Improve water retention

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Improve water retention

Improve water retention01. Water flow quantity improvement

General description

Water retention and runoff can be altered by different factors such as changes in land cover, soil structure and compacting, stormwater management, loss of floodplains and wetlands. Loss of water retention combined with accelerated runoff typically increases the frequency and magnitude of flood peaks and reduces the availability of water to streams during low flow (base flow) periods (Stream Habitat Restoration Guidelines, WDFW). The alternatives for improve water retention may be applied in combination with other restoration measures (e.g. restoring side channels or former meander is firstly done for restore channel morphology and lateral connectivity but also increases infiltration area) at different spatial scale and location (riparian zone, nearby land, etc). Some techniques for improve water retention are:

  • Changes in land use and cover
  • Improve stormwater management
  • Reduce and limit the amount of impervious surfaces in the watershed
• Change land use practices and zoning regulations to limit the allowable percent of impervious surface in the watershed
• Decommission roads
• Use pervious pavement alternatives where feasible


Changes in land use and cover

Forest and vegetation cover have long been recognized as major factors influencing runoff, infiltration and evapotranspiration from shallow water tables. Watershed treatment involving the establishment of tree, bush and other plant cover is widely used as a way of reducing runoff and increasing infiltration. This is frequently assumed to increase recharge and is advocated as a core part of packages to address groundwater overdraft. However, the effect of surface vegetation on groundwater levels is not automatic. It depends on the balance between improvements in infiltration caused by increased vegetation and relative changes induced in evapotranspiration. In some cases, removal of forest cover has caused water levels to rise significantly with major environmental consequences, e.g. in much of New South Wales, Australia. (Groundwater Management: The Search for Practical Approaches. FAO, 2003)


Effects of forest manipulation on water yield
• Carefully executed light, selective harvesting will have little if any effect on streamflow, which increases with the amount of timber removed.
• The data set for the humid tropics supports the general finding of Bosch and Hewlett (1982) that removal of natural forest cover may result in a considerable initial increase in water yield (up to 800 mm per year); possibly more in highrainfall regions, depending mainly on the amount of rain received after treatment.
• Depending on rainfall patterns, there is a rather irregular decline in streamflow gain, with time, associated with the establishment of the new cover. No data have been published regarding the number of years needed for a return to pre-cut streamflow totals in the case of natural regrowth, but it may take more than eight years.
• Water yield after maturation of the new vegetation may: remain above original streamflow totals in the case of conversion to annual cropping, grassland or tea plantations; return to original levels (Pinus plantation after full canopy closure); or remain below previous values (reforestation of grassland with Pinus or Eucalyptus). Coppicing of Eucalyptus after ten years caused even stronger reductions for two years.

Source: Extracted from Bruijnzeel, 1990.

Applicability

Expected effect of measure on (including literature citations):

  • HYMO (general and specified per HYMO element)

Replacement of conventional pipe stormwater systems by bio-filtration stormwater management leads to a decrease on urban runoff, lower peak discharges, delay on stormwater discharge, and shorter duration (Lloyd et al., 2002).

  • physico � chemical parameters

Concentrations of Total Suspended Solids (TSS), Total Phosphorus (TP) and Total Nitrogen (TN) discharged from the bio-filtration system are typically lower than pollutant concentrations discharged from the piped system (Lloyd et al., 2002).

  • Biota (general and specified per Biological quality elements)
BQE Macroinvertebrates Fish Macrophytes Phytoplankton
Effect + + + o

Temporal and spatial response

Pressures that can be addressed by this measure

Cost-efficiency

Case studies where this measure has been applied

Useful references

Lloyd S.D., T.H.F. Wong and C. J. Chesterfield (2002) Water sensitive urban design- a stormwater management perspective. [1]

Other relevant information

Improving Stormwater Detention Basins for Better Stormwater Management (PEC's Stormwater Management Facility Retrofit Program. This fact sheet highlights design concepts for stormwater best management practices (BMPs) in urban areas.[2]

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