An idea that gets talked about a lot, but I've not seen actually tried is to use the cool water that is watering your lawn to cool the house before it goes to the lawn.
Well, Rick has done an actual experiment with this method, and it appears to work pretty well.
This is Rick's experimental setup just to test how well the system works before building a working model, but it would not take a lot to transform this into something that could work on a "production" basis.
The test setup consists of a 20 by 20 inch water to air heat exchanger with a box type fan to force air through it. Lawn watering water at about 53.5F at the input of the heat exchanger leaves the heat exchanger at about 57.5F. The flow rate is roughly 2.5 gpm. The air flowing through the heat exchanger is cooled by about 10F in Rick's current setup.
The heat added to the water (and removed from the air) is (2.5 gal/min)*(8.33 lb/gal)*(60 min/hr)*(57.5F - 53.5F)*(1 BTU/lb-F) = 5000 BTU/hr, or nearly half a ton of AC.
With 5000 BTU/hr of cooling, and with the airflow such that the air is cooled by 10F (as Rick measured), then the airflow needed is (5000 BTU/hr) / ((0.07 lb/cf)*(10F)*(0.24 BTU/lb-F) = 29,700 cf/hr or 500 cfm.
The 5000 BTU/hr of cooling is equivalent to 1465 watts of cooling power, so, If a 50 watt fan is needed to do this, than the COP (Coefficient of Performance) is (1464 watts) / (50 watts) = 29.3, which is equivalent to a SEER of about 125!. Its basically free cooling.
Note that Rick has not had a chance to experiment with different flow rates, and the results might be even better after that.
Once the water cools the house it goes on out to the lawn for watering.
With the current setup, there is about 150 gallons of water use per hour, but if the water is going to be used anyway for lawn irrigation, one might as well get the free cooling.
While this method increases the relative humidity of the air passing through it just because of the temperature reduction, unlike an evaporative cooler, it does not add any water to the air, and this is a plus.
The cost of the setup is not large: Rick paid $180 for the heat exchanger on ebay, and I'd guess a good fan of this type might be $30 if you don't already have one around.
This shows the basic setup with the fan pulling air through an water to air heat exchanger
that is supplied by a garden hose.
Water connection to the heat exchanger.
Rear view of the heat exchanger.
This shows the temperature drop through the heat exchanger with the water flowing.
In operation, there is typically about 10F of air cooling with the fan running on medium. This was enough to cool a 300 sqft space from 82F to 72F in about 1 hour.
Rick plans to do some more experimenting with this setup, and will hopefully let us know what he finds out.
Rick is in SW Montana, so the climate is dry, and high humidity is not normally a problem. Using this method in areas with high humidity would not work as well in that the air coming out of the cooler might be very humid.
As Todd mentions in the comments below, there is another example of this kind of cooling system here...