Kellner, E. and J.A. Hubbart. 2017. Land use impacts on floodplain water table response to precipitation events. Ecohydrology. DOI: 10.1002/eco.1913
It is flood season along the Mississippi River (seen in the cover photo) and as water is piling up in backwater storage areas across the lower Mississippi Valley it is worth taking a look at how the ground absorbs water. This involves taking a closer look at a little known field called soil physics. When I say those two words most people immediately stop listening and try to turn the conversation to literally anything else, but I am here to convince you that if you care about flooding you should care about soil physics. And maybe, just maybe, respect the soil you live upon a little bit more. We will explore the basics of soil physics and how different land cover types impact flood reduction.
Floodplains are located adjacent to rivers and can range in size depending on the climate region and underlying geology of the associated river basin. Floodplains play an important role in the water cycle because they lie between surface waters and groundwater (water in soils). When rivers overflow their banks, floodplains act as buffers, storing that extra water and reducing the potential for flooding at downstream locations. This service is referred to as flood attenuation. The amount of water that a floodplain can store depends on a number of factors, including soil characteristics and the amount of water already present in the soil prior to the flooding event. Soil hydraulic properties, largely determined by specific soil characteristics are an integral part of understanding how much water a soil can store as we shall see.
The details underneath
Soils perform amazing things for us humans. The list is really endless, from supporting the growth of all terrestrial plants, to purifying water, and providing us with important antibiotics such as the well-known penicillin (derived from a soil fungus). Soils are a conglomerate of materials including mineral grains, organic material, gases, water, and other soluble compounds such as salts (ions of potassium, chloride, sodium etc.). Soil characteristics are determined by the relative abundance of various soil components and soil structure. Mineral grains range in size from gravel, which is defined as falling between 2-64 mm, to small clay particles that can be in the sub micrometer range (Fig. 2). In between these two extremes you have sands and silts. Soil structure refers to the tendency of soils to aggregate in various ways depending on specific properties (Fig. 3). For instance you have the loner single grains of dry beach sands, the small groups of aggregated soils seen in many forests, and the massive soils of arid places that form large cohesive blocks.
The makeup of soil determines its characteristics, including its porosity, or how much area in a specified volume of soil is occupied by voids in the soil; bulk density or the ratio of mass to volume of a soil; and hydraulic conductivity or how easily water travels through the soil medium. Properties such as these control water movements and storage within a soil. Given the massive amount of forested land that has been converted to agriculture, particularly in fertile floodplains, it is worth exploring what happens to the flood attenuation capacity of a floodplain that is converted from forested to agricultural land. Should we expect a change in the ability of the floodplain to absorb and store water if it is no longer forested? How might soil characteristics change after agricultural conversion?
Altering land use causes changes in soil hydrology
Kellner and Hubbart (2016) investigated the differences in soil water storage after rain and high river flow events between a forested and agricultural site along a creek in central Missouri. The team installed a grid of 9 piezometers along a creek in both a forested and agricultural land cover type. A piezometer is an instrument used to measure pressure and in this case was used to measure the pressure of the groundwater at the two sites. By analyzing if the groundwater responded to an event and how rapid this response was at each site, the study was able to identify differences in flood attenuation capacity between land cover types. The study found that the water table at the agricultural site responded to a greater number of precipitation events including small rain events relative to the forested site (Fig. 4). In addition, the time it took for the water table at the forested site to respond to a precipitation or high flow event was much longer relative to the agricultural site. Futhermore, the ratio of water table response magnitude (or the change in height of the water table after an event) to precipitation amount was 150% greater at the agricultural site relative to the forested site. These results imply a relatively large difference in the flood attenuation capacity of agricultural versus forested sites, but the question remains, Why?
When land is converted from forest to agriculture the most obvious change is the removal of trees. Through the process of transpiration, trees transport a massive amount of water from the soil to the atmosphere. Transpiration exports a larger volume of water from an area than runoff to streams or deeper infiltration to underlying aquifers. In forested landscapes, due to the high water demand of trees relative to grasses and shrubs, a larger portion of the water is exported out of the site relative to that of say a corn field. During the growing season when plants are actively transpiring, this translates into a deeper water table underneath a forested site as compared to an agricultural site and thus more space for water storage during a flood or rain event.
But this is only part of the story. Agricultural conversion of forested land completely alters soil components and thus soil properties. Soil structure is permanently changed upon conversion. Exposure to heavy machinery and loss of organic material result in a loss of soil structure. Soils become denser and more compact. This reduction in porosity means there is less space where water can be stored during or after an event. A reduction in the amount of organic material from frequent tillage reduces soil water storage. Organic matter has interesting properties that actually allow it to absorb more water than would be predicted by the porosity alone. Picture a sponge swelling as it absorbs an increasing amount of water.
The effects of converting floodplains to farmlands, specifically forested floodplains, can significantly reduce the ability of an area to attenuate floods and in fact can actually increase the severity of flood events by increasing the amount of water that runs off into the stream relative to what falls as rain.
What do we do?
Unfortunately a huge area of land was converted from forest to agriculture in the United States throughout the 19th and 20th centuries. The Mississippi River floodplain and associated tributaries particularly took a big hit. Of the 10 million hectares of bottomland hardwood forest (a type of floodplain forest) that originally occupied the Mississippi Alluvial Valley, only about 20% remains. Fortunately in the past decade, there has been a large effort to reclaim agricultural land and restore forests not only for their important function in reducing floods, but also to provide wildlife habitat and reduce fertilizer loads to river. However, we all need food so eliminating farms is not the only feasible answer. In recent years, a regulatory framework has incentivized constructing vegetated buffers along streams and rivers in densely farmed areas. This compromise will hopefully become more common as we face larger flood threats. We must continue to invest in these types of programs in order to combat future increases in precipitation intensity predicted with climate change. This effort begins by increasing everyone’s understanding of how flood attenuation works so we can better combat this problem together.
cover image: Modis imagery showing river flooding along the Mississippi River and tributaries March 9th 2018