Wetlands: Vital Role in Water Resource Management

Posted on Jan 5, 2025 in Geology

The Role of Wetlands as a Water Resource

A wetland is an area of land with soil that is permanently flooded. It includes freshwater marshes (e.g. Macquarie Marshes in Australia), bogs (e.g. Strangmoor Bog in the USA), and swamps (e.g. The Everglades in Florida, USA). Wetlands are a very important water resource as they provide many ecosystem services. There are marine, coastal, inland, and artificial types, subdivided into 30 categories of natural wetlands and 9 human-made ones such as reservoirs, barrages, and gravel pits. A biome is a specific habitat.

Why Wetlands are Important Water Resources

Water Quality: Wetlands can purify the impurities in water and provide clean drinking water for humans and animals.
Flood Control: Wetlands are a natural defense against floods since they are able to absorb water and release stored water slowly.
Groundwater Recharge: Wetlands can allow groundwater storage, such as aquifers, to recharge since wetlands are able to store large quantities of water.
Fertile Land: High rates of silt deposition result in an increase in soil fertility, this supports the agriculture and timber companies.
Biodiversity: Wetlands provide habitats for many different species.
Erosion Control: Wetlands at the margins of lakes, rivers, bays, and the ocean protect shorelines and stream banks against erosion as they dissipate stream energy.
Carbon Storage: Certain types of wetlands (peatlands) can act as carbon sinks, and thus reduce the amount of CO2 in the atmosphere.
Tourism/Recreation: Wetlands provide opportunities for recreational activities, such as hiking, fishing, and boating. The unique wildlife of wetlands can attract many tourists. For example, alligator airboat tours in the Everglades National Park attract 1.1 million tourists each year.

Role of Wetlands as a Water Resource

Nutrient removal: Phosphorus and Nitrogen.
Removal of biological oxygen demand from surface water to prevent stagnation.
Removal of suspended solids and associated pollutants from surface water.
Removal of metals.
Removal of pathogens.

Loss and Degradation

Increased demand for agricultural land, population growth, infrastructure development, river flow regulation, invasion of non-native species, and pollution.

Case Study: The Kissimmee River

Between 1962 and 1971, the 165km Kissimmee River once meandered through central Florida. Its floodplain, reaching up to two miles wide, was inundated for long periods by heavy seasonal rains. The frequent flooding has caused severe impacts to people. The USA decided to deepen, straighten, and widen the river, which was transformed into a 90km, 10m deep drainage canal. The river was channelized in 1948, to provide an outlet canal for draining floodwaters from the upper Kissimmee lakes basin, and to provide protection, and to construct the Central and South Florida Project.

Impact of Channelization

The channelization of the Kissimmee River had several unintended impacts. Concerns about the sustainability of existing ecosystems led to a massive restoration project.

Loss of 12,000-14,000 ha of wetlands.
Pre-canal average flow was 0.42m/sec, now is 0.05m/sec.
When there were floods, water only stayed in the drainage basin for only one day, before it was 11 days.
Loss of wetland habitats, now 92% fewer wading birds and waterfowl in winter.
It gives a flat, low-lying drainage basin.
Before the canal, the Kissimmee spilled over the floodplain.
The canal reduced the length of the river from 160km to 90km.
Now a series of lakes controlled by locks.
The canal is designed to hold all floodwater.
Two-thirds of floodplain wetlands were drained by the building of the canal.
Nutrients are no longer absorbed by the wetlands.
Loss of animal and fish species as the river became more stagnant, loss of biodiversity.
Less recharge of aquifers.
Groundwater becoming more saline.

The Kissimmee River Restoration Project

Aim: Restore over 100km² of the river and associated floodplain wetland by 2015. Started in 1999 and will benefit over 320 species, including the bald eagle, wood stork, and snail kite. Create over 11,000ha of wetlands.

Restoration Requires Dechannelization

Need to refill approximately half of the flood control channel and re-establish the flow of water through the natural river channel. In residential areas, the flood control channel will remain in place.

The Costs of this Restoration Project

It is estimated that this project will cost $414 million (the initial channelization cost $20 million). The bill is being shared by Florida and the federal government. Restoration of the river will result in higher losses of water due to evapotranspiration during wet periods. In extremely dry spells, navigation may be impeded, but it is expected that navigable depths will be maintained at least 90% of the time.

The Benefits of Restoration

Higher water levels should ultimately support a natural river ecosystem again, creating habitats for species.
Reestablishment of floodplain wetlands is expected to result in decreased nutrient loads to Lake Okeechobee.
Populations of key avian species such as wading birds and waterfowl have returned to the restored area, and numbers have increased.
Dissolved oxygen levels have doubled, which is critical for the survival of aquatic species.
Increased recreational usage on the restored river could significantly enhance local and regional economies.

Irrigation is the artificial addition of water to soil in areas where there is insufficient for adequate crop growth. Water can be taken from surface stores such as lakes, dams, reservoirs, and rivers or from groundwater. Both rich and poor countries, e.g. large parts of the USA and Australia are irrigated. There is evidence of irrigation in Egypt going back nearly 600 years. In Texas, USA, irrigation has reduced the water table by as much as 50m. It causes a change in precipitation, e.g. Texas increases rainfall, hailstorms, and tornadoes when irrigated these areas have moist soils and complete vegetation cover. Irrigation can reduce the earth’s albedo (reflectivity).

Eutrophication is the nutrient enrichment of streams, ponds, and groundwater. It is caused by agro-chemical runoff during irrigation, which is the artificial addition of water to the soil. Fertilizers in the soil (improve crops yield) are leached into surrounding waterways. The fertilizers added to the soil contain large quantities of nitrates and the increased amount of nutrients result in algal blooms, which are the uncontrolled growth of algae as plants respond to greater nutrient availability.

Impact of Eutrophication on Water Quality

Algal blooms shade the water and species below, causing a decreased rate of photosynthesis, which causes submerged plants to die. As they die and decompose, respiration of decomposers increases causing biochemical oxygen demand (BOD) to increase resulting in oxygen starvation. Anoxia can kill many fish and plants within the aquatic ecosystem. For example, in Kunming city in China, algae overgrowth has killed over 90% of native water weeds and mollusks. Since there are high amounts of nitrate in the water after eutrophication, the eutrophic water can cause some serious health issues. For example, “blue baby syndrome” is much higher for pregnant women drinking water containing more than 20mg/l of nitrate.

Management

Eutrophication can be managed to cause less damage. There are three main approaches. Managing the human activities that produce pollutants (e.g. avoid using nitrogen fertilizers in the winter months whilst soil is wet). Regulating and reducing pollutants at the point of emission (e.g. opening sewage plants that remove nitrates and phosphates from waste). Cleaning up and restoration of the polluted water (e.g. pumping mud from eutrophic lakes). However, prevention is better than cure in managing eutrophication as clean-up and restoration is usually more expensive and less effective as eutrophication becomes more of an issue over time due to positive feedback mechanisms. Also, controlling eutrophication can be challenging since the sources of emission are widely dispersed (non-point source pollution). For example, Lake Biwa in Japan in the 1970s, where management of eutrophication was successful. The widespread use of agricultural fertilizer resulted in algae blooms and lowered dissolved oxygen levels. However, an agreement of not using chemicals within 6km of the lake was established. As a result, nutrient levels were reduced by 20% in 1986.

Case Study: Eutrophication in Kunming City in China

In Dianchi Lake near Kunming City in Yunnan Province, algae overgrowth has killed over 90% of native water weeds and mollusks. Alternative water supplies have run short water from Dianchi Lake has been used since 1992 to supply Kunming’s growing population of 1.2 million residents. The city opened its first sewage treatment plant in 1993, but this copes with only 10% of the city’s sewage.

Salinization is the increased concentration of salts in water and affects the quality of water used for irrigation and domestic use. The main cause of salinization is the sustainable extraction of water. Irrigation often leads to an increase in salinization. This is because of increased rates of evapotranspiration from surface stores of water (e.g. reservoirs) and irrigation channels. As water evaporates, salts are left behind, increasing the ratio of chlorides to bicarbonates. An increased concentration of chlorides in the water has a negative impact on aquatic plant life, as saline water is often toxic to plants. As a result, water quality decreases.

Case Study: The Aral Sea

The Aral Sea in Uzbekistan and Kazakhstan is an example where irrigation has led to salinization. Since the 1960s, the size of the Aral Sea has been shrinking as large quantities of water have been diverted from the two source rivers to irrigate the desert and increase the production of rice and cotton. As the irrigation channels were built quickly and are poorly maintained, 75% of this water is lost through evaporation or leakage. By 2007, the Aral Sea was only 10% of its original size and its salinity has increased to 100g/l, causing damage to the sea’s ecosystem and causing shortages of clean water.

Pollution of Groundwater: In areas where surface water is scarce, water may be pumped from the ground for use in agriculture and irrigation. However, over-abstraction of water from these reserves can result in the depletion and contamination of groundwater resources.