Water Flux and Circulation

Flux into/out of He‘eia Fishpond

To quantify the physical movement of water flowing through the mākāhā, both into and out of the fishpond, a suite of current meters and current profilers were deployed at each of the mākāhā and at the break in the seawall (OB).   A Nortek Aquadopp current meter was used to measure single point current direction and magnitude through OB. Sontek Argonaut-SW current profilers, deployed on the seafloor, were used to integrate water column current direction and magnitude at OM1, OM2, RM2, RM3 and TM.  The current meters were deployed over a full tidal cycle in order to capture current direction and flow for both flood and ebb tides.  Co-located pressure sensors (Onset HOBO U20 water level loggers) made simultaneous measurements of overlying water height (HOBO accuracy = ± 1.5 cm with a resolution of 0.41 cm, http://www.onsetcomp.com/products/data-loggers/u20-001-01-ti).  Rating curves were constructed for each mākāhā and the OB by plotting flow velocity versus water height, using the data from flow meters and pressure sensors (Fig. 3, 4) (Timmerman et al. 2014; Young 2011).  With the constructed rating curves, we now are able to determine water flux through each mākāhā with pressure sensors alone.  Ongoingly, we have a single HOBO pressure sensor deployed at each mākāhā (Fig. 3) that provide continuous pressure data which can be entered into rating curve equations (Timmerman et al. 2014) to determine water flux at any point in time.

 Fig. 4. Example of rating curve from OM1 makaha

Fig. 4. Example of rating curve from OM1 makaha

Water circulation within Heeia Fishpond:  To quantify the physical movement (magnitude, direction) of water within the fishpond, a Nortek Aquadopp current profiler was deployed at various points (can Sherrill provide any more detail on this?) within, collecting data at each point in the fishpond for X (time).  Current magnitude and direction data from this experiment are being used to generate a water circulation model (this is Sherril’s work; we should discuss with her including it, and decide what kind of attribution to give…Margaret what do you think).

 

Temperature

One of the most diagnostic signals of source waters to and climate impact on the fishpond is temperature.  In order to reconstruct the distribution of source waters over time, and also to monitor the impact of climate phenomena on He‘eia Fishpond, a series of in situ temperature sensors is maintained within the fishpond as part of HCOOS (TidBit v2 temperature loggers: Onset Corporation).   Accuracy of TidBit sensors is ±0.2°C and a resolution is 0.02°C (TidBit sensor website).

TidBits are deployed 20 cm above the seafloor with a sampling frequency of 15 min. The deployment depth allows all sensors to remain submerged over all tidal fluctuations, and depth is sufficient to be unaffected by daily surface water heating due to solar radiation.  Additional temperature data are collected by the Onset HOBO U20 water level loggers deployed at the mākāhā (HOBO accuracy = ± 0.2°C with a resolution of 0.02˚C, http://www.onsetcomp.com/products/data-loggers/u20-001-01-ti).  During periodic current meter deployments, temperature data is also collected (Sontek accuracy = ± 0.5°C with a resolution of 0.01˚C; Nortek accuracy = ± 0.1°C with resolution of 0.01°C].