From the perspective of Natural Water Retention Measures (NWRM), maintenance is the set of actions or processes that are performed to keep an already existing natural process functioning in the best possible manner. Maintenance can include physical activities, the planning process and communication.

Excavations, typically filled with gravel, designed to store water and allow it to infiltrate into underlying soils or aquifers.ﾠ Soakaways would typically receive point-source inflow (e.g. from roof drainage).ﾠﾠ
- Based on Stella definitions, adapted by NWRM project experts and validated by the European Commission

1) Terrestrial areas where the vegetation complex and microclimate are products of the combined pressure and influence of perennial and/or intermittent water... and soils that exhibit some wetness characteristics. 2) Zone situated on the bank of a water course such as a river or stream. (Source: DUNSTE / GILP96)

"Supplementary" measures are those measures designed and implemented in addition to the basic measures, with the aim of achieving the objectives established pursuant to Article 4 of the WFD. Part B of Annex VI contains a non-exclusive list of such measures. Member States may also adopt further supplementary measures in order to provide for additional protection or improvement of the waters covered by this Directive, including in implementation of the relevant international agreements referred to in Article 1.

The channel containing or formerly containing the water of a river. (Source: BJGEO)

Ditch blocking in managed peatland forests can be used to slow water and trap sediment after forest harvesting. The ditches can be made of wood logs or gabions, for example.
- Elaborated by NWRM project experts and validated by the European Commission

The sloping side of any hollow in the ground, especially when bordering a river. (Source: CED)

Targeted planting of forests in headwater areas (e.g. with a slope) can help to stabilize hillslopes, thereby reducing erosion and potentially leading to greater water retention in montane areas. Afforestation may have beneficial effects on the hydrograph by reducing peak flows and helping to maintain base flows. The potential for water retention must be balanced against the increased ET and pollutant trapping that may be associated with forests.
- Based on Stella definitions, adapted by NWRM project experts and validated by the European Commission

An enclosed body of water, usually but not necessarily fresh water, from which the sea is excluded. (Source: WHIT)

Sacrifice associated to the use of available resources to one means instead of another (so that any economic cost is indeed an opportunity cost) or of following one course of action instead of the best available alternative. As applied to NWRM, it refers to those negative impacts in terms of welfare, either direct or indirect, that may be linked to the implementation of any measure.
The difference between explicit and implicit costs depends on whether there is an unequivocal monetary payment (or at least one which is straightforward to infer) or not. The term ﾓdirect (economic) costﾔ does actually refer to those costs that fall directly on the promoter of the NWRM. Hence, as opposed to direct costs, indirect costs are those incurred by others (those who are not under the direct scope of the NWRM implementation).