U.S. Department of Agriculture: Agricultural Research Service, Lincoln, Nebraska


Date of this Version



Transactions of the ASAE 39(2):459-464 (1996).


To determine if the internal drainage (downward movement of water out of the root zone) was adequate, we measured the movement of water out of the root zone in bordered plots planted to alfalfa. We applied three water quantity treatments: irrigation plus precipitation equal to one, two, and three times the calculated evapotranspiration (1ET, 2ET, and 3ET); and two irrigation water quality treatments: electrical conductivity (ECiw) of 0.1 S/m, and sodium adsorption ratio (SARiw) of 4 and ECiw = 0.34 S/m, SARiw = 16. Each treatment was replicated three times. Internal drainage amounts during the irrigation season (1 July to about 1 October) were as great as 843 mm. For the seven years at one site, the internal drainage averaged 585 mm, or 66% of the water applied (irrigation plus precipitation) to the 3ET treatment. Increased water application resulted in increased internal drainage. Irrigation with the 0.34 S/m water resulted in greater internal drainage (compared to irrigation with the 0.1 S/m water) at one site, but not at the other site. The 3ET treatment maintained soil water content near field capacity for the entire irrigation season, but a persistent perched water table was not detected. Internal drainage from the 3ET treatments exceeded the total water applied (irrigation plus precipitation) to the 1ET plots for 9 of the 12 site years. The tested soils have sufficient internal drainage capacity to allow supplemental irrigation without forming a perched water table.