“How much soil is being lost from the fields in Tarland each year?”

This is a very valid question since the soil is the farmer’s principal agricultural resource. It has taken effort to clear the soil of rocks, to make water conditions and then nutrient levels correct for crop growth.

When soil is lost valuable phosphorus fertiliser and seed is lost too.

The phosphorus and the soil itself (termed eroded sediment) pollutes waters and damages conditions for fish and other ecology.

“How much soil is being lost from the fields in Tarland each year?”

Our calculations from monitoring during 2004-2005 show that 600 tonnes of soil were lost down the river network from the 50 km2 catchment upstream of Coull

and here’s how we worked it out…

The full story can be found in the journal article

“How much soil is being lost from the fields in Tarland each year?”

There are 3 ways to answer this:

a) Using typical values for soil erosion losses from fields under different land uses taken from scientific literature

b) Using detailed field scale experiments, observing volumes of soil lost, which have given us the literature values above

c) Using a landscape scale approach measuring the amount leaving the catchment through the river network over a year

How to calculate annual river sediment loads from the upper Tarland catchment

We will demonstrate method (c), then compare the average catchment values to possible losses under severe examples of localised erosion

River sampling to determine sediment losses

The amount of material moving down a river is referred to as the mass load. The basic calculation is:

This is then scaled up to a year using all the sampled time points and the average river flow rate (kg / year)

Then this can be expressed on an area basis according to the size of the catchment (kg / hectare / year)

Concentration at time of sampling

(kg / Litre)

Water flow rate when sampling (Litres / second)

Mass load (kg / second)× =

River sampling to determine sediment losses

Sediments are sampled by collection of a water sample using an autosampler to fill a 1 litre bottle

The autosampler can either take a sample at a set time each day, or be automatically triggered by a rise in river level

On return to the laboratory samples are filtered and the mass of dried sediment on each filter paper is recorded

River sampling to determine sediment losses

An intensive period of sampling over 2004 to 2005 provides the best data to determine sediment loads

During this year 495 sediment samples were obtained between daily and 4 hourly storm sampling

River flow was measured every 15 minutes

Samples were collected near Coull This defines a contributing catchment area of 50 km2

What do the final figures mean?During 2004-2005 the average suspended sediment concentrations in the river at Coull ranged from 7 mg / Litre in early summer to 44 mg / Litre in late winter

The loss of sediment measured at Coull was596240 kg (approximately 600 tonnes of soil lost) or 116 kg / hectare / year

This carries approximately 600 kg of pure phosphorus equivalent to £1000 of chemical fertiliser each year,

This is not much in cost, but is equivalent to the phosphorus pollution from the septic tanks of ~500 family homes each year

Further considerationsThe fine suspended sediments measured in the river are not the only component of the river mass load. We have not considered here the courser bed material that moves slowly through the river network.

So, whilst our method underestimates the true river mass load:

it is the finer particles considered here by this method that actually carry the bulk of the nutrient load from the fertilised fields,

also the fine particles are most physically damaging to habitats

Further considerations

Local field erosion rates can be much greater, dependant on topography and management

Much of this soil may not make it to the river network, but instead accumulates in depressions, field edges and buffer strips

An example can be seen from our work in the Lunan catchment (Angus). Experiments with a simple 200 m long filter fence at the bottom of a single potato field after harvested trapped 70 tonnes of soil from a single field during two months