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Transpiration

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

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Transpiration

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  1. The Water Balance Rainfall Transpiration Evaporation Irrigation Runoff Root zone Drainage

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

  3. Rainfall • 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 Runoff Et Es Infiltration Soil horizon 1 Soil horizon 2 … Drainage

  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. 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

  8. SoilWater: Runoff and Infiltration Modified USDA curve number runoff model CN starts at CN2bare, reduced to CNred when cover reaches CNcov.

  9. SoilWater: Saturated Flow ∆SWi+1= SWCONi x (SWi - DULi); for layer i, SWi > DULi

  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.

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