Drainage

Question 1

Give a definition of drainage and discuss two different situations in which drainage might be applied to ensure crop production.

Answer 1

• Drainage: natural or artificial removal of surface and sub-surface water from an area in order to increase its productivity. Soil saturation is bad for plants -> they need oxygen in the soil to respire
• Where intensive storms are possible during the growing season, the objective of crop drainage is to quickly re-establish the aeration of the root zone after a storm
• Pros: increase productivity, create new agricultural land, eliminate stagnant waters which present health risk e.g. malaria
• Cons: strong decrease of humid areas and habitats, degradation of surface water quality, Se pollution in California (mobilized by changing saturation and trapped in a wildlife refuge causings birds to die)
• Humid regions: see below
• Arid regions: improve soil aeration, remove irrigation water with high salt concentration and to prevent GW table from rising

Question 2

List 5 reasons why a farmer might consider draining his land in humid regions.

Answer 2

• Improve soil aeration
• Improve structural stability, reduction of chemical toxicity
• Reduction of nitrate losses due to denitrification
• Reduction of runoff (better infiltration)
• Quicker soil heating (water takes longer to heat than air)

Question 3

Say which information and/or data you need to be able to design a drainage system. Give some examples of each category.

Answer 3

• Hydrologic conditions: origin and timing of the water e.g. rainfall to determine how much water comes in e.g. yearly extremes
• Agricultural conditions: ideal water content in the root zone, define depth that needs to remain unsaturated
• Soil conditions: soil type, air-water in the root zone, capillary rise
• Economical conditions: cost-benefit analysis, available technology
• Also important: irradiation, topography

Question 4

Explain which equation this is. Explain which variables are figuring in this equation and describe the different components of the equation. Why do we have to iterate over the equation and can we not solve explicitly the drain spacing?

Answer 4

• This is the Hooghoudt equation - describes flow around a drain under steady state conditions, used to calculate optimal distance between 2 ditches (L)
• N is the steady state recharge rate, K the hydraulic conductivity, D the equivalent depth, L is the drain spacing and h is the depth of the drains minus the depth of the steady state water table (h is the height of water table between 2 drains)
• First one is Ka is hydraulic conductivity above the drain level and the second one is Kb below the drain level
• There is a radial resistance around the drains - replace real distance D to the impervious layer with an equivalent smaller distance De representing all resistances (horizontal and radial). This is needed if the drain is not right at the impervious layer
• De is calculated by iteration of D and L - D is a function of L (Hooghoudt) and L is a function of D (Van der Molen and Wesseling) iterative calculation is required
• De is a function of L calculated using the Van der Molen and Wesseling equation
• The equation can only be used in homogeneous soil profiles or when drain is at interface of two layers. Steady state equation (in = out, recharge = discharge)
• Increased drain spacing = equivalent depth De becomes closer to real depth - eventually drains are so far away, they don’t have an effect on the water depth halfway between them

Question 5

Explain the concept of resistances as applied in the Ernst-Van Beers equation (no formulas need, but the principle).

Answer 5

• When there are two soil layer and the drain is not at the interface, Ernst Van Beers is used
• The equation works on the principle that flow of groundwater to parallel lines, and the corresponding total heads, can be split into vertical, horizontal and radial that work in series
• They work in series meaning h = hh + hv + hr = qRh + qRv + qRr where q is flow and R is resistance which represents the forces opposing the motion of the fluid
• These resistance terms are then calculated based on drainage spacing, hydraulic conductivity, depth etc.

Question 6

Explain the principles of the De Zeeuw nonsteady equation.

Answer 6

• Under non-steady conditions height of water table changes so steady state equations cannot be used
• Under steady state conditions the drainage discharge, q is equal to the recharge, R/N. This assumption is not applicable due to the fluctuations in the water table
• If the recharge across each time interval is assumed to be constant, the change in drainage time is proportional to the excess drainage rate (R-q). The constant of proportionality is represented by the 𝛂 value above where K is the hydraulic conductivity, De the equivalent depth, 𝝻 represents the storage coefficient and L the drainage spacing
• This equation dq/dt = 𝛂(R-q) can then be integrated over one time step to produce the equation for qt

Question 7

Explain what the storativity of an aquifer is and how is it related to the soil water retention curve? Why is it relevant for drainage design?

Answer 7

• Storativity is the volume of water from storage released per unit surface area of the aquifer per unit decline in hydraulic head
• The storativity links the piezometric level to the water balance and is also dependent on the water table depth based on the equation:
  deltah=1/stor. *deltaWg
• Where h is the water level and Wg is the water storage per unit of surface area
• The storativity of an aquifer and the soil water retention curve are related because the storativity of an aquifer depends on the properties of the soil that it is composed of. If the soil has a high water retention curve, then water will be able to move through it slowly, which means that the aquifer will have a high storativity. Conversely, if the soil has a low water retention curve, then water will be able to move through it quickly, which means that the aquifer will have a low storativity.
• It is important for the drainage design as it is inversely proportional to drainage spacing in drainage design. A higher storativity means that drainage spacing will need to be closer together as water moves between pores slowly whereas at lower storativity water is able to move through quicker so drains can be spaced further apart.

Question 8

Formulate the drainage criteria for a given case (eg irrigated area, temperate climate, …)