2.2 Sensors, housing units, and deployment
We selected a waterproof temperature logger with the capacity for
battery replacement by the user for longevity (company: Onset, Bourne,
MA, USA; model: HOBO Pendant, MX2201; diameter: 3.35 cm; temperature
range: -20 ˚C to 50˚C; temperature precision: ± 0.5˚C; cost: $54.00
USD; data retrieval: Bluetooth, battery: user replaceable CR2032 3V
lithium). Our study region is remote with rugged terrain, and deployed
equipment is exposed to variable weather, UV exposure, and potential
tampering from humans, wildlife, or livestock. The intermittent ponds in
our study region are visited frequently by cattle, and equipment must be
able to withstand trampling or tampering. With this in mind, we designed
a rugged housing unit to protect temperature loggers from damage and
ensure long-term durability (Figure S1). We placed a logger inside a PVC
junction box (hereafter called the housing unit) with two nuts between
the box and the lid for increased air or water flow. The logger moved
freely inside the housing unit to increase the chance that it remained
submerged (i.e., fell to the lowest point within the housing unit) if
disturbed after deployment. The housing unit was connected to a concrete
tie or other secure post (e.g., a metal fence post marking edges of
allotments) via a 3/32” (2.381 mm) galvanized, uncoated steel cable
strung through the holes of the junction box. We fastened the cable by
swaging a crimping sleeve. We provide a complete list of specifications
for tools and materials in Table S1.
At each of the 16 ponds, we deployed one logger at the approximate
deepest point of fill within the tank (the pond logger) and one logger
approximately 10 m outside of the high-water mark for the pond (the
control logger). Where possible, we placed control loggers in sunny,
shade-free areas in order to most closely match conditions and exposure
of the pond logger. If the pond basin consisted of fine clay or silt, we
placed the housing unit on a flat rock partially buried to sit flush
with the ground and to avoid it becoming buried in silt upon pond
inundation. We then secured the housing unit to an existing fence post
(typically a metal T post) or to a concrete tie using steel cable looped
through the housing. We used a mallet to drive concrete ties completely
into the ground for protection of livestock. Finally, we covered units
with loosely stacked rocks to minimize livestock tripping risk and to
help camouflage units to avoid tampering (Figure S2). Loggers recorded
temperature at 15-minute intervals with Bluetooth set to manual (i.e.,
not continuously seeking a signal), resulting in an estimated 3.2-year
battery life for each logger. We visited ponds three times after sensor
deployment: 31 Jul – 2 Aug 2018, 31 Mar - 3 April 2019, and 21 – 27
June 2019 (time of data retrieval). During each site visit, we evaluated
logger function, cleared any mud or sediment in the rugged housing
units, and replaced disturbed rock piles (Figure S3).