"Nathan A. Smith" wrote in message
news
On Wed, 01 Sep 2004 13:41:14 +0000, dr-solo wrote:


running waterfall drops temp if air temp is lower than water temp.

Ok,

I talked to my 60+ year old physcist (spelling?) and learned a lot about
water flow. It's very complicated, but I will try to condense down what I
learned. From our conversation, I believe the above statement is false.
Here's why:

when water flows down a waterfall it has some amount of kentic energy
(provided realistically by the pump) this kentic energy in one way or
another is turned into heat. Let's assume for a moment 100% humidity,
then there would be no lose of water or energy due to evaporation. This
means all that KE is passed to the water, warming it up. On the flip side
of this is 0% humidity where almost all the energy will be used in
evaporation. Now the amount of KE is dependant upon the height of the
waterfall or the energy provided by the pump (minus friction and other
fluid loses). Thus it could be assumed that the larger the pump, the more
energy the waterfall *could* pass to the pond, given a high humidity. Now
what does this have to do with the outside tempature... Well if the
amount of energy necessary to drop the water temp is less than the
amount of energy being imparted to the water than there would be a
net gain of heat in the pond.

What does this all mean? From our conversation -- it is possible that
given the right circumstances that a pump ***COULD** (not saying WILL)
increase the temp in a pond.

The amount of heat energy absorbed by water through friction under garden pond
conditions is miniscule. More heat is absorbed by the water running through the
pump (if it is a submersible, it actually needs water flowing around/through it
to keep the motor from overheating - most pumps, if run dry, can burn out) than
by the action of the impellers themselves. But since the water is in contact
with the pump for a very short period of time, and then goes through wind chill
under wintry conditions, the heat gain is negligible compared to the heat loss
at the waterfall/air interface. Given that we are talking about winter
conditions, you will very rarely find 100% humidity conditions that would
prevent heat loss due to wind chill. And in the winter, even at 100% humidity,
the air temperature will play a huge factor in heat loss. If it is 100%
humidity, and the air temperature is less than the water temperature, the water
will lose heat, even if ever so slightly. I've seen mist/fog coming off my pond
in a cold winter drizzle (above freezing). That fog is carrying the heat of the
water out of the pond. And the amount of heat picked up by the water from
contact with the pump is tiny compared to the huge heat sink of an artic cold
front (drawing off more heat from the water than can possibly be replenished by
a garden pump).

Having said that, I keep my pump running (by not my waterfall) until icy
conditions (I only had a very small crust of ice on one end of the pond last
year, and only for two days) prevail simply because I don't want my filter to
stagnate. It is always good to keep fresh water running through the filter at
all times, if possible, especially at the latitude where I live. My pump isn't
strong enough to create currents that would be bothersome to fish trying to
survive the winter, and the pond is large enough that the currents that are
induced by the pump are minimized. The fish didn't seem to notice it at all
last year.

Again this would mean a pump that in imparting greater energy to water
flow (minus friction and the such) than is being removed by evaporation
and tempature cooling could increase a ponds tempature.

See above.