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The value of the North American lawn |
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Page 6 of 11
Lawns enhance ground water recharge
Ground water is the principal resource of fresh water for many homes and municipalities and represents much of the potential future water supply. It is also the source of much of the water used for irrigation and is a major contributor to flow in many streams and rivers. Underground aquifers (or ground water) supply drinking water to about 50% of the US population. In fact, drinking water is a scarce resource. Shortages of water plaque hundreds of cities worldwide especially during the summer months, resulting in bans on the use of water for irrigation of lawns and landscapes. Sub-optimal ground water recharge can cause drying of underground aquifers and reduce summer base flows, leaving streams shallower and warmer thus threatening the aquatic life. Therefore, capturing of storm water run-off and ground water recharge should be a goal of new developments, existing developments, and production agriculture. In this regard, turfgrass offers the most cost-effective means of storm water capture and ground water recharge. Studies in Maryland have shown that surface-water runoff losses from a cultivated tobacco site averaged 6.7 mm per ha per 4 weeks during the growing season (May - September); whereas, the surface-water runoff loss from perennial turfgrass averaged only 0.6 mm per ha per 4 weeks (Gross et al., 1990; 1991).
The exceptional water absorption capacity of turfgrass is mainly due to its ability to trap and hold runoff, enabling more water infiltrating through the soil-turfgrass ecosystem. A mowed turfgrass lawn possesses a leaf and stem biomass ranging from 1,000 to 30,000 kg per ha, depending on the grass species, season, and cultural regime (Lush, 1990). This biomass is composed of a matrix of fine-textured stems and narrow leaves with numerous, random open spaces. This canopy matrix is highly porous, allowing water infiltration, and a resistance to lateral surface water flow. The water absorption capacity of turfgrass lawns is further enhanced by the activity of earthworms supported by this organic matter rich ecosystem. Populations in the range of 200-300 earthworms per m2 are common in lawns (Potter, 1998). Earthworm activity increases the amount of macropore space within the soil that results in high soil water infiltration rates and water-retention capacity.
However, in new developments, topsoil is often removed and soil beneath is compacted, significantly reducing water infiltration. Impervious surfaces like roads, pavements, and roofs detain no water at all. This leads to extremely fast run-off during storms, which erodes surface soil and stream banks and carries urban pollutants into streams. Therefore, in new developments, the retention and reuse of native topsoil, reduction of the construction footprint, minimizing of soil compaction, and restoration of soil infiltration capacity should be the goal. One way to restore water infiltration capacity of the native soil in urbanized areas is to restore soils by incorporating compost and other organic matter. About 2-4 inches of compost tilled into the upper 8-12 inches of soil, depending upon soil types can provide effective restoration of many soil functions including the water infiltration. One study in Washington demonstrated up to 50% reduction in winter storm runoff from plots of glacial till soil amended with compost, as compared to non-amended till soil. Tilling in compost when re-landscaping and top-dressing turfgrass with compost can retrofit existing landscapes, restoring their water infiltration capacity.
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