Farming and the Water cycle

In what ways do farming practices affect rivers?

[A synoptic link that is a requirement of some of the A level specs….]

Agricultural production can take the form of arable farming (growing crops) or pastoral farming (rearing cattle and other animals). Agriculture can have an impact on both the quantity and quality of water in rivers. These in turn can affect the value of rivers for water resources and can impact on aquatic ecology.

This post considers some common agricultural practices, and their impacts on hydrological systems.

Irrigation

In arid areas, soil needs to be irrigated to sustain the crops being grown, often using water from nearby rivers, or stored in the bedrock (groundwater). Most of this water will be used in transpiration by plants and some will evaporate directly from the soil. This means it is lost to the atmosphere as water vapour, so there is less water in rivers as they flow downstream.

Similarly, repeated extraction of groundwater reduces the amount of water held in this store. In areas where rainfall is strongly seasonal, groundwater extracted for irrigation may recharge during the wet season, but in regions which are dry for most of the year, these stores will take much longer to recover. There may come a point where it is no longer possible to extract groundwater, so irrigation will not be possible, and farmland may have to be abandoned.

Salination

Repeated irrigation also has implications for water quality. Most streams contain dissolved salts and other ions, which come from weathering of rocks in the catchment. When this salt-containing water is used to irrigate soils, the water is lost through evapotranspiration, but the salts are left behind in the soil. If there are repeated cycles of irrigation, salts can build up in soils over time. This is salination.

If soils become too wet, and not all the water can be stored by the soil, the excess water may wash some of these salts out of the soil and back into rivers, further increasing their salinity as they flow downstream. A combination of reducing the amount of water in the river, and increasing the number of salts entering the river, can lead to a much higher concentrations of salts in the river downstream of an agricultural area.

Soil saturation

Some farmers leave fields fallow between harvests, to allow the nutrients in the soil to replenish before crops are sown again. Soils can be left bare, or they might have minimal vegetation in the form of stubble. Fields with sparse vegetation cover can also be found on pastoral farms, if they have been overgrazed, the lack of vegetation cover means less water will be intercepted, so a greater proportion of rainfall reaches the soil.

Transpiration is also reduced. The combination of reduced interception and reduced transpiration means that when fields lie fallow more water reaches and stays in the soil. These soils become closer to saturation - all the pore spaces are full of water. During heavy rainfall events, water is less able to enter the soil and instead it moves over the ground as saturation excess overland flow.

Compaction and crusting

The lack of interception on fallow land means that raindrops are not slowed down by hitting obstacles like leaves and branches, and so they hit the ground rapidly. Heavy raindrop impact can redistribute soil particles to form an impermeable ‘crust’ on the soil surface.

The use of heavy farm vehicles and machinery can have similar effects. As machines pass over the soil, their weight compacts it, forcing individual soil particles closer together and reducing the volume of pore spaces that water can enter. High grazing densities can have a similar effect, with soil compacted by cattle trampling the land. Crusting and compaction reduce the infiltration rate, so that during heavy storms not all the rainfall can enter the soil but flows over the land towards rivers as infiltration excess overland flow.

Flow channelisation

Fallow agricultural land is also susceptible to soil loss through erosion. Soil particles are detached by heavy rainfall or erosive overland flows. These soil particles can then be washed over the surface, or in some cases, blown by wind. Soil erosion leads to the development of rills or gullies, which channel water across fields.

Ploughing also can create small channels in the soil - furrows. In areas where soils are very wet, some farmers create surface or sub-surface drainage channels to rapidly remove the water from the soil. These drainage channels usually discharge directly into nearby rivers.

Flooding

The practices described above lead to decreases in infiltration, and increases in overland flow and channelised flow, especially at times of high rainfall. These changes have implications for flood risk.

Overland flow travels faster than throughflow, as there are fewer barriers to slow it down. This is particularly the case over bare soil. The lack of vegetation decreases surface roughness, so there are fewer obstacles for the water to flow around, and it moves faster. Water in channels such as rills, gullies, furrows, and ditches travel faster, reaching rivers very rapidly. This means that large quantities of water can be delivered to rivers in a short space of time, decreasing lag time and increasing peak discharge, and making flooding more likely.

Water quality

Soil particles removed by erosion can get washed into rivers where they may either be held as suspended sediment or fall to the riverbed. Increased levels of fine sediment in streams can be bad for aquatic plants, as it can make the water cloudy (turbid), blocking out the sunlight they need to photosynthesise. It can also be harmful to fish and other aquatic fauna, as fine sediment can disrupt some stages of their life cycles.

Fertilisers and pesticides

Farmers use fertilisers and pesticides to boost crop growth, or to improve grassland for grazing. Fertilisers are high in nutrients, including nitrogen and potassium. Manure or slurry, which also have high nutrient contents, are also used as fertilisers. If fertilisers are applied in large quantities, or if it rains shortly after application, they can get washed from fields into rivers. Once these nutrients enter waterways, they can stimulate the growth of algae and other aquatic plants. This growth can become excessive causing eutrophication. Some species of algae produce substances that are toxic to mammals, including humans. When the excess growth of plants dies back, the bacteria which decompose the dead vegetation use a lot of the oxygen in the water. This depletes the oxygen available for use by other species, which can be fatal for fish and other aquatic life.

Like fertilisers, pesticides can also get washed into waterways. These can be ingested by small organisms, which then get eaten by larger ones. In this way, concentrations of pesticide increase up the food chain. High concentrations of pesticides are harmful to fish and mammals, and can kill them.