RT08A - Lower Laraos

Site Code: RT08A. GPS: S12°33656, W75°825343. Elevation: 2924m asl.

Water Quality Data

The following river and water quality data was collected from this site:

RT08A July 2019 November 2019 November 2021 June 2022
Season: Austral Winter Austral Summer Austral Summer Austral Winter
Date: 11.07.2019 20.11.2019 15.11.2021 28.6.22
Time: 12.30 11.00 10.15 11.15
Width (m) 3.0 3.0 3.4 4.6
Depth (Av.) (m) 0.11 0.16 0.28 0.19
CSA (Av.) (m²) 0.34 0.47 0.96 0.88
Velocity (Av.) (m/s) 0.65 0.34 0.44 1.11
Discharge (Av.) (m³/s) 0.221 0.161 0.420 0.970
Riverbed sediment Gravel, cobbles & boulders Cobbles & Boulders Mainly Cobbles Cobbles & Boulders
Temp. (Atmospheric) (°C) 12 23 20 22
Temp. (Water) (°C) 11.93 12.85 12.28 12.22
Atmospheric pressure (mbar) 697 726   624
Dissolved oxygen (mg/l) 5.24 3.99 6.71 6.95
Dissolved oxygen (%) 67.3 63.3 87.2 106.1
pH   8.68 8.33 8.43
Conductivity (µS/cm) 263 296 371 272
Total Dissolved Solids (ppm) 132 148 186 136
ORP (mV) 350.2 123.6 146.1 156.2
Turbidity Slightly turbid Clear: Clear: Clear:
Alkalinity (mg/l CaCO₃) 119 (1.6N) - 110 (1.6N) 120 (1.6N)
Alkalinity Tester (ppm/ CaCO₃) - - - 102

Data Analysis

Discharge at this point along the Rio Laraos is always low. It is a small tributary and significant water abstraction takes place for the small town of Laraos and the irrigation of extensive Andenes (terraced hillsides) above the site. An additional factor is that a significant quatity of any rainfall in the headwaters is retained in a natural reservoir immediately above Laraos (Laraos sits atop a terminal moraine) which helps to control the river flow throughout the year.

At just over 2900m asl the DO readings at RT08A fall within the range to be expected for both July and November data sets. During the day, atmospheric temperatures are often much higher than water temperatures which vary little on a diurnal basis. The latter can account for the limited reductions in DO levels.

The conductivity readings at this site (263-371 µS/cm) are as expected for a sub-catchment at this elevation.

Average pH readings of about 8.5 are as expected. At these levels the alkalinity of the river is highly suitable for aquatic life and indicative of clean water.

Site Description

Site RT08A lies in the Rio Laraos valley at 2914m asl and just above the confluence with the Rio Cañete. It is located in the temperate zone, above the HEP station, in a picturesque section of the valley. The valley at this point is narrow with a v-shaped profile and a narrow flood-plain. River flow in the Rio Laraos is year-round but delivers a minimal input to the Rio Cañete. This is probably because water abstraction is significant for the settlement of Laraos and the extensive terraces downstream of it.

The study site is dominated by alders, agaves, and tall natural scrub. Above the narrow flood plain, the valley sides are similar to RC11 and all the observations made for that site apply to RT08A as it is only 100 metres away.

Tributary T08 - the Laraos subcatchment. (Eustace Barnes) Tributary 08 - the Laraos subcatchment. (Eustace Barnes)

Site T08A - the study site location. (John Forrest) Site RT08A - the study site location: just beside the main valley highway. (John Forrest)

Site T08A - the study site. (John Forrest) Site RT08A - the study site. (July 2019) (John Forrest)

Site T08A - View upstream from the study site. (John Forrest) Site RT08A - View upstream from the study site. (July 2019) (John Forrest)