The passage of the 1850 Federal Swampland Act
facilitated the drainage of thousands of acres of Illinois land, producing some
of the most fertile cropland in the world.Some
10 million acres, about 35% of the total agricultural area in the state, are
tiled. These drainage systems have a significant effect on the hydrology and
water quality of the watersheds in which they occur.There
is a strong correlation between improved drainage and elevated nutrient
transport from cropped land.
Conservation Drainage is the incorporation of environmentally
friendly practices and structures into existing drainage infrastructure.These
practices lead to the optimization of drainage practices for water quality
objectives. In light of the importance of drainage to agriculture in the state,
conservation drainage practices (CDPs) should
reduce nutrient transport from drained land without adversely affecting
drainage performance or crop production. The
practices mentioned below satisfies
this criterion.
Bioreactors are essentially subsurface trenches filled with a carbon
source, mainly wood chips, through which water
is allowed
to flow just before leaving the drain to enter a surface water body. The carbon
source in the trench serves as a substrate for bacteria that break down the
nitrate through
denitrification
or other biochemical processes (Figure
32).
Bioreactors provide many advantages:
They use proven technology
They require no modification of current practices
No land needs to
be taken
out of production
There is no decrease in drainage effectiveness
They require little or no maintenance
They last for up to 20 years.
To ensure that there is no decrease in drainage effectiveness, water is allowed to bypass the system when the tile flow rate exceeds the flow capacity of the bioreactor. Typically, only 20 – 30% of the annual tile flow passes through the bioreactor and is treated. Most of the flow bypasses the bioreactor during high-flow events.
Bioreactors have no adverse effects on production. They
are
designed
such that they do not restrict drainage. Unlike other edge-of-field practices
such as wetlands or buffer strips, they do not necessitate taking land out of
production.They are also very cost
effective. However, there
are
no financial incentive
for a producer to install one of these systems. Like other conservation
practices such as wetlands and buffer strips, it
is expected
that as these systems are further developed they will be considered worthy of
public funding.
Drainage Water Management
In drainage water management, a control structure (Figure
33
)
is
placed
at the outlet of a tile system to control the level of the water table in the
soil. This practice
can be used
to raise the water level after harvest, thereby reducing nitrate loading from
tile effluent, or to retain water in the soil during the growing season. The
normal mode of operation in Illinois is to set the water table control height
to within 6 inches of the soil surface on November 1, and to lower the control
height to the level of the tile on March 15. Thus, water
is
held
back in the field during the fallow period. In experiments in Illinois,
reductions of up to 82.5 % and 46.6 %
were measured
for phosphate (Figure
34) and nitrate (Figure
35), respectively .
Analysis of phosphorus data
is complicated
by the fact that there are a few events in which inordinately large amounts of
phosphorus are transported through the drains. It
is surmised
that these events result from preferential flow and experiments to further
explicate this phenomenon have been initiated. The dominant mechanism for
nitrate reduction is not
denitrification, but the reduction of flow volumes
that result from implementing drainage water management.
Existing drainage
systemscan
be retrofitted for drainage water managementby
installing control structures at a cost of$20
- $40 per acre. For new systems addition costs
are incurred
by laying out the drainage systems to optimize the benefits of drainage water
management.
In Illinois, drainage systems
are normally laid out
to minimize the cost of installation.
However ,
such installations do notnecessarily
maximize the benefits of drainage water management. Shown in Figure 5 are two
possible drainage systems that
could be installed
on the same field. In all likelihood, this lower cost system would be the one
selectedfor
installation. In this instance, based on average installations costs,the
difference in cost is $75/acre.
Drainage water management systems
can be managed
so that they store water during the growing season (Figure 6). This stored
water can result in increased yields. One relationship between the additional
cost of a drainage water management system and the corresponding breakeven
income
is shown
in Figure 7.A system that cost $100/acre,
for example, has a breakeven income of $7.05. Thus, it does not take a large
yield increase to pay for the installation of a drainage water management
system(Figure
36) (Figure
37) (Figure
38).(Figure
39)
Depth/Spacing Relationships
Originally, tile systems in Illinois were designed for the sole purpose of quickly removing excess water from the plant root zone to prevent wet stress and to improve crop yields.Drainage intensity is expressed as the drainage coefficient, the depth of water to be drained to lower a water table, initially at the soil surface, by a foot in 24 hours.Different combinations of depth and spacing result in the same drainage coefficient (Figure 40 ), but they may be different in their water quality response .
Experiments are
being conducted on several different soil types to determine
depth/spacing combinations that optimize productivity with minimum adverse
water quality effects.Preliminary
results, shown in
Figure 41
,seem to suggest that shallower tiles placed closer together are
more suitable than deeper tiles placed farther apart.
For a specified drainage coefficient, shallower tile are associated
with narrower
spacings,and
thus drainage cost increases with decreasing tile depth. The relationship
between spacing and average installation cost
is shown
in Figure 10. Also shown is the relationship between computer simulated yield
from a silt loam and spacing, using 30 years of weather data from Urbana. While
the yield ratio varies with soil type and location,
andthe
cost ratio varies with the contractor, the general form of the curves are
representative of what can be expected in Illinois. Note that there is
a threshold
spacing beyond which no yield benefits can be obtained by putting the tiles
closer together.
The yield and the cost curves
can be used
to determine the spacing that maximizes the rate of return on the investment in
drainage. In this instance the maximum rate of return was 17%, obtained at a
spacing of 45 ft. If the spacing
were reduced
to 35 ft so as to improve the quality of the drain effluent, the rate of return
on the drainage investment would drop to 15%. This is still a substantial rate
of return for the producer. Further benefits would also accrue from the
improvement in downstream water quality.
Conclusions
Conservation drainage practices are designed to provide water quality benefits. They are most likely to be adopted if they do not adversely affect profits. Some practices can be managed to provide production benefits. If these benefits exceed the costs then there is economic incentive to implement these practices