Remember too that swales and water catchments are only one half of a calculated response to (specific) landscape dynamics. Plants (trees, perennials and annuals) make up the other half of the equation -"converting infiltration to biomass" is the sound bite. The usual argument against water catchment is about depriving surface streams of overland flow. The response to that argument is that; although swales and other catchments do reduce overland flow -the best and most significant thing that infiltration does is to increase "baseflow" of surface waters through groundwater recharge (as opposed to surface runoff) -baseflow is much better quality of water and contributes over a significantly longer duration. As with all techniques and technologies it is in fact only appropriate if it is truly filling a need (solving an issue ie; aforestation and reforestation, reduce flooding problems, etc...) You wouldn't want to infiltrate more than the soil could safely hold. Maybe a better question is when would you not want to use water catchments... I'll tell you some mistakes I have made and have seen -You don't want to work on too steep or too shallow of a soil profile, even in the northeast but especially in the west, bedrock can be quite close to the surface especially on hills...under those conditions, cutting swales for infiltration has caused landslides, flooding and erosion - it can all be mitigated through calculating coefficients of run-off friction and saturation rates to determine the frequency and depth of catchments -to be safer, novices should stick to working on less than 15 degree slopes. One idea that I came up with (after I caused a small landslide) is, instead of making the water soak down quickly into the soil, make it soak up! ..as in wick up into a row of compost or straw bales on contour end to end, allowing a slower release. This technique has proved successful in both a "woods-cultivated" ginseng crop that I installed in upstate New York and a hillside olive grove in coastal California. In addition to WATER FOR EVERY FARM, check out PRINCIPLES AND PRACTICES OF GRADING, DRAINAGE AND ROAD ALIGNMENT: AN ECOLOGIC APPROACH by Richard K. Untermann (available from the bookstore at ) Larry Santoyo San Luis Obispo, California 1.800.469.5857 santoyo@earthflow.com