The Water Balance
Rainfall
Transpiration
Evaporation
Irrigation
Runoff
Root zone
Drainage
Apsim is a one dimensional model.
Two methods of water movement:
•Tipping bucket (SoilWater)
•Richards Equation (SWIM)
Rainfall
SW = Inputs - Outputs
SW = (R + I) – (Et + Es + RO + D)
Et Es
Where:
•SW – Change in daily soil
water
•R – Rainfall
Soil horizon 1
•I – Irrigation
•Et – Transpiration
•Es – Evaporation
Soil horizon 2
•RO – Runoff
•D - Drainage
Runoff
Infiltration
…
Drainage
Bucket size
Saturated
Drained Upper Limit (aka Field Capacity)
Lower Limit (15 bar)
Air Dry (oven)
SoilWater
SoilWater: Runoff and Infiltration
USDA curve number (CN) runoff model
SoilWater: Runoff and Infiltration
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: Saturated Flow
∆SWi+1 = SWCONi x (SWi - DULi);
for layer i, SWi > DULi
SoilWater: Soil Water Evaporation
Water 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 Evaporation
Can change between summer & winter.
SoilWater: Transpiration
Plants can extract water to a crop-specific
Lower Limit (LL).
This LL can represent root distribution,
and/or soil constraints.
SoilWater: Transpiration
0.6
DUL
0.55
The potential daily
rate of extraction is:
sw (mm/mm)
0.5
0.45
∆sw = -kl x (sw – ll)
0.4
0.35
LL
0.3
kl is the fraction of
available water that can
be extracted per day.
0.25
0.2
0
10
20
30
time (days)
40
50
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.