Campbell Scientific HS2 and HS2P (HydroSense II) - Manual (Page 17)

HS2 and HS2P
6.4 Water Deficit Data
Water deficit data help irrigators with water management decisions. By default
the HydroSense II does not show water deficit data on the display screen. To
enable the water deficit data display, turn on Deficit Mode in the configuration
menus (p. A-1).
In water deficit mode, wet and dry references can be stored for up to ten soils.
The current measurement is compared to those reference values and relative
water content (RWC) is calculated on a scale from 0% (dry) to 100% (wet).
While any two volumetric water content measurements can be stored as “wet”
and “dry” references, the normal procedure is to store the wilting point as the
“dry” value and field capacity as “wet”. The relative water content is
calculated as
100 ×
−
−
=
dry wet
dry
VWC VWC
VWC VWC
RWC
where
VWC = the current measurement
VWC
dry =
the dry reference
VWC
wet
= the wet reference.
For example, a particular clay-loam soil may reach wilting point at 18% and
field capacity at 35%. With these references, a VWC measurement of 24.5%
will display as a relative water content of 38.2%. A VWC of 30% is calculated
as an RWC of 70.6%.
An RWC value greater than 100% indicates that the soil has a water content
value greater than the “wet” reference; a negative value indicates that the water
content is below the “dry” reference.
Water required (in mm of applied water) to return the soil to the “wet”
reference is also calculated and reported as water deficit. A negative deficit
indicates that that the water content is greater than the “wet” reference for that
soil type.
Since the sensors average water content along the length of the rods, the
sample volume difference between the 20 cm and 12 cm sensors is significant.
This difference affects the water deficit value. Therefore, the current
measurement and reference values must be made with the same rod length.
6.4.1 Soil Types
The HydroSense II needs three parameters to calculate relative water content
and deficit:
• “wet” water content set point
• “dry” water content set point
• sensor length
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