U.S. Mean Annual Precipitation
U.S. PrecipNet Research Sites
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We are asking participants to tell us about the methods they use at their research site(s). These data will be used to better understand the range of methods used across investigations, and will help to develop a set of standard methods for suggested use at all sites. This information will also help to perform meta-analyses of methods and results across projects. Please download the query form, complete it and send it to Michael Loik, University of California, Santa Cruz. We recognize that each research project has its own goals, and that not all of the questions asked will be relevant to all projects.
Many thanks for your participation!
Questions / Answers
Should I measure soil water content or water potential?
I've measured lot's of water potentials and water contents over
the years. Soil water potential is of course the best data point
to have from a physiological point of view. However, psychrometer-type
WP probes yield point measurements. I have seen way too many papers
that have psycrometers at 10 cm or 30 cm or whatever, and report
that as soil water potential. That data point may have absolutely
no relevance to what the plant root system is "seeing". Because
of that, I've switched to TDR and neutron probe based soil water
content measurements, then try to do the soil work to convert soil
theta to WP. That way you get integrated soil water content and
WP over soil depth gradients.
What sensors should I use for measuring soil moisture?
I refer to Table 3.1 on page 34 of Pearcy et al. (1989). Although
it may be outdated, it is a good general comparison of the advantages
and disadvantages of different sensor types. Some sensor types might
be easier to log than others. Installation might be easier for some
compared to others (e.g. seems most people put their TDR probes
in horizontally to avoid thermal effects, which may require more
disturbance per plot.) In my recent conversations with Austin McHugh
at Campbell, it is apparent that a lot of things come up when starting
to design a TDR system. For example, the wires for some of their
sensors can only go 80 feet, then have to be multiplexed, requiring
multiplexers every so many plots, depending on how far apart your
plots are, etc.
At one of our sites we're using an array of new soil matric sensors
to get really accurate WP data around individual shrubs. It's expensive
and simply cannot be duplicated across a site with lot's of factorial
treatments (at least with a finite budget). However, it's the only
way we can get biologically realistic soil WP data in heterogeneous
soils with heterogeneous root systems. TDR and neutron probes are
based soil water content measurements, they can be converted to
soil theta to WP, using empirically determined moisture release
curves. That way you get integrated soil water content and WP over
soil depth gradients.
There is a new type of sensor available now, called EnviroScan.
It is produced by Sentek (Australia) and distributed in the USA
through Campbell Scientific and Simplot Soilbuilders. The measurement
principle is similar to TDR probes, in that it is based on water
content effects on electrical capacitance, but they are in many
ways simpler to install than TDR probes and may be, because of that,
more precise. For one, they can be top-loaded into PVC pipes, and
need not be horizontally inserted like TDR probes. They can remain
in place for continuous monitoring, or can be moved around between
PVC tubes for spot measurements
We bought some ECH2O soil moisture sensors with data loggers for
the Big Bend Field site. The ECHO probe measures the dielectric
constant of the soil in order to determine volumetric water content.
It does this by finding the rate of change of voltage applied to
the sensor once it is buried in the soil. The probe has low sensitivity
to saline and temperature effects in the soil, has low power requirements
and high resolution. Up to 5 probes can be connected to the EM5
data logger that is designed for use with the ECHO probes. The probes
are about $75 each and the data logger is about $400.