Engineering & Technology · Agricultural, Food & Biosystems Engineering
Soil-water management, irrigation and drainage
A moist soil can still make water difficult for roots to obtain. Some pores drain readily; some retain water strongly. Describing soil simply as a full or empty tank misses both how water is held and how quickly a plant can obtain it.
Use process balances, heat transfer and fluids for further detail; this chapter defines its own elementary accounts and units. Plant biology and chemistry provide useful context. No crop yield, soil value, food-processing endpoint, storage limit or machine operating setting in an example is an instruction for a real system.
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Use a root-zone water account and distinguish total storage, available-water conventions and irrigation delivery quantities.
Use the soil model with its conditions
Volumetric water content θ is water volume divided by bulk soil volume. Field capacity is a practical state after rapid downward drainage has markedly decreased in an appropriate soil. A wilting-point convention describes a much drier reference condition. Their difference is used to estimate total available water in a specified root zone; it is not the total water present.
For a uniform 0.40 m root zone with assumed θFC=0.30 and θWP=0.15, available-water depth is 1000×(0.30−0.15)×0.40=60 mm. The factor 1000 converts metres to millimetres. Rooting depth, soil structure, salinity and conditions affect interpretation. Plant stress can occur before that whole difference is depleted, because uptake must meet changing demand.
A water balance includes storage change
Draw the area and root-zone depth being counted. Over an interval, total storage changes with rainfall and irrigation entering, evapotranspiration leaving, drainage below the zone, runoff and any other relevant exchanges. Capillary rise or lateral flow may matter in a real field. State omitted paths explicitly and avoid counting the same incoming water both before and after a loss.
In the separate original example, gross incoming rain is 10 mm and irrigation 20 mm. Initial storage 80 mm plus these inputs, minus ET 15, deep drainage 5 and runoff 3, gives 87 mm. This is an account over a selected area and interval, not a recommended irrigation amount. The available-water comparison in the figure uses the same root-depth assumption while answering a different question.
Connect the field to the larger system
One millimetre over one hectare is 0.001 m×10,000 m²=10 m³. If a hypothetical root-zone requirement were 20 mm over one hectare, its volume would be 200 m³. If an explicitly assumed delivery fraction were 0.80, source withdrawal would be 250 m³. That bookkeeping fraction needs a defined boundary; it is not a universal efficiency value.
Drainage can relieve excess water that limits aeration or causes other problems, while carrying nutrients or salts to another location. Evapotranspiration includes soil evaporation and plant transpiration; calling all of it waste would ignore the biological function. Compare water quantity, quality, timing and downstream effects. Measurements at a few points need a justified connection to the spatial root zone before they guide a field decision.
Reconcile both units and the storage account
The balance change is 10+20−15−5−3=7 mm, so ending storage is 87 mm. Over one hectare that increase corresponds to 70 m³ of water, under the uniform-area interpretation. The total input 30 mm corresponds to 300 m³; the 23 mm outgoing account corresponds to 230 m³.
Separately, field-capacity storage is 0.30×0.40=0.12 m, or 120 mm. Wilting-point storage is 60 mm, leaving a 60 mm difference. The current 87 mm is not “87 mm available”: the chosen lower reference and the plant’s ability to extract water still matter.
Try the reasoning
Is the soil’s available-water difference all of its stored water?
Show the answer and reasoning
No. It is the difference between specified reference contents across a defined root zone.
Why include storage change when input and output differ?
Show the answer and reasoning
Water can accumulate or be depleted within the boundary. Assuming steady storage would erase that physical account.
Does the hypothetical 250 m³ withdrawal establish pump size?
Show the answer and reasoning
No. Pumping rate, duration, elevation, pressure losses, supply and actual crop/soil requirements are additional questions.
What to carry forward
Define the soil zone, water states and flow paths, then connect the water account to biological availability and delivery conditions.
References
Teaching resources and research supporting this lesson’s explanations and examples.