The daily water use is the amount of water used by a crop in one day.
It is measured in mm per day (or ins/day - see Display Units)
PDWU is calculated from the rate of change of the water content in the primary root
zone - typically 0-70 cm (see Root Zones).
This will only be the actual crop water use if there is no surface evaporation
or through-drainage - if either of these are taking place then PDWU will not
be the crop water use but a combination of...
The probe daily water use is used to predict and schedule the next irrigation - the date and amount of water required.
The estimated daily water use (EDWU) is a value of crop water use that is entered by the user.
It is used to schedule a second irrigation date.
The historical daily water use (GDWU) is a value of daily water use that is calculated from
and is used to schedule a third irrigation. (PR41 called this the Global Daily Water Use). The historical data needs to be entered into a DWU file.
The total water content in three separate soil profiles are calculated from the volumetric soil moisture content at each depth. Each of the root zones
needs the user to enter the top and bottom of the root zone.
The first root zone is typically set to be from 0cm to 70 cm, but will really depend on
the crop and soil type.
The water content of the first root zone (RZ1) is used to calculate the probe
daily water use (PDWU)
The maximum water content of the soil that can be maintained against gravity without through-drainage or surface run-off.
The wettest soil profile that can be regularly obtained after through-drainage has ceased.
Like pulling a saturated sponge out of a bath of water - when the sponge stops dripping it is at the full point.
Expressed as mm of water over a certain depth (e.g. 0-70cm).
Will not be a constant VSW down the soil profile.
Dependant on soil type, but independent of the crop.
The water content in root zone 1 is used as the full point, refill point and to calculate PDWU.
The water content of the soil below which the plant exhibits some form of stress, and a drop in yield.
Expressed as mm of water over a certain depth (e.g. 0-70cm).
Will not be constant down the soil profile.
The advent of stress might be identified by a drop in the daily use water
The consequence might be that the roots start extracting water at a greater depth.
Dependant on both soil and crop.
The water content in root zone 1 is used as the full point, refill point and to calculate PDWU.
The water content at which the crop cannot recover.
Wine grapes can be taken drier than the refill point to improve quality, and the permament wilting point is then used as a reference point.
The amount of water between the full point and the permanent wilting point
The amount of water between the full point and the refill point.