the end of the sensor. One antenna transmits a low powered radio frequency signal that is received by the other antenna. The proprietary microprocessor can determine the moisture by measuring the change in frequency of the signal it receives. The more moisture in the soil, the greater the shift in the frequency of the signal. The digital readout from the radio-frequency sensor is soil moisture, in percent. The moisture sensor has an accuracy of ± 2% and a resolution of 1% (dry is 0% and wet is 100%).

   The Aqua-Pro probe is lowered into a 6.5-ft long polycarbonate tube that is installed in the soil well in advance of the start of the measuring program. The probe is conveniently tagged to measure soil moisture at the following ten (10) tube depths: 6, 9, 12, 18, 24, 30, 42, 54, 66, 78 inches. The field procedure consists of the following steps:

   • Drilling a borehole to a depth of 6 ft [Fig. 14 (a)];

   • Installing the polycarbonate access tube in the borehole [Fig. 14 (b)]; and

   • Inserting the moisture probe in the access tube to record the percent moisture at ten predetermined (10) depths [Fig. 3 (c)].

   The drilling and installation of the polycarbonate tubes was carried out during the month of November 2014. The program of measurements spans the period March 2015 to March 2017.

   Fig. 14  Field procedure:  (a) digging the borehole; (b) installing the polycarbonate tube;
   (c) inserting the moisture probe.

5. Schedule of measurements. The field measurements are carried out forthnightly, i.e., approximately every two weeks, on the day that coincides with the new moon or full moon. This schedule is for the purpose of controlling the effect of the gravitational pull of the Earth and moon on the soil moisture data. The first measurement was carried out on March 20, 2015. Fortnightly measurements are continuing to this date. Measurements are expected to proceed until a 2-year evaluation period has been completed.

6. Storm precipitation. Storm precipitation has been measured at a raingage located in the Morning Star Ranch since 1990. Between fortnightly soil moisture measurements, the total amount of recorded precipitation, in inches, is shown in Col. 11 of Table 2.

5.  PRELIMINARY ANALYSIS

Figure 15 shows a plot of the results to date, covering the eight-month period from 150320 to 151125. Figure 15 (a) shows the depth-average moisture-content readings in Borehole 1, while Fig. 15 (b) shows the depth-averaged readings in Borehole 2. Figure 15 (c) shows the maximum moisture-content readings in Borehole 1, while Fig. 15 (d) shows the maximum readings in Borehole 2. Intervening precipitation (mm) is also shown in Figs. 15 (c) and (d) for comparison.

The plots clearly show that the moisture content in the target points (A) is higher than that of the control points (B). These fact lend credence to the premise that there are differential moisture gradients in Tierra del Sol, attributable to exfiltration from the underlying fractured rock aquifer. Results to date are encouraging; continued measurement to span a period of 2 years (until March 2017) should confirm the trend. More detailed analysis will be forthcoming upon completion of the measurement program.

Fig. 15 (a)  Borehole 1:  Depth-averaged readings.

Fig. 15 (b)  Borehole 2:  Depth-averaged readings.

Fig. 15 (c)  Borehole 1:  Maximum readings vs precipitation.

Fig. 15 (d)  Borehole 2:  Maximum readings vs precipitation.

6.  CONCLUSIONS

The present study warrants the following preliminary conclusions, subject to revision at the completion of the measurement program:

1. The presence of soil moisture gradients in otherwise similar patches of ground has been documented by measurements in the community of Tierra del Sol, in southeast San Diego County, California. These gradients may be explained by leakage from the underlying fractured rock aquifer. In all probability, leakage proceeds along planes of fracture, with a clear linear tendency (Fig. 16).

2. The species of red shank (Adenostoma sparsifolium) has morphological and other phenological traits which strongly suggest adaptability to shallow, more or less permanent, substrate moisture. This fact explains the substantial differences between the two lone congeners: (a) red shank, which presents itself with taller specimens in the immediate vicinity of the fractures, and (b) chamise (Adenostoma fasciculatum), a shorter species known to colonize much drier patches of ground. Thus, the rationale for the linear oasis, which owes its existence to groundwater exfiltration along linear paths. The oasis is able to sustain an arborescent shrub such as red shank, whose water affinities lie loosely in between those of the xerophytes and the mesophytes.

Fig. 16  Fracture in rock outcrop in the Tierra del Sol watershed.

A final word on the effect of groundwater pumping in arid regions

Uncontrolled or unregulated pumping in fractured rock aquifers in arid regions may have a negative effect on the sustainability of a linear oasis and other upland spring-fed vegetation (Ponce, 2014). In extreme cases, the drying up of the springs and the eventual demise of the vegetation may be envisioned.

Shreve, F. (1934). The problems of the desert. The Scientific Monthly, 38(3), March, 199-209.

Google Earth ® Fig. 17  Aerial view of the Tierra del Sol watershed, at and near the location of the linear oasis of red shank, immediately north of the U.S.-Mexico border.