Wetlands - Water Storage

Water Storage

This function reflects the capacity of a wetland to collect and retain inflowing surface water, direct precipitation, and discharging groundwater as standing water above the soil surface, pore water in the saturated zone, or soil moisture in the unsaturated zone. A potential independent quantitative measure of this function would be the amount of water stored in the wetland per a given time (e.g., hectare-meters/year).

This function is critical to the maintenance of the wetland and is often considered as the main forcing function for all other wetland processes. Water storage in wetlands is important for three reasons.

• Water that is delayed or stored in the wetland reduces the amount of runoff down slope, thereby ensuring a decrease in flood crests down gradient.
• It guarantees that sufficient moisture is available to allow the development and maintenance of hydric soils and appropriate hydrophytic plant communities The presence of these plant communities ensures that wildlife habitat is available for a variety of species, both resident and migratory
• Water storage supports the biogeochemical processes that occur in wetlands, such as the removal of nutrients and particulates. This process results in improved water quality.

Analysis of Water Storage

Water that is delayed or stored in the wetland reduces the amount of runoff down slope, thereby ensuring a decrease in flood crests down gradient. Wetlands facilitate detention of runoff because many lack well-defined surface water outlets and, between basins, subsurface flows in glacial till are slow. When runoff is detained in a regionally dispersed manner by wetland basins, pulses of water that eventually enter downstream areas in most cases are staggered (desynchronized). This broadens the storm hydrograph and reduces streamflow peaks.

Characteristics and Processes that Influence Water Storage

The characteristics and processes that influence the capacity of a wetland to store water over an extended period are related natural factors, such as climate, geomorphic characteristics, soils, and vegetation.

Human factors play a significant role on many landscapes. Ditching or the placement of tile drainage, and modifications of the surrounding landscape can alter the timing and amount of water reaching the wetland. Changes in peak flows attributable to a wetland varies according to the interaction among outlet capacity, storage available within the site, and the amount of water coming into a wetland.

The characteristics associated with the performance of this function focus on land use, as it influences the volume and timing of water entering the wetland, the volume of the wetland available for storage, the condition of the soils and plants (evapotranspiration, seepage, and soil storage), and activities that reduce retention time (e.g., artificial drainage). Activities above or within the wetland affect the rate and quantity of surface and subsurface water entering and leaving the wetland. Land use activities also affect erosion up slope and sediment import into the wetlands. An increased sediment load will decrease the wetland’s capacity to store water, sometimes nearly eliminating storage capacity. Finally, the elevation and capacity of any constructed outlet below the storage boundary directly affects the height of the water level and, therefore, the ability of the depression to capture and retain water.

Although accumulation and retention of sediments and particulates are recognized functions of wetlands resulting in improved water quality, it has a negative effect on wetland hydrology. Many wetlands are closed basins; thus, sediment inputs are derived primarily from wind and water erosion of upland soils within the catchment. Upland land use affects the movement of water, sediment, and pollutants into the wetland. Generally, the higher the percentage of catchment under perennial cover, the better the condition of the wetland. Properly managed perennial cover helps to slow the movement of water down slope, which aids in the filtering of sediments and entrapment of pollutants.

The chief negative impact to wetlands of accelerated sedimentation is loss of volume from filling. Precipitation that was once lost through evapotranspiration or infiltration to groundwater before entering wetlands with grassland catchments enters via spates of surface runoff from tilled catchments. The accelerated runoff often brings erosional sediments from the surrounding landscape, contributing to filling the basin with soil. In addition to the alteration of hydrologic inputs, the loss of basin volume from siltation reduces the water storage capacity and flood attenuation benefits of wetlands.

The variables having the greatest impact on the ability of a wetland to perform this function are anthropogenic drainage features (surface outlets or tile drains). Alterations that remove water from the wetland year round have a major effect on hydrology. Simply stated, if the wetland has been so hydrologically modified that it is completely drained, then the wetland no longer has the capacity to retain stormwater runoff.

Source: A Regional Guidebook for Applying the Hydrogeomorphic Approach to Assessing Wetland Functions of Prairie Potholes (HGM), Army Corps of Engineers