Matticus wrote:

A little while ago I went on holidays to find that my garden had
scorched in the heat in my absence. The watering system had worked as
programmed, but this was not nearly enough for the unexpected heatwave
.. In short, this has lead me on a bit of a campaign to find a more
appropriate way of using just the right amount of water at just the
right time.

So far the best way of doing this seems to be to take
evapotranspiration into account - I can get weather data easily enough
from the local airport, and I can set my PC up to control the watering
system .. I'd probably have to determine the co-efficients for my lawn
/ garden beds through some trial and error.. The real question I've got
is - has anyone had any experience with this type of thing?

The problem is likely to be relating remote data to your own soil
moisture content, which in turn will relate to individual crop (plant)
requirements and ground cover, as well as soil type (moisture holding
capacity).

Various instruments are available for direct measurement of both soil
moisture and radiation. Some are possibly cost effective for your
application... http://www.sowacs.com/sensors/sensingaug.html

The challenge might be to use the instrumentation to control the
watering system. Tensiometers and gypsum blocks are probably the most
basic instruments providing outputs that could be used (both of which
have been used by irrigators for decades). It's even relatively easy
to make your own of either.

Many of the commercially available home weather stations also have
optional sensors for both soil moisture and radiation.

--
John H