IRRIGATION SUBSURFACE RETURN FLOWS AND WATER USE IN LYSIMETERS CONTAINING APPLE TREES

For 9 yr, subsurface return flow measurements from irrigated lysimeters containing "Macspur McIntosh" apple trees (Malus domestica Borkh.) on MM 111 rootstocks were collected. Water use with and without a grass cover crop was calculated as irrigation plus summer precipitation minus drainage. Drainages were consistently and significantly lower, and therefore water use higher, under grass than under bare soil but the differences were not large. The times of irrigation and drainage events were not closely linked. Drainage followed irrigation anywhere from a day to many days, stretching at times into weeks or months. By far the largest drainages, however, occurred during the irrigation season. Irrigation amounts provided most of the water for return flows with summer or winter precipitation contributing little to the drainage. At the higher irrigation levels the return flow quantities and calculated water use quantities clearly indicate that sprinkler design flow quantities are too high for the area and that improvements in irrigation efficiencies are possible. Key words: Drainage, irrigation, lysimeters, water use, apple trees