1. TranspirationEvaporation Rainfall Runoff Drainage Irrigation Root zone The Water Balance

2. Apsim is a one dimensional model. Two methods of water movement: Tipping bucket (SoilWater) Richards Equation (SWIM)

3. Soil horizon 2 Et Rainfall Runoff Soil horizon 1 Drainage Es  SW = Inputs - Outputs  SW = (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

4. Bucket size Saturated Drained Upper Limit (aka Field Capacity) Lower Limit (15 bar) Air Dry (oven)

5. SoilWater

6. SoilWater : Runoff and Infiltration USDA curve number (CN) runoff model

7. 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

8. Modified USDA curve number runoff model CN starts at CN2 bare, reduced to CN red when cover reaches CN cov. SoilWater : Runoff and Infiltration

9. SoilWater : Saturated Flow ∆SW i+1 = SWCON i x (SW i - DUL i ); for layer i, SW i > DUL i

10. 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

11. SoilWater: Soil Water Evaporation Can change between summer & winter.

12. SoilWater : Transpiration Plants can extract water to a crop-specific Lower Limit (LL). This LL can represent root distribution, and/or soil constraints.

13. 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.

14. 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.