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:

T8a July 2019 November 2019 November 2021
Season: Austral Winter Austral Summer Austral Summer
Date: 11.07.2019 20.11.2019 15.11.21
Time: 12.30pm 11.00am 10.15am
Width (m) 3.0 3.0 3.4
Depth (Av.) (m) 0.11 0.16 0.28
CSA (Av.) (m²) 0.34 0.47 0.96
Velocity (Av.) (m/s) 0.65 0.34 0.44
Discharge (Av.) (m³/s) 0.221 0.161 0.420
Riverbed sediment Gravel, cobbles & boulders Cobbles & Boulders Mainly Cobbles
Temp. (Atmospheric) (°C) 12 23 20
Temp. (Water) (°C) 11.93 12.85 12.28
Atmospheric pressure (mbar) 697 726  
Dissolved oxygen (mg/l) 5.24 3.99 6.71
Dissolved oxygen (%) 67.3 63.3 87.20
pH - 8.68 8.33
Conductivity (µS/cm) 263 296 371
Total Dissolved Solids (ppm) 132 148 186
ORP (mV) 350.2 123.6 146.1
Turbidity Slightly turbid Clear: Clear:
Alkalinity (mg/l CaCO₃) 119 (1.6N) - 110 (1.6N)
Alkalinity Field Tester (ppm/ CaCO₃) - - -

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). Discharge fell but remained similar from July to November. Any increased rainfall in this period would have been retained in the reservoir above Laraos and/or used for agricultural purposes.

At just over 2900m asl the DO readings at T8a fall within the range to be expected for both July and November data sets, although they are rather variable at this site. Atmospheric temperatures were much higher in November, although water temperatures were only slightly higher. The latter can account for limited reductions in DO levels. Also, atmospheric pressure was lower in July.

Oxygen Saturation in water bodies. Two bodies of water that record 100\% saturation do not necessarily have the same concentration of dissolved oxygen. Absolute DO (in mg/l) will vary depending on temperature, pressure, and salinity.

The solubility of oxygen decreases as temperature increases; warmer surface waters require less dissolved oxygen to reach 100% DO, than does deeper, cooler waters. At sea level, water at 4°C reaches 100% DO at 10.92 mg/L of dissolved oxygen. At 21°C, there would only be 8.68 mg/L DO at 100% air saturation - -dissolved oxygen decreases exponentially as salt levels increase. At the same pressure and temperature, saltwater holds about 20% less dissolved oxygen than freshwater. -dissolved oxygen increases as pressure increases. This is true of both atmospheric and hydrostatic pressures. Water at lower altitudes can hold more dissolved oxygen than water at higher altitudes. This relationship also explains the potential for “supersaturation” of waters below the thermocline. Hydrostatic saturation decreases by 10% per meter increase in depth due to hydrostatic pressure. This means that if the concentration of dissolved oxygen is at 100% air saturation at the surface, it would only be at 70% air saturation three meters below the surface. -as one ascends through the Andes the capacity of water to hold dissolved oxygen increases as temperature falls. This typically takes place at 1-1.5°C for every 100m increase in elevation. At the same time DO decreases as atmospheric pressure falls. As such another variable is required to explain this anomalous measurement.

The conductivity readings at this site (180-185 µS/cm) are as expected for a sub-catchment.

Average pH readings of about 8.5 in November 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 T8a lies in the Rio Laraos valley at 2914m asl and just above the confluence with the Rio Canete. It is located in the temperate zone, above the HEP station G2, in a picturesque section of the valley. The valley at this point is narrow with a v-shaped profile and a narrow flood-plain. Bed load in the river channel is comprised of scattered large boulders, gravels, and smaller stones. River flow in the Rio Laraos is year-round and delivers a minimal input to the Rio Canete. This is probably because water abstraction is significant for the settlement of Laraos and the surrounding terraces.

The study site which is dominated by alders, agaves, and tall natural scrub. Above the narrow flood plain valley sides are as at RC11 and all the observations made for that site apply to T8a 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 T08A - the study site location: just beside the main valley highway. (John Forrest)

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

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