ISIS Report 11/12/12
How Farmers Can Protect Water Quality, Replenish Aquifers and Save the Soil
Scientists work with farmers to find ways to reduce
surface runoff and soil erosion, thereby also reducing water pollution Dr. Mae-Wan
illustrated and referenced version of this paper is posted on ISIS
members website and is otherwise available for download here
A team of scientists and local farmers used a computer
simulation programme to help identify the best ways to reduce surface runoffs
and soil erosion on farms .
The study was done in collaboration with the
local authorities in an area of south-western France that suffers badly from
surface runoffs and soil erosion after rainfall.
To support the work, they used a geographic information
system (GIS) computer simulation model of water flow and soil erosion, STREAM,
to assess the impacts of a spring stormy event under different management
scenarios at two sites chosen by the farmers. The results were jointly analysed
and evaluated by the farmers and scientists, and the farmers discussed the
technical and economic feasibility of each management scenario.
The STREAM simulations showed that a 40 mm
spring rainfall with current cropping patterns led to 3 116 m3 total
water runoff and 335 tonnes of sediment at site A, and 3 249 m3 water runoff
and 241 tonnes of sediment at site B. Growing grass strips at strategic places
could reduce runoff by about 40 % and sediment by about 50 % at site A. At site
B, grass strips could reduce runoff and sediment by more 50 %, but changing the
cropping system could eliminate both runoff and sediment almost entirely.
Agriculture & water
Agriculture is a major user and polluter of water, and this
needs urgent attention in view of the global depletion of fresh water resources
(see [2, 3] World
Water Supply in Jeopardy, SiS 56; Using Water
Sustainably, SiS 57).
The problem started from the
1960s when intensive agriculture was introduced in Europe to increase crops
yields (see ). This required mechanisation and the application of
fertilizers and pesticides, which soon favoured big farms at the expense of
small farmers. And the now well-known environmental problems of runoff, soil
erosion and pollution of water resources started to emerge.
Over the past 20 years, groundwater and
surface water monitoring in Europe revealed significant nitrate and pesticide
contamination, especially in France, where surface water samples often exceed
the drinking water limit of 0.1 mg pesticides/L. For example, 96 % of surface
water in the Department Tarn and Garonne in south-western France was
contaminated by nitrates, phosphorus and pesticides, partly because of erosive
runoff in cultivated fields.
In 2000, the European Community introduced
the Water Framework Directive (WFD) to restore and preserve the quality of all
water resources. It set targets of water quality to be achieved by 2015. The
common agricultural policy (CAP) reform of 2003 introduced the
‘cross-compliance principle’ that linked the full payment of CAP aids to farms to
compliance with agri-environmental standards called “good agricultural and
environment condition”, which include a part of the annual cropped area to have
permanent plant cover to prevent soil erosion and buffer strips (no-cultivated
or grass planted) along water courses to prevent surface water pollution.
According to French decree, the total surface
area of permanent plant cover (PPC) in each farm must be at least 3 % of the
annual cropped area. PPC or grass strips must be planted within fields, most
importantly, those bordering rivers, and the strips must be between 5 and 10 m
wide and must cover at least 500 m2. Designing these agri-environmental
measures (AEM) is not a trivial matter, and will differ for farms at catchment
level as opposed to river level. Therefore modelling could help find the best design.
And working with farmers in real farms would also put the model to proper test.
Farmers chose the sites
The study was done in the French Department Tarn et Garonne
in collaboration with Lomagne district agricultural committee. Soil erosion is prevalent
in these catchments and sediment loads in streams and rivers impact negatively
on water quality.
The region has a humid temperate climate,
with annual rainfall between 700 and 760 mm, and average daily temperatures 10
to 35 ºC. Rainfall is low to moderate in winter, and the most intense rainfall
events are in spring. The soils in the region are very susceptible to surface
sealing. The water table is very deep (> 10 m). The risk of erosive events
is very high in April-May, when intense rainfalls occur (20-40 mm in 2 to 3
hours) and many fields have just been sown.
Figure 1 Sites
selected for study in southern France
In collaboration with the local
farmers, two sites were selected (see Figure 1). The first is a 41 ha hillside
farm with slopes ranging from 0 to 15 %, comprising five large fields
cultivated by two farmers. In 2009, 36 ha were planted with spring crops (maize
and sunflower) and 5 ha with winter wheat. Spring storm causes mud flows in the
fields with spring crops that cover the downhill road nearly every year. The
second site is a 107 ha catchment that supplies the Serre River and comprises
40 fields cultivated by 5 farmers. This site is characterized by a steep-sided
upstream valley with strong slopes (> 15 %), followed by a relatively flat
valley (slope between 0 and 5 %). In 2009, five main crops were cultivated:
winter crops (wheat, barley and rape) on 43 % of the area, spring crops (maize,
sunflower and sorghum) on 41 %. Grasslands account for 12 % of the area mainly
in the upper basin, while forest and set-aside account for less than 4 % of the
Site A was chosen because erosive runoff is
severe and occurs almost every year in spring. Site B was chosen because it is
small and different crops are grown there. Another important factor was that
most of the farmers (5 of 6) in the two sites selected agreed to spend time
with the scientists.
The hydrological model and geographic information
The software package STREAM is a geographic model of runoff
and erosion at the field and event scale, developed by a company in California,
USA. The basic hypothesis of the model is that soil surface properties such as
soil roughness, surface crusting and vegetation cover control water runoff and
soil erosion/redistribution processes in agricultural landscapes. It also takes
account of tillage direction and landscape features such as ditches, hedges,
roads, and developed sites.
To validate a STREAM model requires measuring
runoff and sediment yield. But such figures did not exist on either site. Five
meeting with farmers took place between January and June 2009. The first two
meetings were with the Lomagne district agricultural committee to choose the
sites. The third meeting in early May 2009 was spent by the scientists visiting
the sites with most of the farmers to record cropping practices and the state
of the surface soil. The fourth and fifth meetings were spent validating the
simulated runoff network qualitatively, building scenarios for the two sites
including alternative locations for grass strips and/or changes in cropping
patterns, and analysing the simulated impacts of changes.
The STREAM model was first run with the crops
and soil surface states recorded at each site during the first visit. These
were used to validate the simulation models qualitatively. The farmers compared
the simulated results with what they have observed in the past in terms of
runoff pathways and erosion-accumulation rates. This was the baseline scenarios
(A0 and B0) for comparison with simulated results of proposed changes.
The changes were suggested by analysis of the
baseline scenarios: two for site A: A1 and A2; and three for site B: B1, B2 and
B3. Two kinds of management options were discussed: grass strips inside or on
the border of fields, or grassland fields to slow down runoff and enable water
to infiltrate; and changes in cropping pattern, given that runoff and erosion
on spring crops are higher than on winter crops.
For the hillside at site A, the proposal
concerned the location and dimensions of grass strips: standard 5 m wide strips
bordering ditches (scenario A1) or 10 m wide strips located mid-slope in the
fields where slopes are steepest (scenario A2). No cropping pattern change was
tested at site A because farmers did not choose to test it, but was tested at
site B. In the scenario B1, a long narrow field bordering the river and
standard grass strips bordering rivers and ditches were combined with existing
cropping patterns. Scenario B2 involved a redistribution of spring and winter
crops within the watershed combined with a long narrow field bordering the
river. In scenario B3, all cropped fields were changed to winter crops.
Reducing runoff and sediment in simulated scenarios
The results of the simulations are given in Table 1. As can
be seen, planting 5 m grass strips bordering ditches and roads (A1) reduced
runoff by about 45 % and sediment by about 25 %. Planting 10 m grass strips
where the slope was steepest (A2) reduces runoff by 43 % and sediment by 39 %.
Thus A2 was better than A1; but most of the farmers did not like A2 because it
reduced the size of the fields and tractors are not allowed on the strip as it
would compact the soil.
Simulated runoffs and sediments for different scenarios
For site B, planting grass strips bordering
the rivers (B1), in accordance with official regulations resulted in reduction
of more than 50 % runoff and sediment. The reallocation of winter and spring
crops in the catchment by putting winter crops close to the river (B2) gave a
slightly lower reduction in sediment, but runoff was only reduced by 22 %.
Planting only winter crops (B3) reduced runoff and sediment almost completely.
However, the farmers did not agree with this cropping because it would lead to
a major reduction in their income; though prices on agricultural products can
strongly vary between years. In addition, farmers confirmed that alternating
spring and winter crops enabled better pest and weed control.
As the results of the simulation demonstrated
the major impact spring crops have on erosion, the farmers were willing to
discuss the drastic solution of planting only winter crops in catchments where
erosion is a very serious problem.
The study did not include change in the direction of tillage
because the model had not been validated for it, and because farmers did not
want to change tillage direction in sloping fields, as across-slope tillage is
impossible and even dangerous when the slope exceeds 10 %. Nevertheless, it is
well-known that across-slope tillage slows downhill runoff.
Seeding spring crops directly into mulch
could also be an option to prevent the risk of erosion. Farmers did not choose
to test this option because the topic divided the farmers in the area.
Nevertheless they were willing to discuss this option after the simulation.
They claimed that the option presents two major constraints: it needs high cost
drilling machines and is risky in the local soil because results are very
variable and unpredictable as has been found on imperfectly drained loamy
Fields cultivated with spring crops are the
main source of erosive runoffs. Some mayors in France have issued an order to
limit spring crop acreage in their municipality and oblige farmers to come up
with a collective cropping plan. A collective cropping plan is not easy, as
crop history and different soil fertility have to be taken into account. Here
is where a discussion involving all parties could be aided by the STREAM
Farmers’ practices are influenced by
economics, and their first priority may not be to protect the environment. Thus
CAP subsidies and penalties from non-compliance with AEM need to be applied to
steer farmers towards environmentally beneficial choices.
The STREAM model proved useful in helping farmers, local
communities and scientists to work together in order to find the best means of
reducing runoff and sediment, which are crucial for replenishing underground
water and improving surface water quality as well as preventing the loss of nutrient-
rich top soils.
It is possible to apply the
analysis on larger territories, such as entire river basins that cross national
boundaries by incorporating remote sensing data. This is urgently needed not just
in Europe, but worldwide [2, 3].
There are 2 comments on this article so far. Add your comment
|ken hargesheimer Comment left 11th December 2012 16:04:50|
A computer is not needed to teach farmers how to prevent soil erosion. Use organic, no-till farming. For a free DVD, email your postal address to firstname.lastname@example.org
|Amit Comment left 11th December 2012 16:04:46|
It's still surprising that in 2012 strategies for erosion-preventing tillage is still not well researched. This article mentions that farmers are reluctant to perform cross-tillage because of a 10% slope problem. Yet, tilling diagonally across slopes at a 45 degree angle would produce a checkerboard pattern that would not only slow runoff, but provide catchment areas for rainwater to slowly leach into the soil. The first pass down the slope would use traditional crop spacing. The second pass, at an opposite angle, would be made after seed planting every 10-15 meters apart. Using this method would eliminate the need for additional equipment, requiring only extra fuel and labor.