Transpiration Evaporation

Rainfall

Runoff

Drainage

Irrigation

Root zone

The Water Balance

Apsim is a one dimensional model.

Two methods of water movement:• Tipping bucket (SoilWater)• Richards Equation (SWIM)

Soil horizon 2

Et

Rainfall

Runoff

Soil horizon 1

Drainage

Es

SW = Inputs - OutputsSW = (R + I) – (Et + Es + RO + D)

Where:• SW – Change in daily soil

water• R – Rainfall• I – Irrigation• Et – Transpiration• Es – Evaporation• RO – Runoff• D - Drainage

…

Infiltration

Bucket sizeSaturated

Drained Upper Limit (aka Field Capacity)

Lower Limit (15 bar)

Air Dry (oven)

SoilWater

SoilWater: Runoff and InfiltrationUSDA curve number (CN) runoff model

Modified USDA curve number runoff model

Q = runoff (mm), P = rainfall (mm), S is the retention parameter (mm), derived from - Antecedant soil water content (to 450mm), - Curve Number, - Bill Mockus’ 1954 hand drawn AMC charts

SoilWater: Runoff and Infiltration

Modified USDA curve number runoff model

CN starts at CN2bare, reduced to CNred when cover reaches CNcov.

SoilWater: Runoff and Infiltration

SoilWater: Saturated Flow

∆SWi+1 = SWCONi x (SWi - DULi); for layer i, SWi > DULi

SoilWater: Soil Water EvaporationWater in surface layer may dry down to air-dry water content.

Potential evaporation (Eo) is from Priestly-Taylor

Eo = f(temperature, radiation, albedo, cover)

Actual evaporation is a two stage drying process. - during first stage = potential (ie. eos), until ∑Es = U - during second stage = CONA * √t

SoilWater: Soil Water EvaporationCan change between summer & winter.

SoilWater: TranspirationPlants can extract water to a crop-specific Lower Limit (LL).

This LL can represent root distribution, and/or soil constraints.

SoilWater: Transpiration

The potential daily rate of extraction is:

∆sw = -kl x (sw – ll)

kl is the fraction of available water that can be extracted per day.

0 5 10 15 20 25 30 35 40 45 500.2

2.2

4.2

6.2

8.2

10.2

12.2

time (days)

sw (

mm

/mm

)

DUL

LL

SoilWater: Root Development

Root development (growth) can be modified by XF – it represents a “exploration factor” for root growth in a layer.

A value of 0 stops growth in that layer.