Black and decker makes a selfpriming washer. If the preasure is basicly gravity or a barrell, and you want it electric, then the self priming feature (regardless of brand) is what you will need.
If you want gas power they can also be made with a waterpump, a hose into a bucket or barrell, and a hose out.
Anothet idea, the next time it is dry, try treating the area with "stuff" (I do not remember the name) to keep the area clear. I am sure someone here knows the name of the chemical.
Personally, I just use bleach and a paint roller. Kills everything about 2 days.