reading, the diode temperature is usually very close, if not the same value as soil temperature.
The diode temperature is used by the Hydra Probe to make algorithmic temperature correction to
the electronics.
4.2 Soil Temperature
The user can select Fahrenheit or Celsius. Diurnal (daily) temperature fluctuations between
daytime highs and nighttime lows may be observed with the Hydra Probe’s temperature data.
These fluctuations will become less pronounced with depth. Vegetation, tree canopy, and soil
moisture are factors that will effect the diurnal soil temperature fluctuations. For example, in the
American Southwest, A Hydra Probe buried at a five inch depth will have very pronounced
temperature fluctuations between the nighttime lows and the daytime highs if there is no
vegetation insulating the soil. Seasonal trends can also be observed in soil temperature data. The
soil temperature range for the Hydra Probe is -30 to 40
4.3 Soil Moisture
4.3.1 Soil Moisture Units
The Hydra Probe provides accurate soil moisture measurements in units of water fraction by
volume (wfv or m
3
m
-3
). That is, a percentage of water in the soil displayed in decimal form.
For example, a water content of 0.20 wfv means that a one liter soil sample contains 200 ml of
water. Full saturation (all the soil pore spaces filled with water) occurs typically between 0.3-
0.45 wfv and is quite soil dependent.
There are a number of other units used to measure soil moisture. They include % water by
weight, % field capacity, % available (to a crop), inches of water to inches of soil, and tension
(or pressure). They are all interrelated in the sense that for a particular soil, knowledge of the
soil moisture in any one of these units allows the soil moisture level in any of the other unit
systems to be determined. It is important to remember that the conversion between units can be
highly soil dependent.
The unit of water fraction by volume (wfv) was chosen for the Hydra Probe for a number of
important reasons. First, the physics behind the soil moisture measurement dictates a response
that is most closely tied with the wfv content of the soil. Second, without specific knowledge of
the soil, one can not convert from wfv to the other unit systems. Third, the unit wfv allows for
direct comparison between readings in different soils. A 0.20 wfv clay contains the same
amount of water as a 0.20 wfv sand.
However, the same thing can not be said about the other measurement units. For example, to use
the unit common in tensiometer measurements, a one Bar sand and a one Bar clay will have
vastly different water contents. The wfv unit can also be readily used to estimate the effects of
precipitation or irrigation. For example, consider a soil that is initially 0.20 wfv, and assume a 5
cm rainfall that is distributed uniformly through the top one meter of soil. What will the
resultant soil moisture in the top one meter of soil be? 5 cm is 0.05 of one meter, so the rainfall
will increase the soil moisture by 0.05 wfv to result in a 0.25 wfv soil. For other units, this
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