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.