Land users have a vital role to play in reducing the risk of flooding in catchments. Working with stakeholders, EVO is developing cloud services and tools to help land users of reduce flood risk.
EVO is developing a hydrological modelling tool which allows users to test out different land use and land management scenarios to see the effect on river levels and the potential risk of flooding. To to understand more about the links between land use and flooding please read on, and to find out more about the EVO modelling tools visit the EVO Portal.
Making the linkage between the land and water
Linking land use activities in the rural upstream headwaters of river catchments to changing the risk of flooding on the flood plains downstream is complicated. The size of the rainfall event or its magnitude is clearly an important determinant of whether enough water is present to cause a river to burst its banks and cause a flood. However, the factors that contribute to high rainfall runoff levels which can lead to flooding vary in space and time. For example, the soil moisture conditions are determined by the duration and frequency of rainfall events and can affect the capacity of the soil to act as a ‘sponge’ and soak up high rainfall. Conversely, the paths of storms can result in very uneven rainfall distribution over the countryside. The relative timing of peak flows in smaller tributaries, once the water reaches the river, can also be critical as to whether these flow peaks reach the main river at the same time, thereby creating a much larger risk of flooding occurring.
Slowing the flow and increasing out of bank storage
Can anything be done in the rural farming landscape to reduce the risk of flooding in our villages and towns on the floodplain? An important way to reduce flooding downstream is to hold water back in and on the land; this effectively slows the flow of water and reduces the size of the peak flow in the river. The capacity of a landscape to store water is a function of its underlying geological and climatic characteristics, its land use and how those uses are managed. The factors that relate to how water moves across the land to a stream are the soil infiltration, storage and tillage regimes and the level of flow connectivity. A number of management options are also available to reduce the potential risk of flooding from farmland.
Soil infiltration, storage and tillage regimes
Natural influences on soil storage capacity of water relate to factors like the soil depth, aspect and slope of fields. A surface that allows the water to flow quickly is not desirable and causes flooding, whereas a higher surface roughness and higher infiltration into the ground can slow down the flood response and is desirable. Steeper slopes and shallow soils are more susceptible to surface runoff. The vegetation cover of soils, whether that is permanent grassland or the cover of other crops, during the wettest times of the year in the winter and spring, has an important impact on the ability of the soil to act as a water store. Runoff of rainwater is much more likely on bare fields than those with a good crop cover. Different types of crops and the way they are cultivated can also increase the risk of runoff, for example crops like maize or potatoes result in much more bare soil being exposed than grass or forestry and the traditional ridges and furrows approach to tillage can generate more runoff than direct drilling of seeds into the soil.
The structure and infiltration capacity of soils will also have an important impact on the efficiency of the soil to act as a sponge and soak up water. Different types of soils have differing capacities but they can also become degraded through poor management practices. Some fine soils can be prone to capping where the surface forms a hard crust, preventing water from infiltrating resulting in water running off the surface. Compaction caused by heavy machinery or high stocking densities can also lead to degraded soils.
Adopting best management practices for soils can significantly improve the structure and functioning of the soil as a store for water during wet weather, it is also very cost effective and reduces nutrient and soil erosion.
Flow connectivity relates to how easy it is for rainwater falling on the land to reach the stream. High connectivity can be bad from a flood risk perspective as larger volumes of water will reach the streams and rivers in a catchment quickly, resulting in high levels of discharge increasing the risk of a flood. Natural connectivity is associated with the shape of the hill slope in relation to the stream; steep slopes with no flat area next to the stream have a higher level of connectivity than gentle slopes or flat land. In the agricultural landscape, connectivity can be enhanced by the use of land drains and ditches and the location of tyre tracks, roads and tramlines. Where these features are oriented down a slope, results in their as a conduit for water speeding up its travel over land.
The density of livestock on land, particularly during the wet winter months can impact on flow connectivity through the compaction of land leading to a reduction in water infiltrating into the soil. Compacted ground leads to higher surface runoff so water reaches the stream more quickly.
In order to address the issues of soil structure and flow connectivity and effectively slow down the flow of water from land to streams and rivers, a number of management approaches can be adopted to trap water and sediment before it can reach the stream. Sometimes measures to reduce flooding can also be similar to those that reduce diffuse pollution, so there is a dual benefit here. This has the dual benefit of reducing the potential flood risk and also preventing fine sediments, nutrients and other diffuse polluting substances from being lost from the land. The use of field boundaries, such as hedges and stone walls and buffer strips can act as potential traps for water, increasing the time for infiltration into the soil and reducing the flow connectivity. Slowing down water delivery with wetlands, ponds, waterlogged zones and storage ponds can also help to store more water on the land before it reaches the river. This may be through a slowing of runoff as it passes through a wetland or as a temporary storage area during a high flow event.