Ecosystem Viltality Drainage Basin Condition (DBC) Flow Connectivity (FC)

Longitudinal or flow connectivity is important for the movement of aquatic life such as fish and the flow of organic matter, nutrients and sediment. Flow connectivity can be fragmented by natural obstructions such as waterfalls, and engineered structures such as dams and weirs. Decreased flow connectivity can negatively impact fish migration and reproduction and may prevent sediment and other nutrients from being delivered downstream.

Flow Connectivity is calculated using the combined Dendritic Connectivity Index (cDCI) which measures the longitudinal connectivity of the river network for potamodromous (fish that migrate within freshwater only), or diadromous (fish that spend portions of their life cycles in both fresh and marine waters), or both species.

Combined Dendritic Connectivity Index (cDCI)

Data Preparation:

1. Identify and geo-locate barriers fragmenting the river network:

The locations of barriers within a basin can be compiled or obtained from local agencies, dam operators and/or global databases (if local information is unavailable). Figure below shows SENTINEL-1 SAR data, which can be used to manually identify “obstructions” that show up as bright patches across water relative to the river network (dark).

Figure. SENTINEL 1 SAR image of dam in the Dongjiang.

1. Assign “passability” value for each structure:

This will be based on local input. Cote et al. (2009) assign barriers an associated passability value, p, which ranges from 0 to 1. This value depends on the physical (e.g., dam height), chemical and/or the hydrologic (flow rates, which vary temporally) attributes of the barrier as well as the biology of the organism in question (which can vary by species, age, etc.).

Note: In the absence of data, following Clarkin et al. (2005), each barrier is assigned a binary passability value. That is, either a barrier meets the designated fish passability criteria (p=1) or not (p=0). One can start with p=0 for all structures and allow the user to change this to p=1. At later stages, functionality can be added for intermediate values if found useful.

Calculation in FHI Toolbox:

1. Impact on potamodromous fish species:

For (n-1) structures with p=0, dividing the river into n fragments, DCIp is calculated as:

$$ DCIp = \sum_{i = 1}^{n}\frac{l_{i}^{2}}{L^{2}} $$

where, L is the total length of the river, and is the length of ith fragment

1. Impact on diadromous fish species:

For (n-1) structures with p=0, dividing the river into n fragments, DCId is calculated as:

$$ DCId = \frac{l_{i}}{L} $$

where, L is the total length of the river, and is the length of fragment closest to the mouth of the river system.

1. Combine:

Finally, cDCI can be calculated as:

$$ cDCI = \left( \frac{w_{p}DCIp + w_{d}\text{DCId}}{w_{p} + w_{d}} \right)*100 $$

where, weights wp and wd depend on the nature of the fish species in the freshwater system.

Notes:

1. Suggested weightings for (wp, wd) are (1,0) for potamodromous dominant systems; (0,1) for diadromous dominant systems.

2. For large sub-basins, like the transboundary 3S system (the Sesan, Srepok, and Sekong rivers spanning Cambodia, Lao PDR and Vietnam), which contain potamodromous species, the fish species travelling up from the Mekong main stem will be affected by obstacles to connectivity in same fashion as diadromous species. Hence, calculation of DCId will be appropriate.

Figure. Calculation of DCIp and DCId for a hypothetical river network with one barrier having p in both directions as 0.5. (Source: Cote et al. 2009)