Science - Soil–Water Relations and Irrigation Science
Scope of the Science domain
This domain tests soil–water–plant relations and irrigation science: soil moisture measurement, available water, evapotranspiration, irrigation scheduling, application efficiency, infiltration and drainage.
Expressing soil water
Gravimetric content θm is water mass per dry soil mass; volumetric content θv = θm × bulk density (g/cm³). Depth of water held in a layer = θv × layer depth. For θm = 20% and bulk density 1.4 g/cm³, θv = 0.28, so a 600 mm root zone stores 0.28 × 600 = 168 mm.
Available water and scheduling
Available water capacity AWC = (θFC − θPWP) × root zone depth. Crops are irrigated before the soil dries to wilting point, so a management allowed deficit (MAD, commonly 50% of AWC for field crops) fixes the net irrigation depth: net = MAD × AWC. Interval = net depth ÷ peak ETc, and gross depth = net depth ÷ application efficiency.
Evapotranspiration
ETc = Kc × ETo, where ETo comes from the FAO-56 Penman–Monteith reference and Kc changes with crop and growth stage. Pan evaporation needs a pan coefficient: ETo = Kp × Epan.
Worked example
Root zone 600 mm; θFC = 0.30 and θPWP = 0.12 (volumetric); MAD = 50%; ETc = 6 mm/day; application efficiency = 75%. AWC = (0.30 − 0.12) × 600 = 108 mm. Net depth = 0.50 × 108 = 54 mm. Interval = 54 ÷ 6 = 9 days. Gross depth = 54 ÷ 0.75 = 72 mm. The system must therefore deliver 72 mm of water every 9 days to meet a 54 mm net requirement.
Traps that cost marks
- Using gravimetric percentage directly as a depth of water without multiplying by bulk density.
- Treating MAD as a fraction of porosity or of field capacity instead of a fraction of AWC.
- Reporting net depth when the question asks for water pumped or released (gross depth).
- Substituting Epan for ETo without the pan coefficient, or using Kc from the wrong growth stage.