Groundwater Connection with Landscape
The depth of the water table is partly responsible for different plant species occupying different positions along the slopes from hill to valley, as only the drought tolerant plants can live on the hills in arid regions and water tolerant plants live near streams (Figure 14). In lowland discharge areas of arid climates, water is lost to the atmosphere by evapotranspiration causing salt to accumulate. Such settings develop salt-tolerant vegetation.
a)
b)
The water table occurs everywhere beneath us and when humans change the landscape shape, the vegetation, or purposefully drain groundwater for farming and construction, we change the depth to the water table. This often has problematic consequences for humans and ecological systems. However, we are able to predict consequences before we make changes so as to make informed decisions about our actions. Depending on the predicted consequences, we may decide not to proceed or we may redesign the changes to reduce the adverse effects.
The depth of the water table can have a strong impact on how the land responds to heat. For example, in hot dry areas of Australia where eucalyptus trees are the native vegetation, the natural position of the water table is deeper below land surface than after the eucalyptus trees are cleared for crops. This occurs because the trees consume soil moisture capturing infiltrating water, resulting in minimal groundwater recharge. After the trees are cleared, the crops consume less groundwater so recharge increases and the water table rises. When the water table is near the ground surface, water evaporates leaving dissolved substances behind to form salt precipitates that accumulate in the soil rendering the land unfit for crops. Soil salinization is a cause of cropland loss each year around the globe. In many agricultural regions, management of land use to avoid salinization is key to agricultural productivity.
Another example of water table depth influencing the landscape is the occurrence of wildfires as described by Elbein (2019) in his Pulitzer Prize winning National Geographic article “Tree Planting Programs Can Do More Harm Than Good.” He explains that a shallow water table in wetlands can make the difference between normal wildfires and infernos that cause massive destruction. This was the case in the Fort McMurray wildfire in Alberta, Canada, in 2016, which was the costliest wildfire in Canada’s history. Mossy bogs, a type of wetland known as a peatland, cover an immense part of northern Canada and Russia. Peat is composed of partially decayed organic material such as moss. Peatlands contain large amounts of carbon that is gradually sequestered from the atmosphere over thousands of years. As peat forms, it supports fewer of the typical trees (black spruce in the Alberta case). With fewer trees, more recharge reaches the water table so the water table moves closer to the surface. The shallow water table makes the peatland resilient after wildfires. Peatlands commonly experience low‑intensity fires and are able to recover the carbon lost during the fire in a relatively short period of time because the shallow water table prevents the fire from burning deeply into the peat. When peatlands were drained for the purpose of creating a spruce forest in the Fort McMurray area, an environmental adjustment occurred: the black spruce trees used more water, the water table declined, the shallower peat was replaced by a drier moss species (kindling instead of fire retardant), the large trees became a huge store of fuel, and then the Fort McMurray wildfire ensued. Intensely burned peatlands require long periods of time to recover the carbon released to the atmosphere by the fire (Figure 15). The media attributed this fire to the extremes of nature related to climate change without recognition that human intervention in the shallow groundwater flow system played a key role in the event.
