Surface water abstraction
Surface water abstraction
01. Water abstractions
Water abstractions may be taken directly from the flowing waters in the channel (surface water abstraction), or indirectly from wells by pumping water from aquifers that may be closely connected to rivers (groundwater abstraction). Furthermore, water abstraction from rivers can be achieved through inter-basin flow transfer schemes, whereby the donor river system has its flow reduced below its diversion.
Effect/Impact on (including literature citations)
Dewson et al. (2007) found that water abstraction decreased water velocity, water depth, and wetted channel width and changes in thermal regime and water chemistry in 90 % of the case studies analysed; James et al. (2008) found that flow reduction significantly decreased water velocity (60–69%) in all streams, while depth (18–61%) and wetted width (24–31%) also tended to decrease. Kleynhans (1996) described loss of fast flowing instream habitat types in streams affected by water abstraction. Sedimentation process may increase and fine sediment deposition increases the most in farmland streams affected by water abstraction (James et al. 2008). Also, with decreased flows the Coarse Particulate Organic Matter (CPOM) retention rate is increased (Dewson et al, 2007). If floods are reduced in main stem river channels, fine sediments delivered by less abstracted tributaries may no longer be flushed downstream but may accumulate on the river bed, reducing its permeability (Kondolf and Wilcock 1996). When water abstraction is intense, channel drought impacts may be disproportionately severe, especially when certain critical thresholds are exceeded. For example, ecological changes may be gradual while a riffle dries but cessation of flow causes abrupt loss of a specific habitat, alteration of physico-chemical conditions in pools downstream, and fragmentation of the river ecosystem (Boulton, 2003).
Changes in thermal regime and water chemistry were found in rivers affected by flow withdrawals by Dewson et al., (2007), and James et al. (2008) found that flow reduction decreased the water temperature range by 18–26%, although it had little effect on average surface water temperatures.
Reduced flows within some river reaches may present impassable obstacles for fish migrations, either by decreasing water depths to below critical levels or by completely drying up entire reaches of river, as occurs on the San Joaquin River of California as a consequence of diversions from Friant Dam (Cain 1997). Also, where baseflows are artificially reduced, dissolved oxygen levels fall to lethal levels in reaches affected by eutrophic or high temperature discharges (e.g., Loire River, France), or dredging (e.g., the Lower San Joaquin River, California), preventing anadromous salmonids from migrating upstream to suitable habitats (Kondolf et al., 2006).
Water extraction can also entrain aquatic organisms. For example, Pringle and Scatena (1999) showed that water extraction removes more than 50% of migrating shrimp larvae in a river located in the Caribbean National Forest in Puerto Rico.
In relation to macroinvertebrates, flow reduction has not been observed to impact on the abundance of common pool macroinvertebrates or on the abundance, vertical distribution or community composition of hyporheic macroinvertebrates. James et al. (2008) found that aquatic macroinvertebrates are resistant to short-term, severe flow reduction as long as some water remains. However, in general, invertebrate abundance may increase or decrease in response to decreased flow, whereas invertebrate richness commonly decreases because habitat diversity decreases (Dewson et al., 2007). Furthermore, Muñoz & Prat (1996) found a highly significant reduction of macroinvertebrate density and taxon number at disturbed stations as consequences of increased pollutant concentrations under abstraction conditions.
In dry countries, deterioration of riparian habitat integrity is a widespread consequence of water abstraction: during droughts tree deaths are common (Kleynhans, 1996).
Case studies where this pressure is present
Possible restoration, rehabilitation and mitigation measures
- Improve water retention
- Recycle used water
- Reduce surface water abstraction without return
- Improve/Create water storage
- Reduce water consumption
- Create low flow channels in over-sized channels
- Reduce surface water abstraction with return
Other relevant information
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