As part of my vineyard was planted with high pressure water; I have tried this method also for making depressions. One advantage is that you are not restricted to a round hole and there is no danger of glazing. A deep hole is more difficult to make. I have tried cutting a kerf about 10” square and 4” to 6” deep then using a shovel to extract the chunk of dirt. In porous soil, this may be sufficient? After the chunk is removed, the process can be repeated for more depth. It takes about 10 to 20 seconds to make the square kerf. This can be done in the summer as the high pressure will still cut through dry soil. I built a high pressure tank trailer that produces 4000 psi. And 4 gal/min. This may be a problem in wet soil as a tank of water is very heavy? 300 gallons weighs 2400 lbs. plus the weight of the unit.