Irrigation and groundwater – efficiency, water table, waterlogging and salinisation
GeographyWater & OceansAges 15–16
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The Irrigated field screen is a cross-section through a dry farming district: choose the irrigation method (flood, furrow, sprinkler, drip), the water source (a canal from a river or a borehole), the crop, rainfall, the salinity of the irrigation water, the share of land irrigated and whether there are subsurface drains. Over 25 seasons the model works out the water applied, irrigation efficiency, deep percolation, water table, root-zone salinity and yield: seepage from canals and fields raises the water table and causes waterlogging and salinisation, while pumping more than the recharge lowers it. The Aquifer screen shows the porosity and permeability of gravel, sand, silt and clay, unconfined and confined aquifers, a flowing artesian well where the pressure surface lies above the ground, and a cone of depression around a pumping well that dries a village well and draws in salt water near the coast.
Lesson: Groundwater and water use in farming: irrigation efficiency, groundwater balance, waterlogging and salinisation, aquifers, artesian wells, cones of depression and saltwater intrusion
What it shows
Irrigation feeds a large share of the world, but water lost from canals and fields does not disappear: it soaks down and raises the water table. When the water table comes within about 2 m of the surface, capillary rise carries salt into the root zone and the soil becomes waterlogged and saline. Pumping groundwater faster than it is recharged lowers the water table instead. The field model is a seasonal water and salt balance; the aquifer screen uses the Theis equation for drawdown and the Ghyben–Herzberg relation for the salt-water boundary. Both are simplified, illustrative models.
How to use
On Irrigated field, choose Method, Water source and Crop, set Rainfall, Water salinity and Irrigated area, tick Subsurface drains if you like, then press Run or +1 season. Switch the Graph between Water table, Root-zone salinity, Yield and Irrigation water. On Aquifer, choose Aquifer material and set Pumping rate, Distance to coast and Recharge area height, press Pump water and tap a layer to read its porosity and permeability.
Parameters you can change
- Screen Irrigated field, Aquifer
- Irrigation method Flood irrigation, Furrow irrigation, Sprinkler irrigation, Drip irrigation
- Water source Canal from a river, Borehole (groundwater)
- Crop Wheat, Maize, Cotton, Tomato
- Annual rainfall 0–800 mm/year
- Salinity of the irrigation water (EC) 0.2–4 dS/m
- Subsurface drains
- Share of land irrigated 10–80 %
- Aquifer material Gravel, Sand, Silt
- Pumping rate 0–5000 m³/day
- Distance from the pumping well to the coast 200–1200 m
- Height of the recharge area above the plain 0–40 m
Questions to explore
- Why does flood irrigation with canal water for many years make the soil waterlogged and salty?
- Drip irrigation saves water, so why can the water table still fall when the irrigated area is large?
- Why is a pumping well close to the coast more likely to turn salty than one further inland?
