Improve water retention

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Improve water retention

Category 01. Water flow quantity improvement

General description

Water quantity and the flow regime can be altered by different factors such as changes in land cover, soil structure and compacting, loss of floodplains and wetlands, and stormwater runoff from urban areas. Loss of water retention combined with accelerated runoff typically increases the frequency and magnitude of flood peaks but also reduces the availability of water to streams during the prevailing low flow (base flow) periods.

Preferably, a more natural flow regime can be restored by increasing water infiltration and retention at the catchment scale as described here, by e.g. changes in land cover and floodplain restoration, since this addresses both, increased peak flows and reduced base flows. When space is limited, an alternative local end-of-pipe solution to reduce peak flows caused by stormwater runoff are retention and detention basins (see Improve water storage).

The measures to improve water infiltration and retention may be applied in combination with other restoration measures (e.g. water storage, 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 to improve water retention and infiltration are:

  • Changes in land use and cover
  • 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


Applicability

Changes in land use and cover

Watershed treatment involving the establishment of tree, bush and other plant cover is widely used as a way of reducing runoff and increasing infiltration. However, the effect depends on the balance between improvements in infiltration caused by increased vegetation and relative changes induced in evapotranspiration (ET). There general thought that more trees mean more water is a wrong assumption. Cover change from species of lower to higher ET would lead to a decrease on mean surface runoff and annual flow. A change to species with lower ET would increase mean surface runoff; a reduction of forest cover increases water input to watercourses. However the impact depends on the management practices and the alternative land uses. Mean flow after the development of new vegetation could be higher, the same or less than the original value, depending on the type of vegetation (Bruijnzeel, 1990). The conversion from forest to pasture leads to a decrease of ET, an increase on surface runoff, base flow and total stream flow (Fuhrer et al., 2000). The conversion into forest would have the opposite effect, but when we are thinking on improving water retention,we should consider the role of vegetation for improve soil infiltration (especially deep rooted trees) but remember that the roots are more like pumps that sponges, and that it is the soil that stores water, not the trees (Hamilton et al., 2008).


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.

Minimize the extent and degree of soil compaction and improve soil infiltration and retention capacity

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

Stormwater Best Management practices have a good cost-efficiency. Replacement of conventional pipe systems by bio-filtration systems supposes little increase on project implementation and brings environmental, economical and social benefits (water quality improvement, aesthetic and recreational values).

Case studies where this measure has been applied

Useful references

Bescansa P., M.J. Imaz, I. Virto, A. Enrique, W.B. Hoogmoed (2006). Soil water retention as affected by tillage and residue management in semiarid Spain. Soil & Tillage Research 87: 19–27

Bruijnzeel LA (1990). Hydrology of moist tropical forests and effects of conversion: a state of knowledge review. Paris: UNESCO International Hydrological Programe.

Hamilton L.S., Dudley N., Greminger G., Hassan N., Lamb D., Stolton S. and S.Tognetti (2008). Forests and water. FAO.

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

Peterson, J.T. and T.J. Kwak. 1999. Modeling the effects of land use and climate change on riverine smallmouth bass. Ecological Applications 9: 1391-1404.

Saldi-Caromile, K., K. Bates, P. Skidmore, J. Barenti, D. Pineo. 2004. Stream Habitat Restoration Guidelines: Final Draft. Co-published by the Washington Departments of Fish and Wildlife and Ecology and the U.S. Fish and Wildlife Service. Olympia, Washington.

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]

Watershed Forestry Resource Guide: Reducing stormwater runoff [3]

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