OR/14/018 Appendix 1 - Description of FAO56 calculation within ZOODRM

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Mansour, M M, Hughes, A G. 2014. Land Use, Climate Change and Water Availability: Preliminary modelling of impacts of climate change and land use change on groundwater recharge for England and Wales. British Geological Survey Internal Report, OR/14/018.

The simplified FAO calculates the evapo-transpiration based on the level of the soil moisture deficit (SMD) on a daily basis. It is assumed that crops draw water from soil at the full potential evaporation rate when the SMD value fluctuates between zero and the value of readily available water (RAW). Crops draw water from soil at a reduced rate if the SMD value fluctuates between RAW and total available water (TAW). Finally, crops are not able to draw water from soil if the SMD value reaches the value of TAW. The soil moisture deficit value cannot go beyond the value of TAW.

Total available water (TAW in mm) is calculated by the following equation:
( ........................... )

Where:

is the moisture content at field capacity
is the moisture content at wilting point
is the root depth (m)

The readily available water RAW (mm) is calculated using the following equation:

Where dp is the depletion factor.

Soil storage is given as the difference between TAW and SMD