RT04A - Lower Allauca

Site Code: RT04A. GPS: S12°62956, W75°979014. Elevation: 1745m asl.

Water Quality Data

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

T4a July 2019 November 2019 November 2021
Season: Austral Winter Austral Summer Austral Summer
Date: 10.07.2019 19.11.2019 12.11.21
Time: 16.05pm 9.30am 12.00
Width (m) 5.0 8.0 0.90
Depth (Av.) (m) 0.01 0.05 0.05
CSA (Av.) (m²) 0.07 0.42 0.05
Velocity (Av.) (m/s) 0.22 0.19 0.43
Discharge (Av.) (m³/s) 0.014 0.080 0.019
Riverbed sediment Cobbles & boulders Cobbles & boulders Cobbles & Boulders
Temp. (Atmospheric) (°C) 23 25 26
Temp. (Water) (°C) 16.39 18.10 21.55
Atmospheric pressure (mbar) 825 824  
Dissolved oxygen (mg/l) 5.49 4.95 7.86
Dissolved oxygen (%) 68.1 69.6 107.5
pH - 8.87 8.73
Conductivity (µS/cm) 248 236 365
Total Dissolved Solids (ppm) 124 118 182
ORP (mV) 342.7 132.7 59.3
Turbidity Clear: Clear: slight green colouration Clear:
Alkalinity (mg/l CaCO₃) 111 (1.6N) - 128 (1.6N)
Alkalinity Field Tester (ppm/ CaCO₃) - - 114

Data Analysis

Discharge in the Rio Ayauca is very similar to that in other sub-catchments. Elevated discharge being recorded for the months January to April, followed by a gradual diminution until the rains start in December. The periods we have sampled lie within the dry season.

At just over 1740m asl the DO readings at RT4a fall within the range to be expected for both July and November data sets. Again, the increased elevation and diurnal fluctuations in temperature in narrow, sheltered sections of the valley will produce fluctuations in maximum absolute oxygen capacity and, in consequence, percentage saturation. This then fluctuates with that rhythm accordingly. The DO levels appear to be rather variable at this site, perhaps indicating that the conditions for life are likewise rather variable. However, we did find abundant frog spawn and immature frogs in the river in November 2019.

The Conductivity reading at this site (235-250 (µS/cm) is very low. Since the stream discharge is very low the impact on the quality of water in the main channel is minimal.

pH readings approaching 9.0 in November are slightly on the high side comapred to the main valley. At these levels the alkalinity of the river is still suitable for most aquatic life and not approaching the tolerances of most aquatic vertebrates and invertebrates.

Site description

Site T4a is located adjacent to the settlement of Calachota and above the main Rio Canete valley at 1740m asl in the arid subtropics. This narrow side valley drains one of the larger sub-catchments that lie on the north bank. At this sample point it has a less v-shaped profile. There are significant areas of cultivation, requiring irrigation waters on the south side of the valley. This is taken from higher up the valley and used to irrigate the hillside above the village of Calachota. Irrigation channels carry water along valley side contours extending the area of cultivation significantly. As such the Rio Ayauca has little or no water in it for much of the year.

The use of pesticides has been observed at many sites throughout the valley and it would be useful to know which agrochemicals are being used. It is also a concern to see the individual application of these chemicals without user protections. As such it must also be a concern that their use is indiscriminate and that there are no controls placed upon their use. As elsewhere in the drainage basin discharge is highly variable with a marked seasonality. Discharge measurements from this tributary from January to March are considerably greater.

Above the cultivated areas valley sides are arid, supporting a xeric vegetation. Along the valley bottom, the vegetation is anthropic in origin. There only remain scattered acacia, clumps of river cane and smaller willows along the river edge. The fauna and flora in the valley remains depauperate and largely comprised of synanthropic species. Scrub Blackbird, Black-necked Woodpecker and Bare-faced and Croaking Ground Doves are common as elsewhere up to the temperate zone. It would be interesting to see if there is any bioaccumulation of pesticide residues or other trace metals in the avifauna throughout the valley.

Tributary T04 - the Ayauca subcatchment. (Eustace Barnes) Tributary 04 - the Ayauca subcatchment. (Eustace Barnes)

Site T04A - the study site location. (John Forrest) Site T04A - the study site location: just above the village of Calchota and up from the main valley highway. (John Forrest)

Site T04A - the study site. (John Forrest) Site T04A - the study site. (July 2019) (John Forrest)

Site T04A - View downstream from the study site. (John Forrest) Site T04A - View downstream from the study site. (July 2019) (John Forrest)